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Green roofsand façades

Gary Grant


Green roofs and façades



Green roofs and façades

Gary Grant


Details of all publications from IHS BRE Pressare available from:Website: www.ihsbrepress.comorIHS BRE PressWilloughby RoadBracknell RG12 8FBTel: 01344 328038Fax: 01344 328005email: [email protected]

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EP 74

© Gary Grant 2006First published 2006ISBN-13: 978-1-86081-940-7ISBN-10: 1-86081-940-0

The contents of this book reflect theknowledge and experience of individual andcorporate contributors. However, the authorand his sources, and the publishers, take noresponsibility for the subsequent use of theinformation, nor for any errors or omissions,it may contain.

Cover picture:A living wall at Quai Branly, Paris


‘An invasion of armies can be resisted, but not an idea whose time has come’.

Victor Hugo, Histoire d'un crime, 1852


Acknowledgements

The author thanks all those people who provided information, advice and images for this book, includingthe following:

Information and advice Images (copyright holder: page number, image description)

Emilio Ambasz Emilio Ambasz and Associates, Inc: 64, both images; 65 (Hiromi Watanabe, photographer)Olaf Bierfreund John Buchanan: 8Patrick Blanc Nick Clarke: 32Stephan Brenneisen Earthhomes: xii, earth sheltered buildingJon Broome Dusty Gedge: xii, extensive green roof, brown roof; 14John Buchanan Peter Harvey: 42Hugues Chalopin Alyson Hurt: 29Luke Engleback Kensington Roof Gardens: 11Charles Fentiman livingroofs.org/Derek Brown: 41; 43Andy Foster McAllister Architects/EcoSchemes: 62Mathew Frith Karlheinz Mauerhofer: 66Dusty Gedge Barry Nicholson: xii, simple–intensive green roof; 15; 20Peter Harvey David Shankbone: 12Alyson Hurt Shea She-sang: xi, intensive green roof; 27Mark Loxton Singapore National Parks Board: 26Rod McAllister Hans-Gunnar Skarstein: 5Barry Nicholson Hugh Wheadon: 54Stephen PeartTony Pollintine Drawings and the following pictures have been provided by the author:Shea She-sang Front cover; xi, green façade; 7; 13; 23; 24; 39; 50, all images; 53; 55; 57Hans-Gunnar SkarsteinMike Wells


vii

Contents

Preface ix

Definitions xi

1 Introduction 1

2 History 5

3 Policy 17United Kingdom 17Continental Europe 23Asia 26North America 28

4 Benefits 31Aesthetics 32Green roofs and the water cycle 33Thermal stability and energy conservation 35Air quality 37Noise 38Electromagnetic radiation 38Use of recycled materials 38Increase in open space 39Wildlife 39

5 Design, build and maintain 45Setting goals 45Structure 45Layers 47Planting, seeding and natural colonisation 51Mosses and lichens 54Slopes 56Green façades 56Irrigation 58Maintenance 59


viii

6 The future 61

References and useful websites and other sources of information 67


ix

Preface

Cities occupy 2% of the earth’s land area but devour 75% of the resourcesthat are consumed by humanity each year. The quality of design andmodes of living in cities are therefore key factors in terms of globalconservation. The Greater London Authority has estimated that Londonhas an ecological footprint twice the size of the United Kingdom. Clearlythis pattern of consumption is unsustainable. The problem is nowmagnified by the realisation that city living produces greenhouse gaseswhich are changing the global climate at an unprecedented rate. Thiscould lead to catastrophic instability.

It is against this background that I have come to believe that the builtenvironment must be altered to mimic the natural environment as a way ofrestoring ecosystems, reducing greenhouse gas emissions, andadapting to climate change. As people have known for millennia, buildingscan support soil and vegetation. By incorporating vegetation into thedesign of buildings (including both roofs and walls) and other structuresfrom the outset, the built environment can provide the so-calledecosystem services normally provided by the natural environment,including flood alleviation, food production, cooling and insulation. Roofscan even benefit the conservation of biodiversity. Building-integratedvegetation, the subject of this book, is not an optional extra, gimmick orpassing fancy but one element of a whole suite of measures which shouldbe used to restore ecosystems and help us move towards life withinnatural limits.

GG, October 2006


x


xi

Definitions

* Brownlie S, 1990. Roof gardens – a review. Urban wildlife now, No 7. Nature Conservancy Council,Peterborough.

Intensive green roof (roof garden)

Green façade

Green roof is used to describe both ornamental roof gardens and roofswith more naturalistic plantings or self-established vegetation. The termliving roof is increasingly being used instead of green roof in the UnitedKingdom.

Green roof (or eco-roof) is occasionally used to describe a roof which isgreen in the economic sense. For example, an energy efficient roof withphotovoltaic cells or extra insulation, or made of sustainably produced orrecycled materials; using the word green or eco to describe this type ofroof is questionable.

A roof garden has been defined by Brownlie* as an area of, usually,ornamental planting with a substrate isolated from the natural ground by aman-made structure of at least one storey. A roof garden is sometimescalled an intensive green roof.

Building-integrated vegetation is an all-encompassing term used todescribe any vegetation that has been deliberately seeded, planted orencouraged to establish itself on a built structure. On conventionalbuildings with vertical walls and pitched or flat roofs, building-integratedvegetation may be divided into green roofs and green walls (or greenfaçades).

A green wall (or green façade) is an exterior wall of a building that hasvegetation growing on it. Masonry and other building materials canbecome colonised by lichens, mosses, grasses and flowering plants that,in nature, grow on cliffs and rocky outcrops. Climbing plants may beinduced to grow directly against the building fabric or climb trelliswork;geotextiles also can be attached to walls and be planted or seeded.


xii Definitions

* Jackson S D, 1996. Overview of transport related wildlife problems in Proceedings of the InternationalConference on Wildlife Ecology and Transportation. State of Florida Department of Transportation,Tallahasee.

† Leeson B F, 1996. Highway conflicts and resolution in Banff National Park, Alberta, in Proceedings ofthe International Conference on Wildlife Ecology and Transportation. State of Florida Department ofTransportation, Tallahasee.

Extensive green roof

Brown roof

Earth sheltered dwelling

Simple–intensive green roof

A simple–intensive green roof (sometimes called asimple–extensive green roof) has a simple structure but is intensivelymanaged with, usually, irrigated, low growing vegetation. A mown lawnon a roof is an example of a simple–intensive green roof.

An extensive green roof consists of low maintenance, normally low-growing vegetation. The term eco-roof is sometimes used in connectionwith extensive green roofs.

Where a deliberate attempt has been made to maximise the opportunitiesfor wildlife to colonise a roof, biodiverse roof is a term sometimes used.These roofs are often created using native species of plant and are notnormally irrigated.

Where extensive green roofs have been designed to mitigate the loss ofspecies of brownfield sites, they have been described as brown roofs.Here the intention is to allow ruderal vegetation to colonise low fertilitysubstrates like those found in the rubble of demolished buildings.

A semi-intensive green roof is equivalent to a herbaceous border in agarden. It would normally have a variety of plantings and allow someaccess. Semi-intensive green roofs require occasional weeding andwatering.

An earth-sheltered structure is set into the ground, with a continuousearth cover replacing at least part of what would be the walls and roof ofa conventional building. Such a building is usually well vegetated andblends with the landscape.

A building-integrated habitat describes any deliberately establishedhabitat on a building. It may include substrates, green roofs, greenfaçades, perches, artificial roosts and nesting containers, boxes andvoids.

A wildlife overpass is a bridge designed to allow wildlife to cross amajor road or rail corridor. An overpass is often vegetated to encourageparticular species to use it* and illustrates the great potential forestablishing vegetation on buildings. Examples include highwayoverpasses planted with native woodland for use as bear and wolfcrossings in the Canadian Rockies†, and dormouse bridges over theChannel Tunnel Rail Link in Kent, England, planted with woodland shrubs.


1

A man is now more likely to be the man in the street than a man on thefarm. Approximately 50% of the world’s population now lives in townsand cities, and this is expected to grow to 55% by 2015 [1].

In China, it is estimated that every year 8.5 million people permanentlyleave the countryside to live and work in the booming cities on theeastern and southern coasts. Our current over-reliance in our townsand cities on artificial, bolt-on, climate-changing technologies can onlybe a temporary solution. An example of the contrast between theecosystem service and conventional approaches would be coolingusing vegetation compared with energy hungry mechanical air-conditioners; the need to find new approaches is becoming anincreasingly urgent problem because cities continue to grow in numberand size, and urban lifestyles are already unsustainable.

In 2006 the think tank that put international debt onto the agendas ofthe G7 and G8 summits, the New Economics Foundation [2], estimated

that, if the whole population ofthe world were to live like peoplein the United Kingdom, theresources of 3.1 planets wouldbe needed. A typical moderncity has an ecological footprint(ie the area needed to maintainthe population in terms of food,resources and waste disposal)of between 100 and 300 timesthe area of the city itself. TheGreater London Authority hasestimated that London has anecological footprint twice thesize of the United Kingdom [3].

Chapter 1

Introduction

The natural environment provides so-called ‘ecosystem services’. By mimicking the natural habitats these services can be provided in the urban environment

Habitat

Geneticresources

Soil fertility, health and stability

Clean water

Flood protection

Pollination

Pest controlRegulation ofclimate

Shade andshelter


2

Given this background, and the need to make the built environmentwork even harder, it could be argued that building-integrated vegetation– the subject of this book – is not an optional extra, gimmick or passingfancy, but one element of a whole suite of measures which should beused to restore ecosystems and help people move towards living withinnatural limits.

The various components of the built environment tend to be simple andsingle purpose, and therefore inefficient. The built environment shouldbe designed and operated in a multi-functional way: to make space fornature and its associated ecosystem services. Building-integratedvegetation (ie living roofs and walls) is a very good example of multi-functional urban design. A roof or external wall can be more than just aweatherproof surface or structural element – it can be a living, cooling,cleansing skin. But how is development planned and implemented? Andwhy doesn’t more vegetation feature in the built environment?

Current trends promise a more joined-up approach. However, in thepast, urban development has often been conceived by a number ofspecialists working together but with relatively little ‘trans-disciplinaryinteraction’ – in other words, with little exchange of ideas. Typicallydevelopment follows a series of well established, sequential steps.

Planners envisage neighbourhoods or zones where particular activitiesmay take place, driven by the need to separate homes from industry orto provide easy access to transportation. Certain areas where historicmonuments or rare habitats occur may be identified for preservation or,occasionally, translocation. Transport engineers lay out a grid ofhighways with standard geometries for a new urban district. Architectsfit buildings into the polygons traced on the map by the grid of roads.Drainage engineers plan the re-routing of streams and networks ofdrains. Utility companies reserve corridors for electricity, gas, waterand telecommunications lines. Finally landscape architects are invitedto specify lawns, small trees and shrubs for left over corners –sometimes called SLOAP (space left over after planning). There is abetter way!

The climate, microclimate, geology, hydrology, landscape character,ecology and history of a place can be evaluated more thoughtfully.Where development is appropriate, planning can begin with greeninfrastructure which might involve identifying and providing a network ofopen spaces – which may be walking and cycling routes, but can alsoprovide ecosystem services in the form of watercourses, flood storageareas and wildlife habitats – to link the city or town centre with the widercountryside. The green infrastructure can then be the setting for landefficient development which is closely linked with public transportnetworks. Wherever possible, elements within the urban fabric shouldbe multi-functional.

Introduction2 Introduction


Introduction 3

Buildings can incorporate energy generating technologies likephotovoltaic and small wind generators; but they can also be fully linkedto greenspace networks and, by incorporating building-integratedvegetation, may even form nodes or corridors within greenspacenetworks. This brings landscape architects and ecologists into a closerworking relationship with other professionals dealing at all scales, andadvising before, during and after development.

The International Institute for the Urban Environment, an offshoot of theDelft University of Technology, is one of several organisationspromoting the concept of innovative multi-functional intensive land use(MILU) [4]; where urban sites are used intensively for several functionsboth day and night, MILU offers residents, workers and tourists the bestcombination of services and environmental quality while minimising thewider ecological footprint. It advocates intensifying use, integratingfunctions, building higher and building underground, and optimisingdaily, weekly, monthly and seasonal use of space. This approach leadsto a reduction in travel by car, less time spent travelling, more socialintercourse, better heath, less land take and lower carbon emissions.There are political, legal and technical problems in making thispossible, but, ultimately, building-integrated vegetation is the greenglue that can make high-density living work by linking the builtenvironment to open space networks.

For millennia there have been citizens of towns and cities who haveconsidered their environment to be unnecessarily barren and havesought to improve life by making space for nature in the form of parks

and gardens. Parks and gardensare essential, but a city with toomany parks is not a city. Putsome of the greenery on theroofs though and the city doesnot lose critical compactness orcohesion. Building-integratedvegetation is oftencharacterised as unaffordablebut, as the following chaptersdemonstrate, it restores naturalcapital and reduces runningcosts. Most of the unused spacein towns and cities is on therooftops: for example, buildings(and therefore roofs) cover24,000 hectares or 16% ofGreater London [5]. This iswasted space in a world that canno longer afford that luxury.

Some of the benefits of building-integrated vegetation

Multi-functional building-integratedvegetation provides, shade, cooling,habitat and rainwater attenuation.Could provide food and clean water.Links the built environment withadjacent green infrastructure

Multi-functional green infrastructureprovides food, shade, windbreaks,

drainage, flood protection, habitat, openspace, cycle paths, footpaths and

corridors for mass transit and utilities


4 Introduction

Now, at the beginning of the 21st century, after more than a century oftheory and experiment, there are indications that building-integratedvegetation is about to enter mainstream living. The signs are verypromising. An indication of the widespread interest is the recentestablishment of national and international umbrella groups promotinggreen roofs.

The European Federation of Green Roof Associations was establishedin 1998; and in Canada, Green Roofs for Healthy Cities [6], a non-governmental organisation that promotes green roofs, was founded thefollowing year.

The Scandinavian Green Roof Institute, which is based in Malmo,Sweden, opened in 2001. This Institute boasts a 9500 m2 roof topgarden (the Augustenborg Botanical Roof Garden) that is the world’sfirst such facility, designed to demonstrate to the public its research onvarious substrates, plants, run-off and slopes.

The International Green Roofs Association [7] organises an annualconference and acts as an umbrella body for national green rooforganisations, promoting the idea worldwide and the value of greenroofs as a tool in sustainable urban development. Most recently,Australia, New Zealand and Mexico have joined the list of national greenroof associations.

The American Society of Landscape Architects unveiled a green roof onits Washington DC headquarters building in April 2006, promoting thetechnology as a way of cooling and filtering the air, and reducing theburden on drains [8]. According to the Toronto-based Green Roofs forHealthy Cities organisation, the United States has about 215,000 m2 ofgreen roofs; however, Germany, the country which leads the world inroof greening, has installed about 130 km2 of green roofs and iscurrently installing them at the rate of approximately 13 km2 per year.The green roof industry in Germany is currently worth an estimated

300 million per annum – still a relatively small sector but substantialenough to have its own quarterly trade magazine, Dach + Grün.

The broader environmental benefits of green roofs are well tested andbecoming better known. Green roofs make buildings more thermallyefficient, prolong the life of a roof, ameliorate the extremes oftemperature and humidity, moderate surface water run-off, providegreenspace for people and wildlife, and help to reduce air pollution andnoise. In addition, the vegetation that green roofs provide within anotherwise grey urban setting has psychological benefits for people whooverlook them. All this suggests that building-integrated vegetation hasthe potential to play a significant part in improving the quality of urbanlife and protecting the wider environment from the demands of the cityand its citizens.


5

Some of the earliest known buildings were made by Neolithic (or NewStone Age) peoples living in south west Asia about 10,000 years ago.Although none of the buildings from that time survive, it seems likelythat most of them were made from simple, locally available materialsincluding stone and earth, and were probably vegetated to someextent.

The Nordic tradition of covering roofs with turf – which continues to thepresent day in several northern countries including Norway, Iceland andthe Faeroe Islands (see below) – goes back, at the very least, to theBronze Age (which in Scandinavia began around 3,800 years ago).

There are several remaining examples of relatively sophisticated earth-sheltered and turf roofed structures – even whole settlements from theBritish Bronze Age (from about 2,800 years ago), one of the best

examples being at Jarlshofon the Shetland Islands.Turf is a durable andreadily available buildingmaterial. The warmth fromthe hearth inside thetypical turf covereddwelling meant that theearly growth of grass onthe roof provided a ‘firstbite’ in spring for sheepand goats.

Chapter 2

History

A Nordic turf roof


The Estonian National Museum [9] has described the construction of atraditional turf roof on the Baltic Island of Saaremaa. A woven willow orhazel hurdle was placed on the rafters set at a 30° slope. The hurdleswere covered with three layers of birch bark and the bark covered withtwo layers of turf – the first (lower) layer of turf being grass-side downand the second (upper) layer grass-side up.

The technology has clearly worked very well; as recently as the early1900s, more than half of the population of Iceland were still living in turf houses [10].

Turf was a critically important building material in the American andCanadian mid-west in the early 19th century [11]. Timber was scarce inthe prairies before the establishment of the railways and settlers oftenconstructed houses from turf. Although the walls were usually scrapedclean, the turf roof was often left to grow by default. These so-called'sod houses' of the prairies were warm in winter and cool in summer, in a climate with sweltering summers and bone-chilling winters wherethose without the protective blanket of turf often died. Some of thehouses have survived. Interestingly, the walls of sod houses usuallyincluded voids which would attract snakes seeking places to hibernate– an early, but accidental demonstration of the potential of building-integrated habitat.

In warmer climates, roof gardens are as old as civilisation. People wereprobably aware of the ability of plants to shade their homes during theday while providing valuable sources of food close to hand, especiallyduring times of siege or famine.

One of the world’s first civilisations was that of ancient Mesopotamiawhich developed irrigated agriculture and thrived in the valleys of theTigris and Euphrates rivers between 6,000 and 2,400 years ago.These early farmers built ziggurats which were pyramids of earth, brick and stone. The archaeologist Sir Leonard Woolley, who waslargely responsible for the modern re-discovery of ancientMesopotamia, found evidence of trees and shrubs being planted onthese structures. Also from ancient Babylonia were the HangingGardens, built around 2,600 years ago and listed in the 2nd century BCby Antipater of Sidon as one of the Seven Wonders of the World.According to the Greek historian Diodorus Siculus (writing severalcenturies later), King Nebuchadnezzar II commissioned the building ofthe Hanging Gardens. They consisted of a series of tree coveredterraces supported on masonry vaults and watered by a concealedriver-fed irrigation system.

6 History

OppositeA garden at Mont Saint Michelconstructed above the abbey

buildings. The abbey, built on anisland off the coast of Normandy,

was first established in the 8thcentury, and extended and rebuilt

in the centuries that followed. Inthe 13th century cloisters were

constructed on the roofs ofaccommodation below and it

seems that at that time thebuilders were seeking every

opportunity to create roofgardens, perhaps conscious ofthe vulnerability of the island to

siege. Plantings on the structurenow include lawns, herbaceous

borders, vegetable patches andhedges. It seems likely that morespace was given over to growing

food in the past. The Englishmade several unsuccessfulattempts to seize the island

during the Hundred Years War(1337–1453). Perhaps therooftop vegetable gardens

helped the besieged clerics tosurvive in those years


History 7

Consider the following passage from Chapter 19, Verse 26 of theSecond Book of Kings in the Old Testament which suggests thatvegetated roofs were common on more humble dwellings of the period:‘Therefore their inhabitants were of small power, they were dismayedand confounded; they were as the grass of the field, and as the greenherb, as the grass on the house tops, and as corn blasted before it begrown up.’

Several roof gardens have been identified in the ruins of Pompeii whichwas buried during the eruption of Mount Vesuvius in AD 79. Oneexample, just outside the town on the road to Herculaneum, had aplanted terrace above an arched colonnade. It is believed thatcolonnades would have been used by people seeking shade in the heatof the day, with the evaporative cooling from the soil and plants aboveproviding additional comfort. The roof terraces were probably enjoyedmost often in the relative cool of the morning, evening and night.Mediterranean people continue to enjoy roof gardens to the present

day, with many escaping to the roofto sleep under the stars in scentedgardens.

There are several examples of roofgardens from the Middle Ages. Theabbey at Mont Saint Michel is one ofthese (see left).

The Palazzo Piccolomini at Pienza,Italy, was built for Pope Pius II in thesecond half of the 15th century aspart of a new town to function as asummer residence for the papalcourt. Although altered in someways, it survives to this day. A smallsouth facing roof garden is enclosedon three sides by high ivy coveredwalls. A carefully designed drainagesystem prevents water fromreaching the stables below. Thereare flowerbeds, box hedges, afountain and, shrubs, fruit trees andlaurel trees.


8 History


History 9

Other roof gardens were constructed for the Medici family in Tuscany.One roof garden at Careggi, now ruined, was thought to be used as ashowcase for exotic plants. Over a period of about four centuries,beginning in the 14th century, the Gonzaga family turned Mantua, alsoin Tuscany, into a city of palaces with roof gardens and courtyards. Oneof the most famous medieval buildings in Italy is the Guinigi Tower inLucca (see opposite).

The Aztec city of Tenochtitlan, located at the centre of an area nowcovered by modern Mexico City, was established early in the 14thcentury and destroyed by Spanish invaders in 1521. The Spanishconqueror, Hernando Cortes, described how the leading citizensenjoyed roof gardens. The city once housed 200,000 people on anisland at the centre of a large wetland, Lake Texcoco. As well as usingrooftops, the people created floating gardens on the lake by coveringbundles of sticks with soil.

The roof gardens of Renaissance Italy were probably the inspiration forthose created in imperial Russia. Six hectares of gardens wereconstructed in two phases on adjoining roofs at the Kremlin during the1680s. The gardens included a pond, fountains, fruit trees, shrubs andvines. Soldered lead sheets were used for waterproofing. The gardenswere removed in 1773 during reconstruction works. In 1764, on herascension to the Russian throne, Catherine the Great, commissioned anItalian architect, Rastrelli, to build a roof garden of pavements, lawnsand small trees on the Hermitage, St Petersburg. The gardens are stillin use.

Sir Benjamin Ward Richardson was a prominent Victorian physician,anaesthetist, sanitarian – he founded the Journal of Public Health in1855 – and early advocate of the benefits of cycling. Writing in 1876 [12],he argued that in all large towns the roof of each house should be agarden. He believed that widespread creation of roof gardens wouldresult in a vast increase in the health and the happiness of thepopulation.

In 1905 the engineer and advocate of garden cities, A R Sennett,remarked that there were many elaborate roof gardens in Berlindesigned for outdoor entertainment [13].

In the second half of the 19th century, German architects and engineersbegan to experiment with new materials and building techniques thatwere suited for the construction of flat roofs. The German builder,Rabbitz, developed a waterproof cement (exhibited at the Paris WorldExposition of 1867) which was subsequently used in the creation of anumber of roof gardens on family homes in Berlin.

OppositeThe Guinigi Tower in Lucca, theconstruction of which began in1384. It was named after thethen ruler of the town, PaoloGuinigi. Nowadays it supportsseveral holm oaks, Quercus ilex,growing in 60 cm deep brickplanters more than 40 m abovethe ground. This roof has beenadopted as the emblem of thegreen roof organisation in Italy,the Associazione Italiana VerdePensile


10 History

The industrial revolution brought great change to Berlin. Cheaperhousing for the workers who poured into the city from the countrysidewas provided in the form of flat roofed, five storey apartment blocksknown as mietskasernen*. The roofs of these buildings werewaterproofed with bitumen, which was highly flammable [14]. In order toreduce the risk of fire the bitumen was subsequently covered with sandand gravel. Although it was not the original intention of the builders, themietskasernen roofs were colonised by vegetation. These accidentalgreen roofs were long-lived – some surviving well into the 20th century.

In the building boom of 19th century New York City, high-rise roofgardens gave respite from the sweltering heat at a time before theintroduction of air-conditioning. As in Germany, new materialsstimulated new styles of building which were better suited to creatingthe necessary load bearing and waterproofing characteristics requiredfor roof gardens. According to the eminent modern American landscapearchitect and roof garden designer, Theodore Osmundson [15], the term‘roof garden’ was popularised during the 1890s by the musician andimpresario, Rudolph Aronson, who established gardens on the roofs oftheatres to provide space for outdoor summer performances. This wasa response to the high land costs in New York City.

Parisian-style garden theatres were also popular at the time but couldonly be established in the suburbs where land was relatively cheap. Thefirst roof garden was opened on the Casino Theater on Broadway in1882. Following this lead, the architect, Stanford White, rebuilt theMadison Square Gardens in 1889 to include a roof garden theatre. Atotal of eight roof garden theatres opened in New York City during the1890s. By the 1920s these had all gone – victims of rapidly changingfashions in entertainment, including the onset of cinema – but they wereprobably the inspiration for the many private and hotel roof gardens thatfollowed. The idea of roof gardens spread afar, even reaching the FarEast. The Tokyo Mitsukoshi department store roof garden, which wasfounded in 1914 was destroyed in the 1923 earthquake butsubsequently rebuilt. The replacement survives to this day and was themodel for several other department store roof gardens.

London was never noted for its roof gardens; perhaps the abundance ofparks and private gardens, and the sometimes unsettled summerweather and winter smog, reduced the demand. At the beginning of the20th century, Frances Anne Bardswell [16] noted that roof gardens inLondon were rare but described a few examples including the roofgarden of the Home for Working Boys in Bishopsgate Street in the Cityof London, which had ‘trees up to twenty feet high including,sycamores, limes, nut, cedar, chestnut, holly, fir and plane’.

* Mietskasernen (literally translated as rental barracks) are, typically, wide and deep five-storeyapartment blocks, with central courtyards, built in the early 1900s.


History 11

The Garden Cities Associationwas founded in 1899, a yearafter the publication ofEbenezer Howard’s seminalwork To-morrow: a peacefulpath to real reform (which wasreprinted in 1902 as Gardencities of to-morrow) [17]. Howardand his fellow campaignerswere horrified by the unhealthyconditions of the Victorianslums and wanted healthiertowns with clean air, and withineasy reach of the countryside.Howard’s ideas were put intopractice at Letchworth (founded1903) and Welwyn Garden City(founded 1920). This inspiredothers in the United States andGermany to follow suit. Theabundance of open space inthese utopian towns meant thatthe incentives for establishing

green roofs were absent. Letchworth was the site of the firstroundabout (traffic circle) in the United Kingdom, built in 1909, aportent of the car culture that has since brought a dark side to theseutopian towns. A R Sennett brought a scientific and technological slantto garden cities [13] with suggestions of self-cleansing streets andmoving pavements, however the roof gardens of Berlin were hisinspiration for a call for the widespread use of roof gardens in Britain.Sennett was particularly disappointed when the huge school buildingprogramme of the time missed the opportunity to make more use ofroofs as playgrounds.

One of the 20th century’s the most influential architects was theWisconsin-born architect Frank Lloyd Wright. Wright often incorporatedoutdoor terraces into his designs and some of his public andcommercial buildings have planted terraces. The Monona TerraceCommunity and Convention Center, Madison, Wisconsin, wasconceived by Wright in 1938 but not realised until 1997. It incorporatesa planted roof terrace that overlooks Lake Monona. The Swiss-bornmodernist architect Le Corbusier had a similar interest in providing roofterraces. An early example was the Weissenhof, a dwelling in Stuttgart,Germany, that was built in 1927. He also occasionally incorporatedrooftop lawns into his schemes. An example of the latter is on hisWeekend House which was built in a suburb of Paris in 1935. Famousroof gardens from the 1930s that may still be seen include the Derryand Toms department store garden in Kensington, London (see above),

The Spanish Garden at theKensington Roof Gardens,London. (The building wasformerly the Derry and Tomsdepartment store.) Intensivegreen roofs like this are, in effect,small parks


12 History

and the Rockefeller Center in New York (see above), both described indetail along with many others in Theodore Osmundsen’s book Roofgardens (published in 1999). The designer of the Derry and Toms roofgarden, Ralph Hancock, went on to design two of the roof gardens atthe Rockefeller Center.

Another influential roof garden of the period was the park at UnionSquare, San Francisco, which was constructed above an undergroundcar park in 1942. This structure was the first of its kind and became themodel for dozens of other similar structures built throughout the UnitedStates and Japan in the decades that followed.

In the late 1950s and early 1960s a few office and municipalcomplexes in the United States and Europe began to boast roofgardens. Examples from Switzerland include the Grosse Schanze Parkthat is on top of the bus and train stations in Berne, and the Ciba Geigybuilding in Basel. In the United States examples from this period include

Roof gardens at the RockefellerCentre complex, New York,

consist of a series of five roofgardens on medium-rise

buildings overlooked by high-risetowers. The architect, Raymond

Hood, wanted roof gardens toimprove the outlook for office

workers and argued that higherrents could be charged for this. In the original scheme the roof

gardens were to be linked bybridges, but cost cuts during the

Depression meant that theseplans were never implemented


History 13

the Oakland Museum and Kaiser Center, both in Oakland, California, andConstitution Plaza, Hartford, Connecticut.These early Americanprojects inspired hundreds of similar schemes in rapidly growingmodern cities throughout the world including Tokyo, Singapore, HongKong and Perth (Australia). An example from the United Kingdom fromthis period is Harvey’s Department Store, Guildford, designed byGeoffrey Jellicoe in 1957 and subsequently listed by English Heritageas a Grade II garden. It includes mature trees and shrubs and variouswater features. Another example from the United Kingdom is the WillisBuilding (originally the Willis Faber and Dumas headquarters) in Ipswich.Designed by Lord Foster and built in the 1970s, at the time it was thenewest building ever to be listed Grade 1 by English Heritage; it has aroof top canteen surrounded by a roof garden. Gateway House inBasingstoke, Hampshire (see above), designed by the engineers Arup inthe late 1970s, has roof gardens built on several levels.

In 1960, Reinhard Bornkamm, often referred to in Germany as thefather of modern green roofs, began his study of the extensive greenroofs of the Berlin mietskasernen. His research team found that nearlya century after the roofs were first created, around 50 had survived [18].Inspired by Bornkamm’s rediscovery of the mietskasernen roofs andthe advent of new waterproofing technologies in the 1970s, themodern green roof industry in Germany emerged, led by people likeGerda Gollwitzer and Werner Wirsig with their landmark book Roofareas inhabited, viable and covered with vegetation which waspublished in 1971.

Gateway House, Basingstoke. The planting beds on the roofgardens, which contain soilbetween 225 and 900 mm indepth on several levels, are fullyintegrated into the structure


14 History

Gollwitzer and Wirsig were followed by many other roof greeningenthusiasts in the late 1970s including the Stuttgart based, landscapearchitect, Professor Hans Luz, who argued that roof greening is anecessity rather than a luxury. In 1972 the Austrian artist, architect,nature conservationist and peace activist Friedenreich Hundertwasserappeared on the television programme Wünsch Dir Was (Make a wish),making his wish roof afforestation. (It wasn’t until 1986 with the openingof the Hundertwasser-Haus, a housing development in Vienna which hashundreds of trees planted on it, that his wish became reality).

Although specialist green roof companies were established in Germanyas early as the late 1950s, it was not until the early 1980s thatextensive green roofs became commonplace.

In 1977 the German Landscape Research, Development andConstruction Society (usually known as the FLL) established a group toresearch and publish guidance for the planning, building andmaintenance of green roofs – an indication of the maturity of theindustry in that country at that time. Attempts to establish similarcommittees in the United States and United Kingdom are only beingmade now, some thirty years later.

Important participants in the process of establishing the green roofresearch group in Germany were Professor Hans-Joachim Liesecke ofHannover and Walter Kolb of Veitshoechheim in Bavaria who wereresponsible for much of the pioneering work quantifying the benefits ofgreen roofs in reducing rainwater run-off and cooling. It was the

The roof of a building in Grünwald,Germany, dating from the 1890s.Originally covered with sand, andsubsequently allowed to colonisenaturally, it is now dominated bylichens and mosses


History 15

translation of the work of the FLL and other German speakingorganisations into English during the 1990s that really gave theproponents of roof greening in the United Kingdom and North Americathe evidence and past experience that they needed to bolster their owncases.

In the late 1980s and early 1990s, various charities, institutions,housing cooperatives and individuals in and around Londoncommissioned the architecture firm, Architype, and others associatedwith the Walter Segal Trust [19], to design a number of new timberbuildings. The architects had adopted the philosophy of ‘footprintreplacement’ whereby greenspace lost through development should bere-established on roofs. One of those buildings was the Centre forWildlife Gardening built for the London Wildlife Trust; other similarbuildings were 11 Shaw’s Cottages (see below) built in Lewisham, southeast London in 1993, and the Centre for Understanding theEnvironment (CUE) at the Horniman Museum [20] Extension, Forest Hill,south east London (opened in 1994). All these projects includedspecifications for green roofs. The objective in each case was to createa wildlife habitat and the technique was to use turf plug-planted with wildflowers. Turf was chosen for its stability on sloping roofs.

In 1997, concern for the conservation of the black redstart (which, inthe United Kingdom, is a rare and protected bird often associated withderelict land) was the motivation for conservationists including greenroof advocate, Dusty Gedge, and urban ecologist, Mathew Frith, toinsist that living roofs be incorporated into new buildings in south and

The author (left) with Jon Broome(owner and architect) on the roof of

11 Shaw’s Cottages, Lewisham,south east London, a home built in

the Walter Segal style


16 History

east London. The resulting crushed concrete and brick roofs weredesigned to support self-colonising wasteland wildlife. The first of theseroofs, at the Laban Dance Centre, was completed in 2002. Many more(covering an estimated 15,000 m2) have been built, or are currentlyunder construction, having been secured by local planning authoritiesthrough conditions attached to planning approvals. Recently focus hasturned to the question of whether and how living roofs can providehabitat for rare insects and spiders living on brownfield sites [21],particularly those in the urban fringes of northern Kent and south Essexknown as the Thames Gateway.


17

United Kingdom

To date, building-integrated vegetation has been largely overlooked bypolicymakers in the United Kingdom. Perhaps the equable climate ofthe British Isles and the preponderance of private gardens have createdan intellectual cocoon shielding us from some of the more urgentenvironmental problems facing most other people in the world. Eventhough there are no specific roof greening policies, it could be arguedthat policies on construction, biodiversity conservation, drainage,energy conservation, open space and urban renewal could beinterpreted as being supportive of the idea. Local authorities in theUnited Kingdom are now beginning to issue advice notes that recognisethe value of green roofs, although the emphasis is on persuasion ratherthan the provision of incentives or the establishment of regulations. Thepolicy vacuum in the UK is in contrast to some other countries – mostnotably Switzerland, Germany, Japan and some cities in the UnitedStates – where the value of green roofs has been recognised and arebeing more actively promoted by government organisations.

In the United Kingdom, in common with other developed countries,there has been much consideration of the need to renew towns andcities, particularly where the urban cores and industrial centres havesuffered a decline in population and neglect of the urban fabric duringthe 20th century. Early efforts at urban renewal during the 1980s arenow seen by some as rather crude and rushed, with an over-reliance onthe private car and the widespread use of cheap building techniquesand generic ‘anyplace’ low-rise buildings and landscapes.

In 1999, the Urban Task Force, a government appointed panel ofexperts led by Lord Rogers, issued its finding as a report entitledTowards the urban renaissance. In the same year the House ofCommons Select Committee on Environment, Transport and RegionalAffairs published its 17th Report on the same theme. Following these

Chapter 3

Policy


18 Policy

reports, in 2000, the Government issued its Urban White Paper, thedocument that confirmed a commitment to developing brownfield(previously-developed) sites. Much of the argument for reusingbrownfield land is based on the misconception that it has no ecologicalor landscape value when compared with greenfield sites; yet brownfieldsites like the former oil refinery on Canvey Island (the Canvey Wick Siteof Special Scientific Interest) have been found by the entomologist,Peter Harvey, and others to have some of the richest invertebratefaunas anywhere in the country. Brownfield sites are often set in placeswhere there is a severe shortage of open space; another reason whymany of them should never have been redeveloped.

On the positive side, the Urban White Paper did recognise theimportance of more thoughtful urban design and the value of opengreenspaces in cities. If the urban renaissance continues to promotemoves away from low density, land-hungry suburban developmenttowards more innovative, sustainable, well designed, higher density,multi-functional schemes, it seems likely that building-integratedhabitat, including green roofs, will play an increasingly prominent role inthat change.

The government’s adviser on nature conservation in England, EnglishNature (now part of Natural England), has sought to understand theimpacts of the construction industry through its construction sectoranalysis [22]. That report identified the obvious negative impactsassociated with the construction industry but also recognised thatintelligent development can bring many opportunities including,amongst others, the enhancement and creation of new wildlife habitat.Since the beginning of the new century the construction industry hasbeen undergoing rapid change.

In 1998 the Office of the Deputy Prime Minister (now the Departmentfor Communities and Local Government) set up the Construction TaskForce which identified a series of key performance indicators for amodernised construction sector. Part of the follow-up to that report wasa sustainability working group which identified biodiversity as one of itskey performance indicators. The emphasis in these policy documents,however, is on the avoidance of damage to existing sites ofconservation value and the appropriate management of land. This isessential but needs to be matched with a commitment to restoringbiodiversity and recognition of relevant Biodiversity Action Plans (seepage 21 for more on BAPs).

Architects are becoming more aware of the need for environmentallyresponsible design and there is a long history of pioneers inarchitectural theory integrating their thinking to include the needsinfrastructure, culture and ecosystems. There has been a surge inpublications on green architecture in the last two decades.


Policy 19

Interest in green architecture continues to grow and there is now adeveloping interest in living roofs with conferences on the subjectorganised by the Architects’ Journal and others on an annual basis.Construction industry initiatives, like the Building Research EstablishmentEnvironmental Assessment Method (BREEAM) for assessing theenvironmental performance of new buildings, consider aspects ofnature conservation which encourage the creation of building-integratedhabitat, although at present they are more likely to be interpreted asrequiring the planting of native species in the grounds of developments.

The Construction Industry Research and Information Association (CIRIA)promotes best environmental practice in the industry and manages theConstruction Industry Environment Forum (CIEF). It has producedresearch that proposes the adoption of environmental indicators toencourage habitat creation [23]; by implication that would include greenroofs. In 2006, CIRIA published guidance and launched a website thatexplores the design, construction and maintenance of green roofs, andprovides up-to-date advice [24].

As England’s towns and cities have grown, so local communities havemade an effort to ensure that sufficient open space is provided foroutdoor recreation and visual relief from the built environment. Forexample, local plans normally aspire towards the National Playing FieldAssociation’s standard of 6 acres (2.43 hectares) per 1000 inhabitantsfor playing fields and more modest targets for other types of park.Normally, however, insufficient open space exists in inner city areas tomeet targets. This has led to the adoption of strategies of hierarchicalprovision of open space whereby the objective is to ensure thataccessible spaces of various types and sizes are provided.

To policy demands for open space for sport, relaxation and informalplay now are added new demands for more natural areas which providewildlife habitat. The World Heath Organisation encourages localgovernment to recognise and act upon the links between open spaceprovision and health [25], echoing the ideas of Sir Benjamin WardRichardson in 1876 (see page 9). Open space encourages people towalk or cycle, to work and mingle; it absorbs rain, filters out pollutants,blocks noise, cools the city, and, if not too barren, provides wildlifehabitat. Building-integrated vegetation can be a vital part of the openspace network.

Green networks are interlinked natural or vegetated open spaces inotherwise built-up or intensively exploited areas. They are promotedthrough various policies at European and national levels. In England,official policy (Nature conservation, PPS9) emphasises the importanceof corridors and linkages. Dense development in urban areas interruptsgreen corridors or links; therefore there will normally be limitedopportunities to create substantial new areas of greenspace in built-up


20 Policy

areas without compulsory purchase and demolition which areexpensive and unpopular. Roof greening is the most promising methodof solving this problem, something that was recognised by the TokyoMetropolitan Government in 2001 (see more on Tokyo on page 27).

English Nature has promoted the concept of accessible multi-functionalgreenspace networks [26]. Added to the early ideas of open spacesbeing important for recreation and beauty are the additional functionsof biodiversity conservation, sustainable drainage, pollutionabatement, local transport corridors, climate amelioration and outdoorclassrooms [27,28]. The green network concept has its origins in thelinear parks which link urban parks to rural areas, an approachspearheaded in the United States and then adopted in Europe [29]. In theUnited Kingdom green networks have been promoted through theconcept of green chains and wildlife corridors as pioneered in the newtowns of Telford and Milton Keynes. Green chains figure in a number oflocal plans and are recommended in strategic planning guidance forLondon. Green chain policies seek to link accessible open spaces.Green roofs, vegetated viaducts and bridges and covered cuttingscould be used to improve the more urban sections of chains.

Although the ability of habitat corridors to act as conduits for wildlife infragmented landscapes has been called into question [30,31], they maystill be attractive open spaces for people and can be valuable habitatsin their own right. In 1997, George Barker [32] wrote that, in thedisturbed environment of lowland England, ‘a close mosaic of steppingstone habitat patches may be as effective as a continuous strip inallowing [many species] to permeate the whole area’. This suggeststhat green roofs, being parts of buildings and therefore not normallykey parts of wildlife corridors, could become valuable as componentsof a mosaic of stepping-stone habitats in urban neighbourhoods.

A simple–intensive green roof usedas accessible open space. Thisexample is the Sainsbury Centre forthe Visual Arts at the University ofEast Anglia, Norwich. Opened in1978, this project was LordFoster’s first major public building


Policy 21

In 1994, Rohde and Kendle [33] made the case that contact with natureshould be part of everyday life. Following on from this, English Natureadopted its own standards for the provision of easily accessible naturalgreenspace in urban areas [34]. The recommendations included one that‘urban dwellers should have an accessible natural greenspace within300 metres from home’. Living roofs or roof gardens on redevelopedcommercial or municipal sites are probably the only realistic way ofachieving this objective in many densely developed inner cityneighbourhoods.

Even open spaces which are inaccessible to people are important,absorbing rainfall, constituting secondary elements in a green network(eg in providing visual benefits) and providing refuges for wildlife whichmay colonise or visit adjacent, publicly accessible open spaces. In thisway inaccessible green roofs could provide a valuable role providingadditional habitat and species diversity in urban areas.

The Rio Declaration of the United Nations Conference on Environment in1992 – the Earth Summit – led to the formulation and adoption ofstrategies for sustainable development and biodiversity conservation in150 countries. In 1994 the United Kingdom Government publishedBiodiversity: the UK Action Plan and established the UK BiodiversityAction Plan Steering Group that started to publish countrywide targetsand action plans in 1995. However, very few of the UK BiodiversityAction Plan priority habitats and species are found within towns orcities, or can have their conservation addressed within towns andcities. Even fewer opportunities exist for the conservation of priorityhabitats and species through using building-integrated habitat.

Local Biodiversity Action Plans (LBAPs) have subsequently beendeveloped through many local authority-facilitated partnerships totranslate national targets for species and habitats into effective action.Importantly, they also identify local conservation priorities that reflectthe particular character of each area. Few LBAPs target buildings or thebuilt environment, but there are some – including the City of London,Newcastle and Westminster (an inner London borough) – which do.There are also a small number of species-and-habitat action plans thathave been published which can be directly linked to living roofs and builtstructures. These include species action plans for the black redstart (inLondon, Birmingham and the Black Country), peregrine falcon (London)and long-tongued bumblebee (London).

Local planning authority nature conservation strategies – which mostlydate from the late 1980s and more recently are seen as underlining anauthority’s commitment to a LBAP – recommend the protection of sitesof conservation interest through zoning plans. As well as Sites ofSpecial Scientific Interest (SSSIs, designated by central government)and sites of county or metropolitan importance (designated by county


22 Policy

and metropolitan councils), local planning authorities normally identifyother sites of local value where development would not normally bepermitted. However, it is recognised that local sites will be developed ifthere are pressing needs. In these cases the authorities usuallyrecommend that equivalent areas be created nearby so that the overallarea of wildlife sites is maintained. This approach has been referred toby Birmingham City Council [35] as ‘maintaining or increasing the stockof constant natural assets’. Where sites of recognised value aredeveloped, and the sites lost consist of habitats that can be re-created,green roofs could play a role in ensuring that the overall stock ofgreenspace is maintained or increased.

Various species receive special protection under legislation such as theWildlife and Countryside Act 1981 (as amended) and the EuropeanHabitats Directive 1992. Relatively few of these protected speciesoccur in urban areas in significant numbers. However, there are somespecies that may be associated with derelict buildings, brownfield orother urban areas deemed suitable for redevelopment. Examplesinclude birds such as the black redstart, which in England shows amarked tendency to breed and forage on urban derelict sites, andwhich is listed in Annex I of the European Birds Directive and is includedon Schedule 1 of the Wildlife and Countryside Act 1981. As alreadydescribed in Chapter 2, living roofs are being established to provide thenecessary habitat for this and other characteristic ‘wasteland’ speciesas redevelopment of derelict sites proceeds [22,36,37,38].

Specific UK government policy on roof greening is awaited. The City ofLondon (the original centre of the metropolis), in conjunction with theBritish Council for Offices, is promoting green roofs through thepublication of an advice note [39].

In 2004, the Mayor of London launched his living roofs campaign [40].This is presented as part of the sustainable growth agenda whichemphasises how access to open space and sunshine will becomeincreasingly important given the trend to encourage more people of allages and backgrounds to move into the city. Balconies, roof gardensand terraces are seen as a way of compensating for the lack of privategardens in the inner city. Perhaps further guidance, incentives and evenlegislation will come from London and other cities in the UnitedKingdom as the population continues to grow, and as average summertemperatures continue to rise and comfort levels fall.


Policy 23

Continental Europe

The impressive advances in roof greening in Switzerland and Germanyhave been attributed to the positive policy environment. Most greenroof advocates believe that they will need to persuade theirgovernments to adopt similar regulations or incentives before roof andfaçade greening techniques become mainstream in their owncountries.

In Switzerland, land use regulations require all federal agencies toapply the ‘Swiss Landscape Concept’ when commissioning orrehabilitating buildings and installations. This means that facilities mustbe compatible with natural settings and landscape [41].

Also Swiss federal legislation has been designed to protect endangeredspecies. Interpretation of this policy has resulted in the construction ofliving roofs; examples are to be found at the Sihlpost Railway Station inZurich (see below) where platform roofs have been created as wildlifehabitat.

Sihlpost Station, Zurich. Roofshave been designed to

accommodate wall lizards, a raregrasshopper and several rare

species of bee inhabit thetrackside ballast. When the

platforms were extended, thenewly constructed platform roofs

included stony habitat for thelizards and rare invertebrates.

Lizard ladders, in the form of rock-filled gabions, connect the new

roofs with the ground


24 Policy

A roofscape in Basel, Switzerland, in whichextensive (at top of photograph) and intensive

green roofs (below) are combined


Policy 25

In certain Swiss cantons, most notably the city of Basel, all newcommercial flat roofed buildings must include green roofs. Basel has alocal electricity tax designed to promote energy conservation. Some ofthis tax revenue was used to subsidise green roofs in 1996 and 1997,with 20 Swiss francs paid by the city authorities for every square metreof green roof installed during that period – a total of 135 projectscovering 85,000 m2. Since 2002, regulations in Basel have stipulatedthat flat roofs larger than 500 m2 must incorporate soils of varyingdepths that are appropriate to the surrounding region, primarily toencourage rare invertebrates [42].

Nearly half of all cities in Germany, in a list approaching 120 (includingBerlin, Frankfurt, Karlsruhe, Munster and Stuttgart), offer incentives forroof greening [43]. Direct financial support ranges between 25% and100% of the installation cost. The main rationale for this is thesignificant saving in heating and air conditioning costs. Indirect subsidyis also provided by some German states and cities where drainagecharges are reduced for developments with green roofs. The FederalNature Conservation Act requires mitigation for the ecological impactof building construction. This means that biodiverse roofs aresometimes required under conditions attached to construction permits.

Munster in Germany is typical of several German cities in thatstormwater management has been the main driver for green roofs. The city charges owners for the volume of stormwater that runs off aproperty. Fees received are used to maintain the municipal drainagesystem. If a living roof is installed the fee is reduced by 80%.Stuttgart sits in a valley that tends to suffer from poor air quality andhas suffered in the past from urban development that has left the areadenuded of natural vegetation. There has been considerable interest inpromoting green roofs to alleviate these problems. The city itself hasled the way in Germany by greening 105,000 m2 of its own buildings,but, since 1986, has also provided a 50% subsidy for the installation of55,000 m2 of other green roofs. There is also a regulation that requiresflat and low pitch roofs to be covered by extensive green roofs.Stuttgart is the home of several of the world’s leading roof greeningcompanies.


26 Policy

Asia

For decades the government and citizens of Singapore have plantedand nurtured high quality public open spaces and private gardens. Theyare now turning their attention to greening built structures (see below).This has been highlighted with the publication in 2005 by the NationalParks Board of a guide for the selection of plants for green roofs [44].The publication came out of a partnership between the Housing andDevelopment Authority and the National Parks Board, assisted by anumber of private companies, to establish green roof pilot schemes ona multi-storey housing estate in the Punggol district.

Roof gardens at the Edgefield Plains high-rise housing scheme in Singapore. This arrangement is common in Singapore,Hong Kong and China. Typically roof gardens of this kind are above commercial centres or car parks and provide quiet,safe, traffic-free open space for local residents


Policy 27

Rapid economic growth in China, with burgeoning car ownership,smoke belching industries and dust bowls in the countryside, hascreated serious problems with air quality in the cities. The authoritieshave been increasingly concerned with tackling the problem, but, withthe eyes of the world turning to Beijing for the 2008 Olympics, therehave been a number of initiatives to green the city. Beijing is too denselydeveloped to create many new parks so the Beijing Municipal andForestation Bureau has set a target of greening 30% of high-risebuildings and 60% of low-rise (less than 12-storey) buildings by 2008.The official news agency Xinhau [45] has reported that, by the end of2006, Beijing plans to have between 80,000 m2 and 100,000 m2 ofroof gardens with this rising to 300,000 m2 by 2008.

Greater Tokyo is, arguably, the most densely developed metropolison earth and has suffered from an increasingly severe ‘urban heatisland’ problem as the post-war building frenzy has progressed. In2001, the Tokyo authorities finally recognised that something neededto be done and recommended tree planting and new parks, and set atarget of creating 30 km2 of green roofs [46]. Also in 2001 the Tokyometropolitan government amended its Nature Conservation Ordinanceto compel developers of new private buildings with a footprint largerthan 1000 m2, and new public buildings with a footprint larger than250 m2, to green 20% of their roof areas, or face an annual fine. Thisnew law had an impressive effect with the area of green roofs almostdoubling from about 50,000 m2 to over 100,000 m2 in a year andseveral green roof construction companies being established includingsome by leading conglomerates (eg the car manufacturer, Toyota). In2005, the Japanese government followed Tokyo’s lead, passing a lawthat requires all new apartment or office buildings in urban areas tohave at least 20% vegetated rooftops.

A roof garden in a modernhousing scheme in

Shenzhen, GuangdongProvince, China. In its

burgeoning cities China hassevere problems with air

quality and access togreenspace. Chinese

planners have recognisedthat roof greening will form a

key role in making moderncities more comfortable


28 Policy

North America

In 2000, the City Council of Toronto, Canada, adopted anenvironmental plan that ‘indicated its intention to identify’ how greenroofs could be built and what benefits they could bring. A new policywas adopted in 2002 that recognised the value of green roofs intackling the urban heat island effect, and, following the production of adiscussion paper in 2005, the council finally adopted a green roofstrategy [47] in January 2006. The next steps for the city include theproduction of technical booklets, grants for pilot projects and planningagreements to secure green roofs. The city is also considering thepossibility of reducing water rates for properties with green roofs.

Portland, Oregon, is considered to be one of the leading authoritieswith a roof greening policy in North America. In the early 1980s, the citywas encouraging roof gardens through planning incentives. To datethere are around 30 green roofs covering approximately two hectares,but this total is set to grow. There are now new regulations requiringroof greening of public buildings. All new buildings developed in the citymust have a roof with at least 70% vegetation cover. Whereverpractical, replacement roofs must also be green. The planning systemin Portland now works in a way that allows developers more floor spaceif their project includes a living roof. Management charges are levied forsurface drainage in Portland which are calculated according to the areaof impermeable surfaces on a site. Now there are plans to reduce thesecharges by 35% if a property owner has green roofs covering at least70% of the building footprint. It has been suggested that developmentproposals for the central business district are more likely to enjoysmooth passage through the design review process and planningapproval system if they include living roofs. Furthermore, green roofsfeature in Portland’s stormwater management manual which reflectsthe widespread local concern for the maintenance of good quality waterin the Willamette River, a popular place for canoeing and an importantsalmon river.

In Chicago, Illinois, the drivers for green roofs are concerns over theurban heat island effect and air quality. The Mayor of Chicago has takena personal interest in the issue, and has initiated pilot schemes andprovided test plots. Policies in Chicago that encourage thedevelopment of green roofs include an Energy Conservation Code,passed in 2001, requiring roofs to have a minimum solar reflection(albedo) of 0.25. Although this policy does not specify green roofs, thecity authorities accept green roofs as a way of meeting thisrequirement. There is also a ‘building green’ policy that allows a higherdensity of development for buildings with 200 m2 or 50% roof greening,whichever is greater [48]. There are also small grants for living roofs anda system of stormwater retention credits. By 2004, Chicago had about80 green roofs covering approximately 100,000 m2.


Policy 29

New York City is another conurbation with a serious urban heat islandproblem and rainstorms often cause foul sewers to overflow into thewaterways. The creation of new open space is almost unheard ofbecause of the high value of land, and yet, according to Colin Cheney [49],flat roofs make up to 29% of the land area of Manhattan which, ifvegetated, would cover 500 hectares or an area twice the size ofCentral Park. New Yorkers have a history of creating private roofgardens but the biodiverse green roof is a relatively new concept. Alocal non-governmental organisation, Earth Pledge, created a greenroof on their headquarters building and launched a green roofs initiativein 2001 that led to the establishment of a green roofs policy task forceand the New York Ecological Infrastructure Study in 2002 [50]. Fifteenstate and federal agencies and departments have participated in theinitiative. Research on how green roofs might help with stormwaterproblems is underway and extensive green roofs are planned or underconstruction at Pace University in Lower Manhattan, in Queens and inthe South Bronx. The Queens Botanical Gardens is planning a wildflowermeadow roof. The 27-storey Solaire development in Battery Park Cityincludes two green roofs – an accessible roof garden and anotherextensive living roof. Private initiatives include a refurbishment of theNassau Brewery Icehouse in Brooklyn which incorporates a green roof.

A New York City roof garden. With 29% of the land area ofdistricts such as Manhattanbeing flat roofs, theopportunities for roof gardensare immense


30


31

The various benefits of building-integrated vegetation echo those whichhave been identified for vegetation, parks or open space. The closeproximity, though, of vegetation to buildings is likely to have moreeffect on the interior comfort of a building than an adjacent park,however grand. The various benefits of green roofs and green façadesmay be physical, psychological, ecological and economic, and includethe following:

● attenuation of rainwater run-off● evapo-transpirative cooling● reduction in the urban heat island

effect● thermal insulation● wildlife habitat● food production● amenity

In addition, living roofs may becomponents of water conservationschemes; increase humidity in dryweather; absorb air pollutants anddust, electromagnetic radiation, andgreenhouse gases (eg CO2); act asnoise barriers; make use of recycledmaterials (eg from demolition waste);provide tranquillity, green screening,and extra space for relaxation andrecreation, close gaps in greenspacenetworks; and protect buildings fromharmful ultra violet radiation.

Chapter 4

Benefits

Some of the benefits of building-integrated vegetation

Noise barrier

Biodiversity

Protection fromUV radiation

Thermal insulation

Restful views

Evapo-transpirative cooling

Attenuation of rainwater run-off


32 Benefits

Aesthetics

Large-scale architectural projects are often presented to clients and thepublic as models that are viewed from above, and, even though theymay contain barren roofs, they are presented as exemplars of gooddesign. Roofscapes tend to be rather ugly with each roof oftencontrasting in a discordant way with its neighbours. Close up,conventional roofs tend to be grey and cluttered, with vents, airconditioning plant and equipment often installed with no considerationfor appearance. This may be in striking contrast with building entrancesand façades where every care is taken by the architect to ensure adramatic or aesthetic effect. Building-integrated vegetation is theeasiest way to screen ugly equipment, and harmonise and soften theroofscape. Many dull and uninspiring façades could benefit fromgreening. Bland exteriors – for example, like the concrete walls of someof the housing blocks, and municipal and cultural buildings of the 1960sand 1970s – have provoked some to call for their demolition even if thebuildings function very well in terms of the activity within. An easy wayto revitalise, colour and improve these buildings is to vegetate them.

An extensive green roof on atoilet block in Cradle Mountain

National Park, Tasmania,reduces the visual impact of the

building on the naturalenvironment


Benefits 33

Green roofs and the water cycle

The predominance of impermeable surfaces in our expandingconurbations creates rapid run-off and higher peak flows followingheavy rainfall. These increasingly large volumes of dirty water arecontaminated and can damage infrastructure, farmland and wildlife,sometimes a long way downstream.

Toxins and contaminants (eg chlorinated organics, hydrocarbons, silts,heavy metals and excess nutrients) from buildings surfaces and streetsfind their way into watercourses. Studies by the US EnvironmentalProtection Department in the 1970s demonstrated that the first flush(ie the initial washing of surfaces by rainfall after a dry spell) containsthe worst pollution. Urban peak flows can overload and damage drains,cause flooding, carry huge loads of sediment, erode unprotected banks,and even cause sewage to overflow into streets and watercourses.Dissolved oxygen levels in streams may then fall and this, combinedwith the increased silt load, kills fish and aquatic invertebrates. Thedifference between urban and natural run-off quantity and quality can bevery pronounced: the majority of rain falling on conventional roofs runsoff before it can evaporate, but in most situations the majority of rainfalling on soil soaks into the ground where it is filtered.

Climate experts are predicting that global warming will cause anincrease in the intensity of rainfall in some areas, including, forexample, south east England where the infrastructure is alreadyoverloaded and where further housing development is set to continuefor decades. It is predicted by climatologists working for the UKgovernment’s Department for the Environment, Food and Rural Affairs(DEFRA) that winters in the UK will become wetter and summers drier.Sustainable drainage systems (SUDS) are now being promoted as away of reducing the effects of urban development on drainagesystems. SUDS use a variety of techniques to filter, absorb andmoderate rainwater run-off, thereby helping urban designers andengineers to mimic the drainage characteristics of natural catchments.Permeable surfaces, soil, vegetation, swales and ponds all feature indrainage systems that are designed using the SUDS approach. Someproponents of SUDS have been rather slow to acknowledge theimportant role that living roofs can play as part of sustainable drainagesystems, but, in the UK at least, the latest guidance provided by theNational SUDS Working Group [51] in 2004 does refer to green roofs as acomponent of SUDS.

The drainage characteristics of green roofs vary considerablyaccording to the depth and type of substrate and recent weather, withdeeper and drier substrates retaining most water. However, experiencein Germany over many years has shown that 75% of rain falling on atypical extensive green roof can be retained in the short term, with asmuch as 15–20% of this being held in the soil for up to two months [52].


34 Benefits

In an extended trial at the Fencing Academy of Philadelphia [53], 10 mmof rain fell in 20 minutes on an 70 mm-thick extensive sedum roof whichwas already saturated following an earlier extended period of rainfall;even in the most severe five minute period of the storm, only 20% of therain was lost as run-off. Multiplied many times across a city, green roofscould have a dramatic effect on run-off rates, perhaps reducing them togreenfield levels if used in combination with other run-off mitigationtechniques. In inner city areas, where there is insufficient room forswales or ponds, or where permeable pavements may not beappropriate, green roofs or roof gardens may be the only componentsof SUDS.

If predictions of climate change for the United Kingdom are correct,there may be more water shortages which will create a demand forwater reuse and recycling schemes. No doubt there will be watershortages in many other countries. Approximately one third of potable,

Green roofs can play a valuablerole in sustainable drainagesystems (SUDS). They provideSUDS features in locations whereotherwise there might not besufficient space and increase theamount of water lost throughevapo-transpiration, improving thelocal microclimate

Natural catchment Urban catchment

Urban catchment with living roofs

Urban catchment withSUDS and living roofs

40% evapo-transpiration

40% evapo-transpiration40% evapo-transpiration

30% evapo-transpiration

55% run-off10% run-off

45% run-off 10% run-off

15% infiltration50% infiltration

15% infiltration 50% infiltration


Benefits 35

domestic mains water in the United Kingdom is used for flushing toilets.More water is used on lawns. The huge waste of potable water could bereduced by purifying grey water – ie water used in buildings other thanfor toilets – for use on vegetation, and for flushing and irrigation.

Roofs provide the necessary space for water recycling schemes.Constructed wetlands (most often reed beds) are commonly used totreat effluent. The cleansing occurs in the root zone of the reed plantswhere microbes break down impurities. Constructed wetlands of thiskind can be installed on walls or roofs. An experimental vertical reedbed was established on a residential block in Berlin some years ago. InLondon, Water Works UK Ltd is promoting a constructed wetland watercleansing system designed to operate on the roofs of buildings [54]. It isessentially a convoluted channel that allows grey water to flow througha sward of low-growing native wetland plants to produce pathogen-freewater with low suspended solids and low biological oxygen demand. Aswater shortages bite and the costs of potable water rise, theseschemes will begin to make commercial sense. The ‘sky swamps’ of thecities of the future will not only conserve water but will be particularlyeffective in providing evaporative cooling in summer heat. They will alsobring wetland wildlife to the rooftops.

Thermal stability and energy conservation

Most roofs are constructed from materials like asphalt, slate or claytile, concrete or steel that heat up quickly in sunlight. If this is coupledwith poor insulation, or the absence of cooling ventilation,temperatures in the space below the roof, including living space, maysoar. If air conditioning is used to combat high temperatures, electricityconsumption (and associated net carbon production if the powersource is a fossil fuel) increases. Planting on and above a roof canreduce energy costs by providing summer shade. Deciduous plants canwork well in temperate climates, screening unwanted summer sun butallowing winter sun to warm buildings or provide welcome light.

Water stored in growing substrates and vegetation is lost throughevaporation and transpiration. Evaporation increases as temperaturerises. Transpiration is the process where water is lost from plantsthrough pores (stomata) that allow the movement of gases in and out.The most important characteristic of evaporation and transpiration, interms of the ability of vegetation to maintain comfort, is the change ofstate of water from liquid to vapour; this process requires energy (theso-called latent heat of evaporation) which has a cooling effect. Therate of evaporation depends on the humidity and temperature of the airbut, in most climates and situations, it is vegetation that providesrespite from the summer heat.


36 Benefits

By the 1970s, meteorologists were beginning todescribe the phenomenon of cities becomingsignificantly warmer, sometimes several degreeswarmer, than the surrounding countryside, as the urbanheat island effect [55]. Higher temperatures in citiesincrease the demand for air conditioning and lowers airquality. The main cause of urban heat islands is concrete,masonry and tarmac absorbing and storing radiant heatfrom the sun during the day and re-radiating it at night.The temperature of a conventional dark-coloured roof(with low reflectivity or albedo) may exceed 60 °C on ahot summer day. In contrast vegetation has a higheralbedo than the building materials, does not store heatwell and actually loses heat through evapo-transpiration.

In the past, climatologists have suggested painting roofswhite, but the simple solution to the urban heat islandeffect is to bring more vegetation into the city. This hasbeen demonstrated admirably by the city of Chicago thathas compared the roof temperatures of similar municipalbuildings – one with a green roof and one without – andfound significant benefits with the green roof [50]. On a hotsummer’s afternoon in August 2001, the difference intemperature between the conventional and green roofswas 28 °C! With that kind of benefit multiplied throughoutChicago, the city authorities have calculated that annualsavings of $100 million could be made in reducedelectricity demand for air conditioning. A sophisticatedcomputer model, which uses Toronto as its study area,predicts that if 5% of the roofs were vegetated the urban heat islandeffect at midday in June could be reduced by between 1 and 2°C [56].This suggests that a fairly modest adoption of green roofs could havereal benefits in terms of improving comfort and saving energy.

Green roof systems can improve the thermal insulation of a building,helping to keep heat out in warm weather and thereby reducing thedemand for air conditioning. In winter the extra insulation that a greenroof provides helps to keep warmth in, thereby reducing heatingdemands. Most designs for green roofs include air gaps and otherinsulating layers between the main structure and growth medium thatcan further improve insulation. In Germany, in 2000, savings in heatingoil from extensive green roofs were estimated at 2 litres/m2/year [57].

A study by the City of Chicago, comparingconventional and green roofs, found daytimedifferences in roof temperatures of 28 °C

Conventional roof

Green roof

Day Night

Tem

pera

ture


Benefits 37

Air quality

Cities continue to have air quality that does not meet minimumstandards thought to be necessary for good health. The GreaterLondon Authority has estimated that in the United Kingdom anestimated 24,000 people die prematurely each year from the effects ofair pollution. In Asia there are blankets of brown air which threaten thehealth of millions and may even be weakening the annual monsoon(which brings rain essential for agriculture) by reducing the sunlight thatreaches the seas. Air pollution needs to be tackled primarily at source.However, large scale planting programmes can help to alleviate theproblem, and living roofs and other building-integrated vegetation canhelp to provide the extra space required for this purpose.

Vegetation has been shown to reduce atmospheric pollution by filteringparticulates and absorbing gaseous pollutants. The ability of vegetationto filter and absorb pollutants depends on weather conditions, the typeand concentration of the pollutants, the local topography, the locationof the planting, and the nature of the vegetation. Vegetation provides alarge surface area for filtering particulates (dust and soot), which caninclude heavy metals (including cadmium, copper, lead and zinc) andpathogenic microbes. Plant species with a high surface to volume ratio– those with leaf hairs, for example – are best. Noxious gases, whichhave adhered to particulates, may also be filtered out by vegetation.Some air pollutants that are filtered by plants are then bound to the soil.This shows the mechanism whereby green roofs can reduce heavymetal and other pollution in run-off. Bill Wolverton, working for NASA onself-contained artificial ecosystems, demonstrated how atmosphericpollutants, including nitrates and volatile organic chemicals, may beabsorbed through the leaf stomata and subsequently metabolised byplants [58,59].

A major component of photochemical smog is ozone which is formedwhen nitrous oxides (NOx) and volatile organic compounds (VOCs) reacton hot sunny days. High temperatures, which may be increased by theheat island effect, exacerbate the problem. Stratospheric ozone formsa layer many kilometres above the earth where it filters out dangerousultraviolet light, but near the ground ozone can aggravate breathingdifficulties and damage vegetation, even in low concentrations (lessthan 40 parts per billion). Even rural areas down wind from cities haveincreasingly high concentrations of ozone that are believed to causecrop damage estimated at $2 billion per annum in the United States [60].Tackling high temperatures helps to reduce photochemical smog.


38 Benefits

Noise

It has been estimated that up to 20% of the population of the EuropeanUnion (ie about 80 million people) suffer from noise levels that healthexperts consider to be unacceptable [61]. Noise barriers are usuallymade from solid materials like concrete or earth. Narrow belts ofvegetation are not an effective barrier to noise; however, the soil orsubstrate of roof gardens is useful in this respect. The deeper thesubstrate, the better the roof at deadening noise. Unpublished researchby Kalzip (a roof manufacturer) suggests that a roof of standardconstruction (ie unvegetated) can reduce sound which reaches theinside of a building by 33 dB. With a 120 mm deep extensive green roofthis reduction can be extended to 41 dB when the roof is dry and 51 dBwhen the roof is wet (saturated). This compares with a typical reductionof 43 dB for a 100 mm concrete wall. These figures suggest that anextensive green roof can reduce sound within a building by 8 dB ormore when compared with a conventional roof. One of the statedreasons for using a living roof on the headquarters building of The Gapin San Bruno, California, designed by architects William McDonough &Partners (opened in 1997), was that it absorbs sound emanating fromnearby busy highways and flightpaths [62].

Electromagnetic radiation

Vegetation is known to absorb electromagnetic radiation. Although acontroversial subject with an absence of convincing evidence, there isincreasing concern over the possible negative effects ofelectromagnetic radiation associated with radio transmissions andpower supplies. Vegetation, including that on green roofs has thepotential to be part of future strategies for reducing exposure toelectromagnetic radiation.

Use of recycled materials

Many of the materials used in green roof construction (eg membranesand drainage mats) are manufactured from recycled plastics. Extensivegreen roofs in London are often made from recycled building materials(eg crushed brick and concrete). By using these materials, developerscan avoid charges incurred through disposal at landfills. Growing mediamay contain composts made from household waste.


Benefits 39

Increase in open space

Most roofs are unused for anything except protection from theelements. By adding green roofs to buildings, the area available forleisure, recreation or wildlife can be increased. The total area ofbuildings (and therefore roofs) in London is 16% of the total land area orabout 24,000 ha [63]. Through the process of redevelopment andrefurbishment, a substantial part of this potential could be realised forrecreation and wildlife habitat while retaining development density andurban amenity at street level.

Wildlife

There are, currently, very few examples of living roofs where habitatcreation is a primary objective, with the exception of some of the morerecent projects in Basel, and the stony and grass roofs in London; butinterest is growing and many projects are planned. Habitats that havebeen deliberately re-created at ground level include grasslands,

woodlands and scrub, heath, coastalhabitats and wetlands, and it istechnically possible to create some ofthese on roofs. In theory, any terrestrialhabitat that can be created in aparticular location can be created on aroof in the same area, although theremay be constraints associated withlimited areas available on typical roofs,extremes in microclimate anddifferences in hydrology. Where ponds,trees and shrubs have been establishedon buildings – for example, inconventional roof gardens – they arenormally isolated or in small groups andoften dominated by non-native species.The ability of these plantings to provideecological functions associated withtrue woodlands or scrub is limited, butthe potential is there to do more ifnative species of trees and shrub areplanted in natural associations and inlarge enough stands.

In practice, cost considerations meanthat living roofs are normallyconstructed using relatively shallowsubstrates which support low growing,open or sparsely vegetated areas

Roof top species-richgrasslands in the foreground atthe Wollishofen waterworks,Zurich, Switzerland, created in1913. Local soil was placedonto the roof to provideevaporative cooling in summer.An accidental side effect wasthe conservation of the species-rich grassland, which supportsseveral species of orchid. Thishabitat has now disappearedfrom the surrounding districtthrough urban developmentand agricultural intensification


40 Benefits

where vegetation succession is slow or arrested. With no ground waterreserves and exposure to desiccating winds, these roofs tend to dryout completely in drought conditions. This places a severe limit on thespecies of plant that are able to thrive. In addition, soil-dwelling orburrowing organisms that tend to migrate to damper ground or burrowdeeper during extended dry spells may have no refuge on a living roof.

Another limitation is the lack of shade and cover provided by largerwoody plants or tall herbaceous vegetation, both of which are normallyabsent from living roofs. The limited space available is anotherproblem: a typical urban roof is rarely large enough to provide sufficientcatchment area to feed a permanent pond or sufficient space for somespecies of bird to hold a breeding territory. Elevation may cause arooftop habitat to become isolated, making it inaccessible to somespecies that cannot fly, be dispersed by the wind or climb. There mayalso be a loss of connectivity with adjacent habitats.

There are ways to reduce these limitations. Even if thin dry soilspredominate, it is possible to provide refuges where soil is muchdeeper or water absorbent, or where water collects – perhaps in placeswhere the structure is capable of withstanding more weight. It ispossible to include artificial refugia (eg buried chambers or log piles)designed for particular species. Studies by Stephan Brenneisen [44] inSwitzerland have proven that varying soil depth does increase overallinvertebrate diversity. In order to improve connectivity between roofs,and between roofs and the ground, consideration might be given toconstructing vegetated bridges between green roofs (as can be seen inthe Roppongi Hills development in Tokyo) and vegetated ramps andbuttresses linking living roofs with the ground.

Conventional roofs will become vegetated if left undisturbed for longenough. Pioneer soil-forming plants like algae and lichens eventuallycreate suitable conditions for other plants to colonise. More than 600species of lichens have been recorded in the built environment. Many ofthese grow on roofs and some are nationally scarce. New concrete isusually too alkaline (pH 11) for most organisms to survive on it but, as itweathers and is neutralised, it may be colonised by a range of algae,mosses and lichens. Most roof materials have a much lower, morebenign pH. The rougher the surface, the easier it is for plants tocolonise. Drip and drainage lines, areas beneath bird perches andplaces where leaves and other materials collect are colonised first.

Moss cushions, pecked at by birds looking for food, frequently becomedislodged from steep roofs, so that the process of succession isconstantly being pushed back.


Benefits 41

The botanist R M Payne [64] surveyed the self-established flora of 639roofs and wartime pillboxes in East Anglia over an eight year period andfound a total of 135 species. The ten most frequently encounteredspecies were biting stonecrop (Sedum acre), rue-leaved saxifrage(Saxifraga tridactylites), annual meadow grass (Poa annua), commongroundsel (Senecio vulgaris), common chickweed (Stellaria media),white stonecrop (Sedum album), hairy bittercress (Cardamine hirsute),sycamore (Acer pseudoplatanus), ivy-leaved toadflax (Cymbalariamuralis) and reflexed stonecrop (Sedum reflexum). Of the 30 mostcommonly encountered species, 24 were considered to be dispersedby wind, 8 were dispersed by both wind and birds, and 3 by birds alone.Other studies reviewed by Payne suggest that most roof plants aredispersed by the wind. Wind dispersed plants include moss and lichenspores and species with plumed seeds such as many composites andwillowherbs. Bird sown plants include berry-bearing species in theRosaceae and other families. Ant dispersal may be another importantmechanism. Plants such as snapdragon, wallflower, ivy-leaved toadflax,herb-robert, annual mercury and white dead-nettle are all examples ofant-dispersed species that occur on walls and roofs. Ants have beenknown to carry seeds distances of up to 60 m.

There are many species of invertebrate (for example Linyphiids ormoney spiders) which are highly mobile and widely dispersedthroughout urban areas, and which will colonise any roof. Caution needsto be exercised in interpreting lists of species captured on green roofs.Some of them may not persist and others might be ubiquitous, perhapsbeing found on conventional roofs if anyone was to look. Neverthelessstudies of invertebrates on extensive green roofs in Basel by StephanBrenneisen [44], and in London by Richard Jones [65] and Gyongyer

Kadas [66], have found aremarkable diversity includingmany rare or endangeredspecies.

An extensive green roof on afactory near Zug, Switzerland,seeded with native wildflowersand designed to benefitinvertebrates. A welcome side-effect is the reduction insummer interior temperaturesreported by the workers sincethe green roof was installed


42 Benefits

Brenneisen’s study in Basel of 11 roofs found a total of 172 species ofbeetle. Older roofs had more species. Roofs with the highest structuraldiversity had the highest density of beetles. The study showed that theability of a roof to retain water was a key factor in attracting beetles.Spiders were also studied on the 11 roofs with 60 species found. Ofthese 70% were from a common group that are known to disperse well.Older roofs had a higher diversity of spider species.

In London, Kadas found 51 species of spider on 10 roofs (51 species isequivalent to 19.5% of the Greater London spider fauna). Most specieswere from groups like the money spiders and hunting spiders thatdisperse well (even reaching the top of a 70 m high tower at CanaryWharf) and tolerate low growing vegetation. Smaller numbers of largeorb-web spinning spiders (Lycosidae) were found. This isunderstandable since these spiders require taller vegetation with whichto support their webs. Other species found on roofs by Kadas were agroup of spiders from the Tetragnathidae family. This was surprisingbecause these spiders are normally associated with wetlands. Therewere some rarities, including species never previously found in London.

Kadas also collected a number of bees, including bumble bees(Bombus lucorum, B. terrestris, B. lapidarius) and Andrena bicolor(a bee that nests in a hole in the ground). It was noted that bees wereattracted to the flowers on the London green roofs but were unlikely tofind suitable nesting habitats because the substrates are normally toothin. Another surprise was butterflies (eg the small white) and moths (egthe silver y), with the latter seen on the 51st storey of Canary Wharf. In1980, Malin [67] reported frequent use of a 23rd floor roof garden bybees so it is clear that a range of flying insects can reach green roofson tall buildings.

Bumble bees (Bombus) is onegenus of insects that may benefitfrom green roofs


Benefits 43

Jones sampled invertebrates on the roof of 11 Shaw’s Cottages,Lewisham, as part of a study of eight extensive green roofs in London.Although none were endangered, a total of 54 species were found, themost for any of the roofs studied. Species singled out for specialmention were Metabletus foveatus, a ground beetle of dry sandyplaces; Scolopostethus decoratus, a ground bug of open sandy heaths;and Pseudoeuophrys erratica, a spider normally found under stonesand on walls in the north of England and Scotland. Jones noted that thediversity of invertebrate species is related to roof age, substrate depthand substrate structure, a pattern that had previously been establishedby Brenneisen in his study of Swiss roofs.

Jones found 48 species of invertebrates on three sedum roofs atCanary Wharf, the new office district in London’s Docklands. Notablespecies included Helophorus nubilis, a scarce ‘crawling water beetle’;Chlamydatus evanescens, a nationally rare leaf bug; Erigone aletris, aNorth American spider recently naturalised in the United Kingdom; andPardosa agrestis, a nationally scarce wolf spider. It is suspected thatC. evanescens, perhaps with others, was imported into the country with

the pre-grown sedum mats. Jones hasobserved that green roofs in London includemany invertebrates species which in the wildare associated with sparsely vegetated, dry,warm places like chalk downs, heaths, cliffsand dunes, and man made habitats on urbanwastelands.

In Basel, Brenneisen has made a study ofbird use of living roofs. Twenty five speciesof bird were observed foraging on insectsand seed, preening, searching for nestingmaterial, singing and nesting. The height ofthe roof had no apparent effect on bird use.It was suggested that green roofs had asignificant benefit for black restart, whitewagtail and house sparrow. Species thatmade frequent use of green roofs werecollared dove, carrion crow, magpie,goldfinch and tree sparrow. The presence ofblack redstart on the green roofs in Baselwas of special interest to those in Londonlooking for replacement habitat for thisspecies when its favoured breeding sites areredeveloped.

The black redstart for whichseveral biodiverse ‘rubble’ roofshave been created in London toprovide habitat


44 Benefits

A study by livingroofs.org [68] of the five-hectare complex of green roofsat the Rolls Royce factory near Chichester, West Sussex (whichincludes the largest green roof in the United Kingdom), revealed songthrush, linnet, ringed plover, little ringed plover and skylark. The littleringed plover was probably breeding and there were five pairs ofskylarks breeding on the roof in 2005. Other birds observed usinggreen roofs in the UK include blackbird, robin, wren, goldfinch, linnet,greenfinch, chaffinch and house sparrow. There is concern over thelong term decline of the house sparrow in England [69]. Perhaps greenroofs could help to reverse this decline by providing alternative feedinghabitat, especially for the small insects needed during the first few daysof the house sparrow’s life?


45

Setting goals

For maximum impact and cost effectiveness of a living roof, it isimportant that the designers set carefully considered and preciseobjectives. There may be several objectives and not all of these will beentirely compatible. This should lead to a process of prioritisation; forexample, is the objective a ‘must have’, a ‘should have’ or a ‘would like’?Objectives can be centred on the creation of a particular habitat that isbeing lost or may target particular species. Other importantconsiderations may include water absorption capacity, coolingperformance, appearance, and the need for rapid establishment,stakeholder consultations and research needs.

Structure

Any roof needs to be strong enough to support its own weight, thewaterproofing and any load that may be put upon it (eg snow). Add theweight of a green roof to an existing roof and additional strength is likelyto be required. When specifying a green roof, architects and engineersmust take account of the dead load of wet soil, plants and othermaterials and the potential for live loads of people or movingmachinery, all of which must be safely supported. Each jurisdiction hasits own standards for roof strength that varies according to localconditions and past experience.

A standard domestic roof constructed from sawn timber andsupporting tiles is normally designed for a loading of between 100 and150 kg/m2. Where heavy snowfall can be expected, roofs may bedesigned to support 200 kg/m2. The weight of sedum roofs 50 mmthick may be 70 kg/m2. Erisco Bauder [70] produce geotextile blankets25 mm thick; one sown with sedums or mosses has a saturated weightof 30 kg/m2 and a coir fibre fleece sown with herbs, grasses, sedums

Chapter 5

Design, build and maintain


46 Design, build and maintain

and mosses has a saturated weight of 42 kg/m2. The low weights ofthese and other similar products mean that it is possible to retrofitgreen roofs to some structures without additional strengthening.However, the advice of a structural engineer should always besought and local building codes checked beforehand.

At the other end of the scale an intensive green roof with trees and soil600 mm deep may need to support more than 1 tonne/m2, requiringsome kind of heavy steel or reinforced concrete structure. Althoughreinforced concrete structures are expensive they are usually severalstoreys in height and little, if any, extra strength would need to be addedto these structures to support a green roof. Stephen Scrivens [71] hasargued that the roof garden at the Willis Building (formerly the Willis,Faber and Dumas Building) in Ipswich added nothing to the overallcapital cost of the building. In the case of a low-rise lightweightstructure, an intensive green roof would require substantial additionalstructural support which would normally make such an option tooexpensive.

The deeper the substrate on a green roof, the more water that it canstore, the greater the options for establishing vegetation and attractingwildlife, but the greater the cost. Extensive green roofs are constructedwith shallow substrates, primarily to make them affordable. Typicalextensive green roofs with a depth of around 150 mm may have aloading of 100–200 kg/m2; the exact weight depends on the materialused (see chart aside). Pebbles weigh 5 or 6 timesas much as light expanded clay aggregate. Thewet weight of vegetation and a substrate on agreen roof with substrate depth of 180 mm isreported to be 220 kg/m2 by the green roofmanufacturer, ZinCo [57].

However, waterlogged turf and sod roofs can bemuch heavier than the lightweight materialssupplied by manufacturers. For example, the wetweight of a turf roof at the Findhorn eco-village with150 mm of soil was reported [72] to be 510 kg/m2.Turf roofs with 100 mm of soil at the Centre forAlternative Technology in Powys weigh around500 kg/m2 when wet, requiring rafters double thestandard thickness of 100 mm [73] (ie 200 mm).When designing and specifying a green roof, it maybe possible to increase the depth of the substrateor to place a heavy feature like a tree whereadditional support is located in the structure (egabove a column or heavy beam).

Saturated weight of various types of green roof

Coir fibre fleece

Sedum on felt mat

Extensive sedum roof

Turf roof

Roof garden

30

50

180

150

600

30

70

220

510

1000

Green roof

Type Depth Wet weight

(mm) (kg/m2)


Design, build and maintain 47

Layers

A typical green roof supplied by one of the larger manufacturers is oftendescribed as a ‘system’, perhaps because it is constructed from manylayers. Occasionally it has been suggested that the various layers in agreen roof mimic the layers in a natural soil profile, although naturalsoils do not usually behave in such a uniform, predictable andhomogeneous way.

A key component of a green roof is a waterproof layer, usually amembrane. In some cases this may be placed on top of a fleece whichprotects the membrane from any imperfections in the structural deckbelow. A liquid-applied membrane is sprayed or brushed directly onto aroofing deck.

Waterproof layers have been made from many different materialsincluding asphalt, bitumen, polyvinylchloride (PVC), polyurethanes,butyl rubber (polyisobutylene) and ethylene-propylene-diene monomer(EPDM). There is much rivalry between the various competing suppliersof waterproofing materials and each material has its pros and cons. It iscertainly true to say that waterproofing technology is more reliable thanit once was. Manufacturers are now able to test membraneselectronically and can find pinprick flaws. Certainly the common publicperception that roof membranes are unreliable and that flat roofs are tobe avoided at all costs is outdated and based on anecdotes of poorlyconstructed roofs that usually date from the 1960s and 1970s. Despitethe failures of the 1960s and 1970s, many older roofs are sound, withsome of the oldest green roofs surviving for the best part of a centurywithout problems.

Waterproof membranes in traditional roof gardens were often coveredwith a concrete screed to provide protection from mechanical damageor penetration by roots. More often a barrier membrane coated withcopper or other biocide protects the waterproof membrane from roots.Some species of plant have roots that are capable of piercing virtuallyany waterproof material and the softer organic materials like bitumencan be easily penetrated by roots.

Sometimes a layer of insulating material like glass fibre or urethanefoam is placed above the waterproof membrane. Inverted or ‘upsidedown’ roofs like this are commonly encountered on commercialbuildings, with the insulation boards usually covered with paving slabsor aggregate to weigh them down and prevent the wind from blowingthem away. An extensive vegetated roof can easily be established as areplacement for the paving slabs or aggregates, with no additionalloading on the roof structure. This creates a strong economic argumentfor vegetated roofs where inverted roofs would otherwise beconstructed without concern for cost. Using an insulation board has the



benefit of protecting the membrane from degrading extremes of heatand cold. Where a green roof is retrofitted onto an older building, thisadditional layer can bring the building up to a more modern standard ofinsulation.

Above the insulation or root barrier is a drainage board or combineddrainage and reservoir board, usually manufactured from a mouldedpolyethylene. This assures unimpeded drainage over the whole roofwhile, in some designs, allowing water to be retained in reservoirpockets. Very deep drainage boards are available for intensive greenroofs which are intended to be capable of withstanding the weight oflarge volumes of soil and masonry, and of trees, without becomingdeformed. Clearly if the objective was to create a habitat that requiressaturated soil, a drainage board would be inappropriate. Mouldedpolyethylene boards are usually manufactured from recycled materials.

Where downpipes and other features penetrate waterproofmembranes, there is a need for careful detailing during installation, andfor protection from mechanical damage and weathering. Perimeterchannels are usually filled with gravel or pebbles to increase drainagecapacity in the vicinity of outlets and to provide vegetation-free zones(typically 500 mm in width) required in some countries to preventrooftop grass fires from spreading to the building proper. (Also pebblesare often piled deep along the margins of a roof to help prevent highwinds from lifting the roofing deck.)

Drainage boards are covered by a geotextile filter fleece to prevent siltfrom entering the drainage layer and causing blockages or allowingmaterial to be washed into downpipes. Filter fleece is usually madefrom lightweight polyester, polypropylene or polyethylene non-wovenmatting. Occasionally a moisture retention blanket is placed above thedrainage layer; where substrate layers are especially thin a moistureretention blanket can help vegetation to survive dry spells for longer.

Sometimes a layer of aggregate or granular material is placed abovethe filter fleece to promote free drainage. Above this aggregate layerthere is a substrate (if required) with the growing medium on top of that.To date the emphasis in developing green roof growing media has beento provide a sterile, absorbent and lightweight material capable ofanchoring plants and supplying water and nutrients to the roots. This isa very horticultural approach where the intention is to support a smallnumber of cultivated varieties of plants rather than a habitat or soilecosystem. In such a simplified situation, wild flowers may beconsidered weeds and invertebrates, pests. A very lightweight growingsubstrate can be pumped onto a roof, making it cheap to install andpresenting a smaller weight demand on the roof structure. If it is toolightweight, though, it is vulnerable to being blown away.



The developers of artificial growing media formulate materials that arestable, drain well, do not shrink or degrade, hold water withoutbecoming saturated, and contain nutrients. Materials that are used asgrowing media in green roofs include peat, wood chips, sand, perlite (asiliceous mineral of volcanic origin) and calcined clay aggregate.(Calcination occurs when a substance is heated to a high temperatureto drive off water and carbon dioxide.) Typically a growing medium willbe a mixture of an inorganic material like calcined clay that will make upabout 80% of the mixture with sterile humus making up the remainder. Itis common practice to avoid using topsoil on green roofs because of itsheavy weight, vulnerability to compaction and tendency for acidity toincrease, and because of the presence of weed seeds. None of theseproblems need be of concern if the long term aim is to allow a self-established sward to develop and the roof is of adequate strength. Theorchid-rich waterworks roofs at Wollishofen in Zurich (built in 1913)were made using local topsoil (see photograph on page 39).

Where there is a slope to ensure efficient drainage, a living roof build-upcan be relatively simple. Many modern Scandinavian turf roofs consistof timber planking or plywood sheathing covered by a waterproof layerand thick turf, making three layers. Even where the roof is not sloping, asimple approach with a few layers can work; the roofs at Wollishofenare constructed on concrete decks sealed with bitumen, and coveredwith 50 mm of gravel and 150 mm of local top soil. An alternative, offour layers, comprises deck, waterproofing layer, drainage gravel andsoil, with the last two mixed together. Localised waterlogging –something that most green roof manufacturers mistakenly try to avoid– has helped to increase the biodiversity of these roofs.

Where the intention is tomimic a habitat which hasbeen lost to development,the original soil (or a closeapproximation to it) mightbe placed on the roof,ideally as deep turf – but ifnot, without too muchmixing of the variouslayers in the soil. Chalkrubble mixed with topsoil,sands, gravels, pebbles,crushed concrete andbrick and turf have all beenused to promote theestablishment of species-rich swards.

A cross-section through atypical extensive green roof

Vegetation

Substrate (growingmedium)

Filter fabric

Drainage/reservoir board

Root barrier

Waterproof membrane

Roof deck



Above A sedum roofRight Sedum stonecropBelow Sedum matting in production near Orléans, France



Planting, seeding and natural colonisation

After placing the growing medium comes the planting or seeding. Overthe years, roof greening companies and horticulturalists have identifieda number of plants that are suitable for planting on roofs. Full lists areprovided by Nigel Dunnett and Noel Kingsbury [74]. The commonlyspecified green roof plants need to be able to thrive in shallow soils,resist drought, survive heat and exposure and require low maintenance.Sedums (stonecrops) are the plants most usually specified on extensivegreen roofs. They are members of the Crassulaceae, a family of about1,500 species worldwide of mostly succulents (ie water storing) plants.Succulents store water to enable them to survive high temperaturesand drought. In the wild, sedums are often associated with dry, sunny,sandy or rocky places.

Plants of the Crassulaceae are one of 34 families of plants,encompassing more than 20,000 species worldwide, which employcrassulacean acid metabolism [75] as a form of photosynthesis; thishappens when stomata (leaf pores) close during the day (whenevaporative losses would be at their highest in the case of conventionalplants) and open during the relative cool of night time to fix carbondioxide. Sedums used in green roof planting are small, low growingplants having attractive white or yellow flowers with five petals. Speciescommonly used on green roofs in Europe include white stonecrop(Sedum album), reflexed stonecrop (S. reflexum) and Spanishstonecrop (S. hispanicum). These plants can be established by seedingor from cuttings. They can be pre-grown on geotextiles to form matsthat can be easily delivered and installed to produce an instant effect.They give a uniform appearance to a roof and provide year-roundcolour, although they are not immune to the effects of drought. As thesubstrate or mat that they are growing on dries out they change colour,turning red or brown (see opposite).

Although the flowers of stonecrops attract insects, including butterfliesand bees, the uniformity in design and monoculture of sedum blanketsdoes not exploit all possible opportunities to maximise biodiversity. Ifan ecologist wants to create a habitat on a roof, a different approachwould be adopted. As Oliver Gilbert and Penny Anderson have written [76],long before plants are selected basic factors need to be consideredincluding local conditions, climate and habitats, and local strategies,initiatives and biodiversity targets. By specifying a pre-grown sedummat without proper forethought, a designer is bypassing this essentialprocess.

Most ecologists would agree that to allow natural colonisation is adesirable way to create new habitat. On a roof this would involve putting a locally appropriate substrate onto a roof and waiting to seewhat colonises. Colonisation can be slow, and there are uncertainties



about what will colonise first and what the appearance of the new roofwill be in the short term; but, with patience, a diverse community ofplants and animals will result. They will be self-selected as suitable forthe location.

In certain settings for new green roofs, the surrounding area may not besufficiently rich in wildlife to allow rapid colonisation; or the architects,owners or occupiers of the building may no be sufficiently patient orconfident to wait for what nature will certainly do by itself. In thesecases seeding or planting will be required. In creating habitat, theadvice is to use native species of local provenance planted or seeded innatural associations. The reasons for this are that there is a need torestore habitats and associated wildlife that has been lost from townand country. In restoring this lost wildlife, though, there is a danger,which has been eloquently articulated by organisations like FloraLocale [77], in using exotic species or plants of non-native origin that aregenetically quite different. Our native flora has already suffered throughhybridisation with continental strains that are often more robust orshowy but less well suited to the local climate. Non-native species arerelatively easy to identify, but exotic strains of native species are moreeasily overlooked. There is insufficient knowledge of native plantgenetics to make any firm conclusions regarding the acceptability ofusing any particular plants; that is why conservationists have adopted aprecautionary approach and urge people to use native species of localprovenance. By avoiding the use of non-native species we also reducethe likelihood of introducing and spreading plants which may becomeaggressive and damaging to native ecosystems.

Although the trend to use native species is growing there are still manywho like to use non-natives in all planting, including planting on roofs.Dunnett and Kingsbury [74] have reviewed the reasons people have givenfor using non-native plants on roofs. These include the suggestions thatthere may not be locally-occurring native species that will grow well,that native plants may not occur in communities that resemble rooftopenvironments, and that native species cannot provide attractive yearround flowers or foliage. It has also been suggested that non-nativeplants may provide an abundance of nectar or fruit that cannot bematched by native plants. Detailed studies of the wildlife of privategardens in Sheffield have indicated that high plant and invertebratediversity can occur [78] and it seems that this may also be cited as areason for people to continue to plant primarily for appearance sake, orfor capricious or whimsical reasons. In practice, most locations wherea green roof is planned will be in the middle of a highly disturbedlandscape full of non-native plants, some planted, others self-sown andsome long naturalised (meaning that they have established breedingpopulations in the wild). Despite the prevailing situation, it should not betaken as a reason to ignore an opportunity to restore a more naturalassociation of species.



For reasons of cost, most living roofs will comprise relativelylightweight constructions supporting shallow, free draining, low nutrientsubstrates. There are species-rich native communities that occurnaturally in these conditions which can be established using wildflowerand grassland seed mixes. The exact composition of the seed mix willdepend on the substrate used, the local climate and area in which theroof is located. Materials that have been used to make substrates forspecies-rich roofs include mixtures of crushed brick and crushedconcrete, chalk rubble, gravels and pebbles. Semi-natural chalk andlimestone grasslands are well known as species-rich areas in the widercountryside, and much work has been done on the re-creation of thesehabitats [21]. This knowledge can be applied to the creation of species-rich swards on roofs. Seed mixes should be specified to take accountof substrate type and local conditions. Where perennial wildflowershave been seeded onto roofs, they have often been mixed with annualswhich may not persist as the sward closes, but they will create an extraburst of colour for the first year or two.

The term ‘brown roof’ has been used to describe living roofs designedto replace wasteland habitats on brownfield sites which have becomecovered with often diverse, self-established vegetation. Some of thefirst brown roofs established in London (eg on the roof of the LabanDance Centre, Deptford) were constructed using a crushed concretesubstrate and left to colonise naturally: a process which has beendeemed too slow by most observers. In that particular case thedesigner, Dusty Gedge, has seeded the roof with a mixture that hasbeen drawn up with consideration for the substrate, using only speciesknown to occur in the London area. In Basel, Switzerland, low-nutrient

vegetated roofsubstrates, based onlocally sourced riveraggregates, have beenmixed with varyingpercentages (generallyless than 40%) of soil toincrease water-holdingcapacity and then sownwith local native seedmixes to accelerate plantestablishment.

An extensive pebbly roof inBasel, Switzerland. Allextensive green roofs in Baselare seeded with local mixes ofnative wildflowers


Another approach, which has been specified for various buildingsdesigned by Architype and the architect Jon Broome, is to placecommercial turf above a layer of topsoil and plant plug or pot-grownwildflowers into the turf. The turf can be laid upside-down and seededwith annuals to give instant colour. Although some grasses cansuppress plant diversity, on sloping roofs this approach has theadvantage that the turf helps to maintain stability and reduce erosionduring establishment. The green roof on the Centre for Understandingthe Environment (CUE) building at the Horniman Museum wasconstructed using turf; this roof was considered to be species-richwhen surveyed more than ten years after establishment [79].

Mosses and lichens

Mosses and lichens colonise almost all roofs, particularly where guanoor leaf litter collects. These are often thought to be a problem byowners even though these plants do not normally damage the fabric ofbuildings. Mosses are a group of low growing photosynthesising(green) plants that do not flower or fruit but produce spores. Theyrequire such small quantities of nutrients that many species can survivein the most inhospitable places, clinging to masonry and tiles,apparently dead during drought but springing back to life following rain.

In some respects mosses are beneficial for buildings. Like all livingroofs they shield the building fabric from ultra-violet light. Moss andlichens, however, can be established on even the most lightweight ofstructures and still provide some of the benefits associated with moresubstantial installations. They absorb rainfall and therefore provide


A grass roof on the Centre forUnderstanding the Environmentbuilding at the Horniman Museum,Lewisham, south London. Recentecological surveys found this roofto be the most biodiverse inLondon



some limited cooling. A ‘moss forest’ offers cover for thousands ofmicroscopic animals such as water bears (tardigrades), and providesfood for birds and nesting sites for insects such as carder bees; it istherefore a valuable wildlife habitat. There is a centuries-old tradition ofcultivating moss in Japan which has inspired a small but growing bandof moss gardeners in the West [80]. Self-established moss carpets canbe encouraged on a layer of sandy soil, 20 mm deep or less; if keptdamp, these moss communities will establish themselves throughairborne spores. A French company supplies pre-grown Ceratodonpurpureum (a very common moss associated with the builtenvironment) in the form of roof and wall panels. An English company(Fentiman) has recently entered the market with a durable, sticky (whenfirst applied), cement-based coating designed to be covered with mosscollected from gutters or rescued from building sites. It is worth notingthat various species of moss flourish on sedum mats (perhaps to theannoyance of those who expect sedum to continue to dominate).

Lichens are composite symbiotic organisms made up of fungi (whichdominate) and algae or cyanobacteria. Carbohydrates manufactured bythe alga or cyanobacterium through photosynthesis is passed to thehost fungus. Lichens are able to survive extremes of temperature anddrought, and can colonise surfaces that are too sterile for most otherorganisms; they have been known to colonise metal, plastic and evensmooth glass. The yellow crust to be found on many tiled roofs isusually a species of the lichen genus Xanthoria. Although it is possibleto encourage or cultivate lichens and mosses, patience is requiredbecause these are relatively slow-growing organisms, especiallylichens which may grow less than a millimetre a year.

A self-established moss roof at Par Docks, Cornwall. This

demonstrates that even the mostlightweight structures can be

greened



Slopes

There is a common misconception that green roofs must be flat; butslopes of up to 40° can be vegetated with relatively simple measures,and even walls can be vegetated using meshes and mats, with stabilityimproving as roots mature. With sloping roofs there can be a problemwith vegetation or substrate slipping over membranes; in order to avoidthese problems, battens or grids are used. Slopes are more vulnerableto erosion and desiccation than flat roofs, especially duringestablishment.

Green façades

The simple approach of growing self-supporting ivy or creepers up awall probably dates back centuries. This is a perfectly good way ofgreening a wall, although there are limitations to the height thatclimbers will grow (typically about 10 m, occasionally 20 m, but rarely30 m). Accessing the underlying structure may be difficult if repairs arenecessary. The most commonly encountered species in temperatecountries are ivy (Hedera helix), Boston ivy (Parthenocissustricuspidata) and Virginia creeper (P.quinquefolia). The latter two aremuch admired because they turn red in autumn.

The modern approach to creating a green façade is to train vegetationon trellis-work or wires, and there is an ever-increasing variety of(usually stainless steel) proprietary cables, nets and meshes designedto train climbers and vines up façades [74]. These ‘green screens’ shieldbuildings from sunlight and create cool convection currents. Trellis-work also creates gaps that can provide cover for nesting or roostingbirds. It is relatively simple to design and fix trellis so that, when itbecomes necessary to work on the wall, it can be unfastened with theplant still attached; and once the work is completed to refasten it to thewall without damaging the plant. Deciduous climbers like Boston ivy andvines (Vitis sp) can be particularly useful in temperate climates,providing summer shade but allowing buildings to be warmed by wintersunshine. By providing planters at different levels and correspondingtrellis-work, it is possible to green the façades of tall and complexstructures, including some that might otherwise be ugly or sterile. Pre-seeded, sometimes irrigated, geotextile tiles and mats have been usedto clothe buildings in vegetation. An example is the Almeida Theatre inKings Cross, London (designed by Clarke Associates). Other methodsof creating green walls include gabions (wire baskets filled with stones),dry stone construction, stacked bags and planted geotextile pocketsfixed to walls or frames.

OppositeA vegetated wall created by theFrench botanist, Patrick Blanc,

at Quai Branly, Paris. Severalspecies of plant, which

originate from all around theglobe, grow in an irrigated,

water absorbent composite offelt and geotextiles secured toa rigid board which, in turn, is

fixed to a steel frame





The French botanist, Patrick Blanc, has installed eighty hydroponicmurs vegetaux (vegetated walls) since 1988. Most are in France, butthere have been installations in Belgium, Italy, Spain, Switzerland, theUSA, Japan, South Korea and Thailand. A variety of carefully selectedplants are individually planted into a felt mat backed by geotextiles andstructural boards. The species used vary according to orientation ofthe wall and the local climate. Plants used in the vegetated wall at theChaumont-sur-Loire Garden Festival in 2005 included more thanthirteen exotic (to France) species including Begonia sutherlandii, Ficuscarica, Lonicera pileata, Corydalis ochroleuca and Pelaargoniumendlicherianum. Water and nutrients for Blanc vegetated walls arepumped from a reservoir tank to the top of the wall from where theydrain back down to the tank. The unlimited supply of water means thatthese installations have very high potential to provide cooling, althoughthey require regular maintenance. This technique appears to be mainlyused for aesthetic purposes, but biodiversity benefits could beincreased through using native species. These walls provideevaporative cooling and could be adapted to clean grey water.

Irrigation

Irrigation that wastes water is undesirable and unnecessary; extensiveliving roofs are, therefore, normally designed to rely solely on rainfall.Extensive roofs change their appearance through the seasons,blooming after rain or when temperatures rise, dying back following dryweather or after frost. It is better for the environment to learn to acceptand enjoy seasonality. Having said that, there may be situations – forexample, on formal roof gardens or where evaporative cooling isrequired – where irrigation might be acceptable. Wherever possible, itis preferable to use stored rainfall or recycled grey water. Whereirrigation is used, sprinkler systems should be avoided because theytend to be wasteful. For efficiency, the timing and volume of irrigationwater used may be controlled by software linked to a weather station.Power for pumps should be provided by renewable energy, normallyphotovoltaic or wind power.



Maintenance

Extensive green roofs with thin soils are deliberately designed to be lowmaintenance, but they do require some attention if they are not tobecome colonised by trees. Tree and shrub seedlings (mainly willow,birch and Buddleia) have the potential to damage membranes and(unless the intention is to create woodland and the design of the roofstructure has taken account of tree growth) should be removed during asingle annual maintenance visit.

Blocked drains can be a problem on green roofs. An annual inspectionshould include clearance of debris from drains. Intensive roof gardensrequire frequent attention – just as any garden does. Conventional flatroofs, covered with aggregate or paving slabs also need an annualcheck to remove seedlings and weeds, so the maintenance costs ofextensive green roofs are comparable to those associated withconventional roofs and are therefore not a substantial reason forrejecting an opportunity to create a living roof.


60


61

There should now be enough evidence and successful examplesworldwide to be able to convince more legislators, planners, architects,landscape architects, engineers, developers and builders that building-integrated vegetation has real benefits, at local and city-wide scales.Nevertheless, more research is needed to confirm some of the morecontroversial claims and to quantify other more obvious benefits.

In the United States, academics are repeating and refining work onrainfall run-off attenuation rates pioneered in Germany. Ecologists inEurope are continuing to catalogue the biodiversity of green roofs andare experimenting with new combinations of substrate and vegetationas habitat. In Switzerland, research on the diversity of invertebrates onolder established extensive green roofs is being used to inform newdesign. The variety of extensive green roofs will increase in line with thedesire and ability to create habitats on roofs. There will be a widerrange of growing materials used and more use of deeper substrates.Combinations of habitats on a single roof will become more common,and there will be a greater emphasis on targeting the conservation ofparticular species and habitats.

Elsewhere In North America and in the Far East, urban geographers andengineers are developing more sophisticated computer models to helpcity authorities predict how varying levels of adoption of green roofswould perform in moderating the effect of urban heat islands and inreducing carbon emissions. The authorities in Beijing are promotingroof gardens as a technique to improve air quality. It seems likely thatthese initiatives will be repeated in many of China’s other cities; andIndia will follow China’s lead.

As climate continues to change and the need to cool cities becomesmore widely recognised, there will be a greater emphasis on using livingroofs as a means of increasing evaporative cooling. The collection andrecycling of grey water for this purpose will become commonplace.

Chapter 6

The future


62 The future

Little research has been done to date on how a verdant outlook canpromote good mental health. No doubt urban designers will learn muchmore about this and it will help people to plan neighbourhoods. Mostpeople believe that high quality environments increase health andhappiness. The economic argument for this is that the extra cost of highquality green environments would be recouped through improved health(and lower healthcare costs) and higher rates of employment (withagain lower healthcare costs and higher tax revenues). The recentCABE Space [81] document, Does money grow on trees? hasdemonstrated that traditional urban greenspace boosts propertyprices. Such research is expensive, but given the promising indications,governments surely have a duty to invest.

As the evidence continues to build and knowledge spreads to the widerpublic, those who currently believe that it is inappropriate to vegetatebuildings may become the minority. It is predictable that attitudestowards vegetated architecture will change, that building-integratedvegetation will become more commonplace – following furtheradoption of new guidance on urban design by central and localgovernment – and more mainstream, as it already has in parts ofGermany and Switzerland.

A proposed design for WestKowloon cultural centre by

McAllister Architects andEcoSchemes. A concrete deck

would provide a new public parkabove the cultural centre,

surrounded by inaccessibleslopes to encourage wildlife.

Perforations in the deck wouldallow natural light into the space

below. Evaporative coolingfrom the park would reducedemand for air conditioning


The future 63

More governments will offer incentives to promote the construction ofgreen roofs. These normally involve an authority allowing, say, thedevelopment of more floor space if green roofs are installed, or offeringa reduction in drainage charges in line with falling rates of run-off. It maybe that drainage charges will be calculated more precisely in the futureto provide encouragement for owners who capture and recycle rainfalland wastewater. This would also have the effect of increasing theamount of building-integrated vegetation.

Many drainage engineers who, having spent the first half of theircareers straightening and culverting watercourses, are now reversingthat work. There is now a realisation that, by changing drainagepatterns to take water away from sites as quickly as possible, in manysituations the likelihood of flooding downstream has increased,groundwater reserves have reduced, and much beauty and wildlife inthe landscape has been lost. Water quality in rivers has fallen as theresult of increased surface run-off and fish diversity in urban rivers hasfallen catastrophically.

A start has been made on the mammoth task of restoring rivers andstreams, but restoration of whole catchments is necessary if waterquality and flow characteristics are to be returned to previous, morenatural conditions. In rural areas this might mean the re-wetting of bogsand marshes, and reafforestation; but in cities vegetated architecturewill be seem as an integral component of restoring catchments andbringing back fish to rivers – a concept that has already been graspedin Portland, Oregon.

Artists, architects and landscape architects will take an increasinginterest in using vegetation as a medium. Perhaps more widespreaduse of naturalistic planting on buildings will have the side effect ofencouraging more experimentation in planting and managing groundlevel parks and gardens. There has been a tendency towards uniformityin the planting of open space, with a loss of habitat and speciesdiversity, and loss of local distinctiveness. Ecologists will work withdesigners to reverse this trend, perhaps leading to wider use of nativespecies, and tolerance of dead vegetation (which is an essentialelement of the suite of habitats needed for the full diversity of life).

There has been a growing consensus amongst urban planners that, fortowns and cities to be efficient and therefore environmentally friendly,the density of development needs to be high, meaning multiple storeybuildings arranged in close proximity. This maximises the use of landand services, and makes public transport more efficient and affordable.Dense development, however, can be sterile and gloomy. Paintingmulti-storey blocks in several colours may brighten the inner city scenebut vegetated architecture will be seen increasingly as the key to thelong term success of high density urban planning.


64 The future


The future 65

New forms of building will emerge. Earth-sheltering, and vegetatedterraces and ramps, will help to blur the distinction between buildingsand the open spaces in which they are set, and increase the potentialfor the movement and spread of wildlife and people between buildingsand their surroundings, and throughout cities. An example of this is theFukuoka Prefectural International Hall (Asian Crossroads Over The Seaor ACROS building) in Japan, designed by the Argentinian-born, NewYork City-based architect, Emilio Ambasz [82], and constructed in 1995.It is both a building and a park, with a giant staircase of fifteenaccessible vegetated terraces that overlook the adjacent Tenjin Parkand which effectively extend that public open space. Initially, buildingsthat fully embrace the possibilities of building-integrated vegetation willbe public, institutional or large commercial schemes like the ACROSbuilding, but architects will increasingly find ways of achieving similareffects in residential situations.

Opposite topA cross-section of the ACROS(Asia Crossroads Over the Sea)building, Fukuoka, Japan. Amodern hanging gardens

Opposite bottomA section showing detail of howlush vegetation is combinedwith access. A verdant man-made mountain trail

BelowDesigned by architects EmilioAmbasz & Associates andopened in 1993, the ACROSbuilding complements andeffectively extends the adjacentTenjin Central Park


Another landmark building that may inspire architects to work more withvegetation is the Hundertwasser-Haus in Vienna designed by the lateartist and architect, Friedenreich Hundertwasser, and opened in 1986.This building includes hundreds of tonnes of soil planted with hundredsand trees and shrubs in rooftop groves. The concept was repeated atthe KunstHausWien, a museum and exhibition centre opened in 1991.

Not all building owners,however, can afford thenecessary structures tosupport very large volumesof soil. In the industrialfringes of cities, moderncommercial buildings tendto be lightweight with adesign life of decadesrather than centuries.These structures will beclad with lightweight, oftenmodular pallets, shallowboxes or mega-tiles ofvegetation that will mimiclow growing naturalhabitats, and which can beeasily demounted andmoved to another nearbybuilding when a site isredeveloped.

Building-integrated vegetation will make the cities, dwellings andworkplaces of the future, greener and cleaner, cooler in summer andmore tranquil, with healthier and happier citizens sharing the space witha greater number and variety of plants and wildlife.

66 The future

The Hundertwasser building,Vienna


67

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A single copy of this document is licensed to On This is ... · vegetation may be divided into green roofs and green walls (or green façades). Agreen wall (orgreen façade) is an