Performance, diagnosis, maintenance, repair and the avoidance … · 2014-05-08 · 3.4 Fully supported built-up bitumen felt and felt strip slates 127 3.5 Rigid sheets 132 3.6 Shingles

ROOFS AND ROOFINGPerformance, diagnosis, maintenance, repair and the avoidance of defectsTHIRD EDITION

H W Harrison, P M Trotman and G K Saunders

BRE Building Elements series

ROOFS AND ROOFINGPerformance, diagnosis, maintenance, repair and the avoidance of defects

Third edition

H W Harrison, P M Trotman and G K Saunders

BRE Building Elements series

Preface to the first edition vPreface to the second edition viiiPreface to the third edition ixAbout the authors x

1 INTRODUCTION 1 1.1 Background 2 1.2 References 11

2 THE BASIC FUNCTIONS OF ALL ROOFS 13 2.1 Strength and stability 14 2.2 Dimensional stability 24 2.3 Exclusion and disposal of rain and snow 25 2.4 Energy conservation and ventilation 29 2.5 Control of solar heat and air temperature 34 2.6 Fire safety and precautions and lightning protection 42 2.7 Daylighting and control of glare 47 2.8 Sound insulation 49 2.9 Durability, ease of maintenance and whole-life costs 51 2.10 Functions particular to pitched roofs 58 2.11 Functions particular to flat roofs 62 2.12 Extensive lightweight green roofs 66 2.13 Modern methods of construction 68 2.14 Roof-mounted photovoltaic systems 70 2.15 References 73

3 SHORT-SPAN DOMESTIC PITCHED ROOFS 77 3.1 Concrete and clay tiles 78 3.2 Slate and stone tiles 108 3.3 Fully supported metal 120 3.4 Fully supported built-up bitumen felt and felt strip slates 127 3.5 Rigid sheets 132 3.6 Shingles 136 3.7 Thatch 139 3.8 Thermal insulation in lofts 147 3.9 Loft conversions 148 3.10 References 157

4 SHORT-SPAN DOMESTIC FLAT ROOFS 161 4.1 Built-up felt 163 4.2 Mastic asphalt 172 4.3 Inverted flat roofs 177 4.4 Fully supported metal roofs 180 4.5 Single-layer membranes 183 4.6 References 186

CONTENTS

iiiCONTENTS

iv CONTENTS (CONT’D)

5 MEDIUM-SPAN COMMERCIAL AND PUBLIC ROOFS 189 5.1 Pitched roofs 191 5.2 Flat roofs 204 5.3 Vaults and other special shaped roofs such as glazed atria 212 5.4 Roof gardens (intensive green roofs) 219 5.5 Swimming pool roofs 221 5.6 References 223

6 MEDIUM-SPAN INDUSTRIAL ROOFS 225 6.1 Sheeted portals, north lights, monitors and saw-tooths 226 6.2 Patent glazing 237 6.3 Temporary and short-life roofs 241 6.4 References 245

7 LONG-SPAN ROOFS 247 7.1 All kinds of long-span roofs 248 7.2 Skeletal structures 254 7.3 References 258

8 INDEX 259

The third edition of this book is being published at a time when the UK construction industry is facing a significant reduction in its work load, and nearly a decade after the second edition was prepared. That decade has seen massive changes in public awareness of the need for sustainability in construction, and the introduction of the Code for Sustainable Homes in November 2006, which since May 2008 has formed a basis for assessment of the acceptability of the design of new housing in England and Wales.

But similar needs exist for the whole of the UK’s future building programme, new-build hospitals, factories, educational buildings and other long-life buildings which will provide challenges to designers in meeting the conditions brought about by anticipated climate changes and the need to be carbon-neutral and to conserve our dwindling natural resources. There have also been significant changes in British Standards which increasingly reflect those taking place in Europe.

However, the UK cannot afford year-on-year to renew more than a very small percentage of the stock of existing buildings, and the need for intelligent conservation and upgrading of the old stock is arguably of equal if not more importance.

It is against this background that this third edition of Roofs and roofing has been prepared. In addition to thorough revision of the chapters on the more traditional forms of construction, such as tiling and slating, completely new chapters have been prepared on:

PREFACE TO THE THIRD EDITION

ixPREFACE TO THE THIRD EDITION

extensive lightweight green roofs,modern methods of construction,roof-mounted photovoltaic systems,thermal insulation in lofts,loft conversions,single-layer membranes.

New sections have been introduced as appropriate into existing chapters, including:

new forms of metal roofing,siphonic roof drainage,new materials technologies,improved protective finishes for timber, metals and concrete.

Where appropriate, each chapter now contains a section dealing with provisions that may become necessary to accommodate climate change (eg increased rainfall, stronger winds and higher temperatures).

There have been considerable changes too in the standards covering roof drainage which have been reflected in the revised text.

Approximately one-quarter of the photographs are new to this edition.

HWHPMTGKSJune 2009

••••••

••••

773 SHORT-SPAN DOMESTIC PITCHED ROOFS 77

Short-span roofs are normally defined as being of less than 8–9 m span. Pitched roofs are conventionally defined as those roofs with slopes greater than 10°, whereas roofs of slope 10° or less are defined as flat.

For the purposes of this book, pitched roofs have been categorised into those covered in relatively small overlapping units, dealt with in Chapters 3.1, 3.2 and 3.6, those covered in sheet materials, dealt with in Chapters 3.3–3.5, and thatch, which forms a category of its own, dealt with in Chapter 3.7.

Small overlapping units consist of the following types:

clay tile,concrete tile,

••

natural slate,manmade slate,shingles.

Figure 3.2 shows the current market share held by the first four categories. Following increased appreciation of the need to protect the environment, there has been limited use of recycled materials such as rubber. The increased popularity of PV systems is envisaged which will affect the type of roofing chosen to support them.

Some general information about defects in pitched roofs in housing can be found in Assessing traditional housing for rehabilitation[1].

•••

3 SHORT-SPAN DOMESTIC PITCHED ROOFS

Figure 3.1 This steeply pitched plain tiled roof was built in 1880 but re-covered after bomb damage in the 1939–45 war

Figure 3.2: Roof tile market by value, 2006. Data from Roofing Market Report[2]

Slate20%

Concrete tiles 61%

Fibre-cementproducts

7%Clay tiles 12%

87PAGE HEADER RIGHT – Page header subtitle 873 SHORT-SPAN DOMESTIC PITCHED ROOFS

BRE site inspections. Drawings have been seen that specifically state that the design of gutter systems should be left to the site staff to sort out! The pitch of valley gutters is less than the pitch of the roof they join, and valley gutters on pitches of less than 20° are particularly prone to leaking. Poor detailing was commonly found in the BRE quality assessments.

BRE site inspections revealed a number of cases where gutters had not been installed on porches and bay windows: decisions which may be marginal where the drips cause no inconvenience to the occupants, but less acceptable for doorways (Figure 3.27).

A room in the roofWhere a room is formed within a pitched roof void, it is sometimes difficult to achieve adequate ventilation for the roof to the outside (Figure 3.28). BRE Defect Action Sheets 118[22] and 119[23] deal with this problem, particularly where it involves the careful placing of thermal insulation to ensure an adequate ventilation gap. Thermal insulation: avoiding risks[24] is also helpful. The most important points to watch are:

that a vapour control layer (eg of at least 500-gauge polyethylene) is installed in the sloping part of the roof under the insulation,that a vapour permeable sarking is used in new construction,that cupboards should be within the insulated envelope, ensuring continuity of the lining.

MAIN PERFORMANCE REQUIREMENTS AND DEFECTS

Choice of materials for structureTimber has been used as the main structural material in the vast majority of tiled roofs: those dating from before the early 1960s for the most part designed with strutted purlins (Figure 3.29), more recent designs using trussed rafters. The structure of most of these is in good condition (only 1 in 8 of these structures being reported as faulty[5]), provided attention has been paid to routine maintenance of the covering.

•

•

•

Box 3.1: Calculations of rainwater run-off from short span domestic roofing and gutter size

Using Figure 3.25, the effective catchment area that will discharge to each gutter is:

for the slope of a pitched roof, the plan area, A (m2), plus half the elevation area, B (m2) (Figure 3.25a),

for a pitched roof abutting a wall, the plan area, A, plus half the elevation area, B, plus half the wall area, C, above the roof slope (Figure 3.25b),

for a flat roof, the relevant plan area.

The run-off rate to each gutter is the total catchment area for the gutter divided by 48. This produces the run-off in litres per second using the recommended rainfall (thunderstorm) rate of 75 mm/hour.

The size of the guttering is shown in Table 3.5 using the flow capacity that will accommodate the run-off rate. As part of this process, the number of outlets should be considered: more outlets from a run of guttering spreads the total loading on the gutter, but the loadings will vary according to where the outlets are positioned (Figure 3.26).

•

•

•

Figure 3.26: Spacing between outlets to reduce water load on gutters

Table 3.5: Flow capacities of standard eaves gutters (when level)

Flow capacity Size of (litres/sec) gutter True Nominal (mm) half round half round 75 0.38 0.27100 0.78 0.55115 1.11 0.78125 1.37 0.96150 2.16 1.52

Figure 3.25: Catchment areas for calculating rainwater run-off

This gutter needs to have twice the flow capacity of . . .

. . . this gutter, and four times the

capacity of . . .

. . . this gutter,

(a)

B

A

(b)

BA

C

BRE Building Elements

Floors and flooringPerformance, diagnosis, maintenance,repair and the avoidance of defects

P W Pye, MRIC, CChem

H W Harrison, ISO, Dip Arch, RIBA

iii

Contents

Preface vReadership vScope of the book vSome important definitions viAcknowledgements viiSecond edition vii

0 Introduction 1Records of failures and faults in buildings 1BRE publications on floors and flooring 5Changes in construction practice over the years 5

1 The basic functions of all floors 111.1 Strength and stability 121.2 Dimensional stability 171.3 Thermal properties 201.4 Control of dampness and condensation, and waterproofness 251.5 Comfort and safety 321.6 Fire and resistance to high temperatures 441.7 Appearance and reflectivity 511.8 Sound insulation 531.9 Durability 561.10 Inspection and maintenance 65

2 Suspended floors and ceilings 692.1 Timber on timber or nailable steel joists 702.2 In situ suspended concrete slabs 952.3 Precast concrete beam and block, slab and plank floors 1062.4 Steel sheet, and steel and cast iron beams with brick or concrete infill 1162.5 Masonry vaults 1242.6 Platform and other access floors 128

3 Solid floors 1333.1 Concrete groundbearing floors: insulated above the structure

or uninsulated 1343.2 Concrete groundbearing floors: insulated below or at the edge

of the structure 1513.3 Rafts 154

iv Contents

4 Screeds, underlays and underlayments 1574.1 Dense sand and cement screeds: bonded and unbonded 1584.2 Floating screeds: sand and cement 1664.3 Levelling and smoothing underlayments 1704.4 Lightweight screeds 1724.5 Screeds based on calcium sulfate binders 1744.6 Matwells, duct covers and structural movement joints 177

5 Jointless floor finishes 1815.1 Concrete wearing surfaces 1825.2 Polymer modified cementitious screeds 1865.3 Granolithic and cementitious wearing screeds 1885.4 In situ terrazzo 1915.5 Synthetic resins 1945.6 Paints and seals 1995.7 Mastic asphalt and pitchmastic 2025.8 Magnesium oxychloride (magnesite) 206

6 Jointed resilient finishes 2116.1 Textile 2126.2 Linoleum 2156.3 Cork 2186.4 PVC flexible 2206.5 PVC semi-flexible or vinyl (asbestos) 2266.6 Rubber 2296.7 Thermoplastic 232

7 Jointed hard finishes 2357.1 Ceramic tiles and brick paviors 2367.2 Concrete flags 2457.3 Natural stone 2477.4 Terrazzo tiles 2527.5 Composition block 2567.6 Metal 260

8 Timber and timber products 2638.1 General 2648.2 Board and strip 2688.3 Block 2728.4 Parquet and mosaic 2758.5 Panel products 277

Appendix A How to identify less recognisable floorings and their substrates 283

Appendix B How to choose a flooring 284

References and further or general reading 285

Index 295

First edition

It has been said that most problemswith floors occur because peopleinsist on walking on them, pushingtrolleys over them, placing largeobjects on them and dropping thingson them – if only they were ceilingsthey would never wear out! A smallwitticism that reflects the way somepeople, including professionals inthe construction industry, see floors.Or, rather, don’t see them! After all,what is there to a floor: floorboardsnailed to joists. What can go wrongwith that? And if it happens to form a ceiling, even better. But the factsbelie this perception.

BRE’s figures on faults inbuildings of all types (given ingreater detail in the introductorychapter which follows) show that asubstantial number concern floors.Despite the advice that has beenavailable to the industry from the1920s, faults in flooring, such ascracking, detachment and entrappedwater, recur frequently. If some ofthe errors appear elementary, thisonly reflects what happens in thedesign office and on site. All that wecan do is show to those who work inthe flooring industry what is beingdone incorrectly and how to takecorrective action – preferably beforefaults or defects lead to costlydamage.

This book describes the materialsand products, methods and criteriawhich are used in the construction offloors and flooring. It draws thereader’s attention to those elementsand practices which ensure goodperformance or lead to faults andfailure. There is sufficient discussion

of the underlying structure to enablean understanding of the behaviour ofthe whole floor without going veryfar into engineering designprinciples. It does not purport,though, to be a book of constructionpractice; nor does it provide thereader with the informationnecessary to design a floor, but,mainly through lists andcomprehensive illustration, showshim or her what to look for as goodand bad features of floors andflooring. It also offers sources offurther information and advice.

ReadershipFloors and flooring is addressedprimarily to building surveyors andother professionals performingsimilar functions, such as architectsand builders, who maintain, repair,extend and renew the nationalbuilding stock. Lecturers and othereducators in the building field willalso find it to be a useful adjunct totheir course material.

Scope of the bookAlthough books on flooring are few,there is no shortage of industryguidance on floors and flooring. Theproblem is that people do not use theguidance that exists.

To try to remedy that situation,the contents of this book areconfigured so that the principles,features and functions of floors andflooring are described first (Chapter 1). There needs to besufficient discussion of principles toimpart understanding of the reasonfor certain practices; without thatunderstanding, practitioners willhave difficulty following correct

procedures – or until they make themistakes, or overlook precautions, asprevious generations have done. The criteria presented in Chapter 1are then related to the different types of floors and their finishes(Chapters 2–8).

The text concentrates on thoseaspects of construction which, in theexperience of BRE, lead to thegreatest number of problems orgreatest potential expense if carriedout unsatisfactorily. It follows thatthese problems will be picked upmost frequently by maintenancesurveyors and others carrying outremedial work on floors. Althoughmost of the information relates toolder buildings, surveyors may becalled upon to inspect buildings builtin relatively recent years. It istherefore appropriate also to includemuch material concerningobservations by BRE of newbuildings under construction in theperiod 1985–95.

Many of the difficulties which arereferred to BRE for advice stem from too hasty assumptions aboutthe causes of particular defects. Veryoften the symptoms are treated, notthe causes, and the defects recur. It isto be hoped that this book willencourage a systematic approach tothe diagnosis of floor and flooringdefects.

The case studies provided in someof the chapters are selected from thefiles of BRE Advisory Service andthe former Housing DefectsPrevention Unit, and represent themost frequent kinds of problems onwhich BRE is consulted. They arenot meant to be comprehensive inscope since the factors affecting

v

Preface

individual sites are many and varied.As has already been said, this

book is not a textbook on buildingconstruction. Hence, the drawingsare not working drawings but merelyshow either those aspects to whichthe particular attention of readersneeds to be drawn or simply providetypical details to support text.

Other more specific aspects of thesubject not deemed to be relevant tothis book are mentioned briefly inappropriate chapters – usually in theintroductory paragraphs.

Passive fire protection measuresare those features of the fabric, suchas structural frames, walls and floors,that are incorporated into buildingdesign to ensure an acceptable levelof safety. These measures, so far asthey affect floors, are dealt with inoutline in this book. Measures whichare brought into action on theoccurrence of a fire, such as firedetectors, sprinklers and smokeexhaust systems are referred to asactive fire protection, and are notdealt with in this book.

The standard headings within thechapters are repeated only wherethere is a need to refer the readerback to earlier statements or wherethere is something relevant to add towhat has gone before. We haveassumed that readers will knowmany of the more commonabbreviations used in the industry –DPC, PVC etc – and we havedeclined to spell them out.

This book deals with all kinds offloorings (ie floor coverings),including both in situ andmanufactured products, and theseare covered in detail in Chapters 5–8.A classification of floorings for useinternationally has been publishedby the European Union ofAgrément, and this is examined inmore detail later in the book.

Ceilings, whether suspended orapplied directly to soffits, are treatedas integral parts of the element offloors since many aspects ofperformance, such as fire and sound,affect all parts of both floors andceilings. Ceilings are mainly dealtwith in Chapter 2.

Ramps, landings and stair treadsare included as elements of floors,but not staircase enclosures. There isan argument for dealing withstaircases in conjunction with wallssince, in most cases, it is necessary toconsider the enclosures for stairs inconjunction with stair flights;enclosures for staircases, and suchmatters as protected shafts, aretherefore considered as elements ofwalls.

Much that is relevant to ordinaryfloor finishes applies also to stairtread finishes.

The weatherproofing aspects ofbalconies, insofar as they are similarto those of roofs, are dealt with in thebook on roofs. Thresholds arehandled as parts of external walls.

In many places through the bookwe have quoted British Standardsand codes of practice which havebeen withdrawn; however theywould have been current at the timea particular floor or flooring was laid.We have done this deliberately sincethey often gave better specificationsthan those now current. Indeed, insome cases, standards and codeshave been withdrawn and notreplaced. Copies of old standardsand codes are often retained by BREfor use in disputes; they can also beseen in the British Library.

In the United Kingdom, there arethree different sets of buildingregulations: the Building Regulations1991 which apply to England andWales; the Building Standards(Scotland) Regulations 1990; and theBuilding Regulations (NorthernIreland) 1994. There are manycommon provisions between thethree sets, but there are also majordifferences. Although the book hasbeen written against the backgroundof the building regulations forEngland and Wales, this is simplybecause it is in England and Walesthat most BRE site inspections havebeen carried out. The fact that themajority of references to buildingregulations are to those for Englandand Wales should not make the bookinapplicable to Scotland andNorthern Ireland.

We have deliberately not providedmore than an outline of the majorpoints which specifiers will need totake into account in the cleaning andmaintenance of floorings since this isnot a topic which has been studied indepth by BRE or its predecessor, theBuilding Research Station; in anycase there is suitable literatureavailable from industry sources andother publishers.

Some important definitionsSome of the more general termsused in floors and ceilings will befound in Section 1.3, Subsection 1.3.3of BS 6100-1 (Glossary of buildingand civil engineering terms: Generaland miscellaneous).

Since the book is mainly about theproblems that can arise in floors, twowords, ‘fault’ and ‘defect’, needprecise definition. Fault describes adeparture from good practice indesign or execution of design; it isused for any departure fromrequirements specified in buildingregulations, British Standards andcodes of practice, and the publishedrecommendations of authoritativeorganisations. A defect – a shortfallin performance – is the product of afault, but while such a consequencecannot always be predicted withcertainty, all faults should be seen ashaving the potential for leading todefects. The word ‘failure’ hasoccasionally been used to signify themore serious defects.

By ‘floor’ we mean the whole ofthe horizontal elements of a building(excluding roofs but includingceilings); ‘flooring’ refers simply tothe finish of the upper surface of thefloor.

Where the term ‘investigator’ hasbeen used, it covers a variety of rolesincluding a member of BRE’sAdvisory Service, a BRE researcheror a consultant working undercontract to BRE.

vi Preface

‘Topping’ has been used todescribe an in situ material laid toprovide good abrasion resistanceand to provide the wearing surface. It has also been used to refer to acementitious mix used to locktogether components of a structuralconcrete floor, such as hollow pots,where it is called a structuraltopping.

‘Underlay’ is a layer used betweenthe structural deck or slab and theflooring, either prefabricated (egplywood or hardboard) or a thick insitu layer (eg mastic asphalt oraggregate filled latex cement).‘Underlayment’ is in situ materialused to smooth or level the baseprior to laying the flooring. (Theterm underlay seems to havebecome restricted to preformedsubstrates and the termunderlayment to those formed insitu, and we have adopted thisdistinction. However, in otherindustry publications and documentsthe former term will be found toapply to both applications.)

‘Wear’ is the progressive loss fromthe surface of a material orcomponent brought about bymechanical action.

AcknowledgementsPhotographs which do not bear anattribution have been provided fromour own collections or from the BREPhotographic Archive, a uniquecollection dating from the early1920s.

To the following colleagues –many from the BRE ScottishLaboratory and the Fire ResearchStation – and former colleagues whohave suggested material for thisbook or commented on drafts orboth, we offer our thanks:D R Addison, Dr B R Anderson, R W Berry, Dr A B Birtles, Dr P Bonfield, P J Buller, Dr R N Butlin, A H Cockram, Dr J P Cornish, S A Covington, R N Cox, Dr R C deVekey, Diane M Currie, R J Currie, Maggie Davidson, Dr J M W Dinwoodie, Dr B R Ellis, Dr V Enjily, Dr L C Fothergill, E Grant, G J Griffin, C J Grimwood, J H Hunt,Dr P J Littlefair, T I Longworth, K W Maun, N O Milbank, Dr D B Moore, W A Morris, Dr R M Moss, F Nowak, E F O’Sullivan, R E H Read, J F Reid,M R Richardson, J Seller, A J Stevens, P M Trotman, C H C Turner and Dr T J S Yates, allof BRE.

We have also drawn upon somenotes prepared by the late Dr FrankHarper and the late Wilfred Warlow.In addition, we acknowledge thecontributions of the original, thoughanonymous, authors of Principles ofmodern building, Volume 2, fromwhich several passages have beenadapted and updated.

PWPHWHAugust 1997

Second edition

This revised edition of Floors andflooring embodies a considerablenumber of changes from the firstedition, particularly with respect tochanges to standards and codes, andto building regulations.

Major changes will be found tothe section concerned with revisedguidance for identifying radonaffected areas (Chapter 1.5); to thesection on tests for sound insulationwhich has been amended by theintroduction of pre-completiontesting (PCT) by ApprovedDocument E (2003) of the BuildingRegulations (Chapter 1.8); and to the chapter on panel products(Chapter 8.5) following thepreparation of performancespecifications and confirmation offormer draft International Standards.

The opportunity has also beentaken to update references to thehundred or so British Standardsmentioned in the first edition.

PWPHWHJuly 2003

Preface vii

This is a general chapter whichincludes information relevant to allkinds of flooring – informationrelevant to a particular kind offlooring, such as typical wear rateson a particular surface, will be givenin the appropriate later chapter.

Floor finishes may be consideredto provide for the following mainfunctions:� protection of the structural floor� better appearance� increased comfort and safety

The relative importance of thesefunctions varies according tocircumstances and budgets.However, one of the most importantattributes is the maintenance of thefunctions over time; in other words,the finishes must be durable. Becauseof its importance, durability hasoften been regarded as a basicproperty of a floor finish, but itrepresents only the length of timethat the chosen properties persistand depends as much on theconditions of use as on theproperties of the finish(98).

Many factors influence the life of a floor finish:� wear� water and other liquids� indenting loads and impacts

(Figure 1.61)� sunlight� insects� moulds and fungi� high temperature

as well as the fundamental propertiesof the materials and adhesives used,and the compatibility of these withother parts of the structure and itsbehaviour in use.

The European Union ofAgrément (UEAtc) have givenpriority, in their Method ofAssessment and Test coveringinnovative floorings, to assessing theconditions of service for flooringsusing what has been called theUPEC system. UPEC is the Frenchacronym for wear (usure) index 1–4,indentation (poinçonnement) index1–3, water (eau) index 0–3, andchemicals (chimiques) index 0–3(UEAtc Method of Assessment andTest No 2(25)). Given the existence ofan Agrément certificate for aparticular product, therefore, asurveyor should be able to make ajudgement on its performance.UEAtc and the British Board ofAgrément (BBA) have also issuedfurther guidance on the assessment

56 1 The basic functions of all floors

Chapter 1.9 Durability

Figure 1.60The durability of any flooring is governedultimately by the kind of use it gets,particularly by heavy wheeled vehicles

Figure 1.61This industrial floor has suffered very heavywear in spite of the robust finish. It hasbeen subjected to piecemeal replacement.The wheel tracks of mechanical handlingequipment can be seen

of plastics floorings (UEAtcMethods of Assessment and Test No 23(99) and No 36(100), and BBAInformation Sheet No 2(101)). Theseare referred to in greater detail in theappropriate later chapters.Experience has shown that there is arelationship between wear andindentation of PVC floorings; thesefloorings, invariably, have a C indexof 2. For PVC floorings, it hastherefore been found possible toomit the C rating, and to combinethe U and P ratings into a single Gclassification (1–5).

Of all the performance factorsdescribed above, wear is perhaps themost influential because:� it is unavoidable in normal use� it applies to all kinds of flooring� in many cases, it can be very

obvious to users when it occurs

Expected life of flooringsThere is a British Standard ondurability, BS 7543(102), which appliesto floors and flooring. It givesgeneral guidance on required andpredicted service life, and how topresent these requirements whenpreparing a design brief.

The service life of a floor coveringdepends as much on the conditionsin the building and the degree of useas on the inherent properties of thematerial and the techniques adoptedin laying it. However, even bearingin mind the comparative ease ofreplacement of many thin floorings,building users should have areasonable expectation of aminimum service life for a properly

selected flooring, taking allcircumstances into account. Whatthat minimum might be is a matterfor debate, but UEAtc have decidedon 10 years, and BRE would notdispute that figure(25).

Durability is affected by the careand skill with which the floor is laid,and the behaviour of the subfloor, aswell as by the choice of materialitself. Failure to provide adequatedampproofing and ventilation, and asound screed, can lead to seriousdeterioration in finish; for exampleas buckling of wood block floors orloss of adhesion in tiled or paintedfinishes (Figure 1.62).

Resistance to wearAll floor surfaces wear to somedegree when subjected to foot orwheeled traffic. There may also beother factors including themovement of furniture, which mayscrape or cut the surface, and themovement of heavy loads such as inwarehouses. However, using thedefinition of wear as the progressiveloss from the surface of a bodybrought about by mechanical action,it is not just the quantitative loss ofmaterial which is important but alsothe qualitative assessment of thecondition of the worn surface.Where changes in appearance are

involved, the assessment becomesmore and more subjective.

Wear and damage can beconsidered to arise from a number ofcauses:� mechanical action (leading to

progressive loss of substance)� abrasion caused by fine solid

particles� cutting by the action of vehicles

(eg trolleys) and furniture� corrosion from spillages� degradation from soluble salts

rising from the subsoil orentrapped moisture in concrete

� fatigue in the surface material� movement or rocking from

uneven or friable bedding causingchipping of arrises of slabs or tiles

There has been considerableconcern since the 1960s with thedegree of wear of floors in heritagebuildings. This is dealt with in detailin Chapters 5.2 and 5.5. In somecases, depending of course on thematerial involved, it is evident thatwear of around one millimetre in tenyears is occurring(103).

1.9 Durability 57

Figure 1.62This paint finish is breaking up. There islittle substance to resist further wear

Figure 1.63Measuring the forces applied by foot traffic to flooring. Although these tests wereconducted in the late 1950s, the results still apply


Walls, windows and doorsPerformance, diagnosis, maintenance,repair and the avoidance of defects


R C de Vekey, PhD, MRIC, CChem, DIC

BRE

Garston

Watford

WD2 7JR

iii

Contents

Preface vReadership vScope of the book vSome important definitions viAcknowledgements vi

0 Introduction 1Records of failures and faults in buildings 1BRE Defects Database records 3BRE publications on walls, windows and doors 9Changes in construction practice over the years 9

1 The basic functions of the vertical elements 151.1 Strength and stability 161.2 Dimensional stability 241.3 Exclusion and disposal of rain and snow 281.4 Thermal properties and condensation 381.5 Ventilation and air leakage 441.6 Daylighting, reflectivity and glare 461.7 Solar heat, control of sunlight and frost 491.8 Fire 541.9 Noise and sound insulation 581.10 Safety and security 611.11 Durability and maintenance 651.12 Anthropometrics and dynamics 74

2 Loadbearing external walls and units 772.1 Brick, solid 782.2 Brick and block, cavity 902.3 Stone rubble and ashlar 1032.4 Precast concrete 1082.5 In situ concrete 1162.6 Earth, clay and chalk 120

iv Contents

3 External cladding on frames 1253.1 Masonry on steel or concrete frame 1263.2 Precast concrete on steel or concrete frame 1323.3 Timber frame 1373.4 Sheet cladding over frames 1483.5 Glass fibre reinforced polyester and glass fibre reinforced cement 163

4 Glazing and curtain walling 1674.1 Domestic windows 1684.2 Non-domestic windows 1834.3 Curtain walling and glass blocks 187

5 External doors, thresholds and shutters 1935.1 Hinged, pivoting and sliding pedestrian doors 1945.2 Thresholds 1995.3 Garage doors, shutters and gates 203

6 Separating and compartment walls 2076.1 Brick and block, precast and in situ concrete 2086.2 Framed 2156.3 Stairway enclosures and protected shafts 218

7 Partitions 2217.1 Masonry partitions 2227.2 Framed, movable and relocatable partitions 227

8 Internal doors 2338.1 Doors for normal traffic 2348.2 Fire doors 239

9 Applied external finishes 2439.1 Tiling 2449.2 Rendering 2489.3 Timber and plastics cladding 2599.4 Paints and other liquid or plastic finishes 262

10 Applied internal finishes 26710.1 Tiling 26810.2 Plastering and rendering 27110.3 Sheet and board finishes 27510.4 Paints and other liquid or plastic finishes 279

References and further or general reading 281

Index 295

This book is about all the mainvertical elements of buildings, bothexternal, including walls, windowsand doors, and internal, includingseparating walls, partitions andinternal doors. It deals in outline withthe achieved performances anddeficiencies of the fabric over thewhole age range of the nationalbuilding stock. Of course, manyperformance requirements arecommon to buildings of whatever age,the main difference being thatoccupants tend to make allowancesfor deficiencies when the building is old.

The construction of a wall and itsconstituent materials must haveadequate strength, be fire resistant,offer the necessary acoustic andthermal properties, and resist erosionand corrosion for the life of thebuilding, so that the occupants can usethe building safely and conveniently.In addition, the external envelopeshould have adequate resistance to theelements, and must allow themaintenance of a suitable indoorenvironment.

Information on faults in buildingsof all types show that nearly halfconcern walls, windows and doors.Despite the advice that has beenavailable to the industry from the1920s, faults which formerly occurredon a substantial scale, such as rainpenetration through windows and doors, still recurfrequently. Some of these faults mayappear to be elementary in character,but this only reflects what happens in practice. Recognitionof faults before they occur on site isobviously much more preferable tocorrection after the event. In some

cases an underlying cause is lack ofknowledge, in others a lack of care.

ReadershipWalls, windows and doors isaddressed primarily to buildingsurveyors and other professionalsperforming similar functions – such asarchitects and builders – whomaintain, repair, extend and renew thenational building stock. It will alsofind application in the education field.

In spite of the current explosion of information, or perhaps because ofit, people do not use the guidance thatexists. In order to try to remedy thatsituation, the advice given in Chapters2 to 10 of this book concentrates onpractical details. However, there alsoneeds to be sufficient discussion ofprinciples to impart understanding ofthe reason for certain practices, andmuch of this information is given in Chapter 1.

Scope of the bookAll kinds of external walls,encompassing loadbearing and non-loadbearing, curtain walls, andovercladding are dealt with first; thenwindows and external doors,including thresholds. Later chaptersinclude separating walls, partitions,and internal doors and stair enclosuresincluding protected shafts. Inprinciple, all types of buildings areincluded, though obvious practicalconsiderations of space decree thatinformation on heritage buildings islimited in scope.

The book is not a manual ofconstruction practice, nor does itprovide the reader with theinformation necessary to design awall. Both good and bad features of

walls, windows and doors and thejoints between them are described,and sources of further information andadvice are offered. The drawings arenot working drawings but merelyshow either those aspects to which theparticular attention of readers needs tobe drawn or simply provide typicaldetails to support text.

Excluded from the scope of thisbook is consideration of foundationsfor walls, or basements, which will bedealt with in another publication.Wall-to-roof junctions at eaves andverges, and fabric or flexible plasticssheathings to buildings were dealtwith in the companion book, Roofsand roofing.

As with the other books in thisseries, the text concentrates on thoseaspects of construction which, in theexperience of BRE, lead to thegreatest number of problems orgreatest potential expense, if carriedout unsatisfactorily. It follows thatthese problems will be picked up mostfrequently by maintenance surveyorsand others carrying out remedial workon walls, windows and doors.Occasionally there is informationrelating to a fault which isinfrequently encountered, and aboutwhich it may in consequence bedifficult to locate information.Although most of the informationrelates to older buildings, muchmaterial concerning observations byBRE of new buildings underconstruction in the period from 1985to 1995 is also included.

Many of the difficulties which arereferred to BRE for advice stem fromtoo hasty an assumption about thecauses of a particular defect. Veryoften the symptom is treated, not the

v

Preface

cause, and the defect recurs. It is to behoped that this book will encourage asystematic approach to the diagnosisof walls and walling defects.

The case studies provided in someof the chapters are selected from thefiles of the BRE Advisory Service andthe former Housing DefectsPrevention Unit, and represent themost frequent kinds of problems onwhich BRE has been consulted.

The standard headings within thechapters are repeated only where thereis a need to refer the reader to earlierstatements or where there issomething relevant to add to what hasgone before.

Since it is necessary to consider theenclosures for stairways inconjunction with stair flights,enclosures for stairways (includingsuch items as protected shafts) aretherefore considered as elements ofwalls and are dealt with in this book inChapter 6.3.

In the United Kingdom, there arethree different sets of buildingregulations: The Building Regulations1991 which apply to England andWales; The Building Standards(Scotland) Regulations 1990; and TheBuilding Regulations (NorthernIreland) 1994. There are manycommon provisions between the threesets, but there are also majordifferences. This book has beenwritten against the background of thebuilding regulations for England andWales since, although there has beenan active Advisory Service forScotland and Northern Ireland, thehighest proportion of site inspectionshas been carried out in England andWales. In addition, the technicalaspects of the book are affected moreby exposure due to location and heightabove sea level than by national oradministrative boundaries. The fact

that the majority of references tobuilding regulations are to those forEngland and Wales, should not makethe book inapplicable to Scotland andNorthern Ireland.

There is insufficient space in thisbook to deal with the highlysophisticated new forms of externalwalls, such as the so-called ‘smart’ orintelligent skins, employing variableexternal fabric, to improve solarcontrol and daylight utilisation. It isintended that these form part of afurther book in this series.Information relating to thesetechniques may be sought from BRE.

Some important definitionsSince the book is mainly about theproblems that can arise in walls,windows and doors, two words, ‘fault’and ‘defect’, need precise definition.Fault describes a departure from goodpractice in design or execution ofdesign: it is used for any departurefrom requirements specified inbuilding regulations, BritishStandards and Codes of practice, andthe published recommendations ofauthoritative organisations. A defect –a shortfall in performance – is theproduct of a fault, but while such aconsequence cannot always bepredicted with certainty, all faultshave the potential for leading todefects. The word failure hasoccasionally been used to signify themore serious defects (andcatastrophes!).

Where the term 'investigator' hasbeen used, it covers a variety of rolesincluding a member of BRE'sAdvisory Service, a BRE researcheror a consultant working under contractto BRE.

Because the term ‘separating wall’has been used in the constructionindustry from the earliest days, and is

still in current use, we prefer to use itin this book as a generic term despitethe comparable term ‘compartmentwall’ which is found in the nationalbuilding regulations.

AcknowledgementsPhotographs which do not bear anattribution have been provided fromour own collections or from the BRE Photographic Archive, a uniquecollection dating from the early1920s.

To the following colleagues, andformer colleagues, who havesuggested material for this book orcommented on drafts, or both, weoffer our thanks:

M J Atkins, P Bonfield, R Cox, E J Daniels, Maggie Davidson, C Grimwood, W H (Bill) Harrison, CHolland, M Howarth, Dr P Littlefair, Penny Morgan, F Nowak, Dr R Orsler, M Pound, P W Pye, R E H Read, J Reid, Justine Redshaw, B Reeves, J Seller,A J Stevens, C M Stirling, N Tinsdeall, P M Trotman, P Walton,and Dr T Yates, all of the BRE.

We wish to acknowledge a specialdebt to P M Trotman for providing themajority of the information in Chapter2.6.

In addition, acknowledgement isgiven to the original thoughanonymous authors of Principles ofmodern building, Volume 1, fromwhich several passages have beenadapted and updated.

H W HR C de VJuly 1998

vi Preface

This first chapter deals in turn withthe basic functions which affect all ofthe vertical external envelope of abuilding, whether large (Figure 1.1)or small, and all of its verticalinternal subdivisions. In laterchapters each of these functions isconsidered in greater detail whererelevant to a particular component.The chapter takes its cue from apassage in Principles of modernbuilding : ‘It will be convenient toconsider the whole range of wallfunctions together, even though anygiven wall, external or internal,loadbearing or non-loadbearing,may not have to perform all of them’.

15

Chapter 1 The basic functions ofthe vertical elements

Figure 1.1Tay House, Glasgow (Photograph by permission of Stoakes Systems Ltd)

The predominating factor in thedesign of a wall is whether it has tocarry an imposed load, for examplefrom floors and roofs. Ever sinceRoman times the loading on a non-framed wall has largely governed itsthickness, and over the yearsconventions became established fortheir construction.

However, it is only comparativelyrarely that the wall can stand by itselfwithout receiving support or restraintfrom the remainder of the building.Even the smaller domestic scalebuilding of loadbearing brick oftenrelies on the floor to provide lateralrestraint to the external wall. All thatthis chapter can do is to draw attentionto some of the more importantconsiderations in relation to thecontribution which walls make to thestructure as a whole, but not to providesufficient information to allow thestructural design of walls to be carriedout.

All walls need to be sufficientlystrong to carry the self weight of thestructure, together with imposedloads; for example those due tofurniture, equipment or the occupantsof the building.

Current requirements as far as thestructure of walls is concerned areembodied in the various nationalbuilding regulations. Taking theEngland and Wales Regulations(15) asan example:

‘(1) The building shall beconstructed so that the combineddead, imposed and wind loads aresustained and transmitted by it to theground

(a) safely; and(b) without causing such deflection

or deformation of any part of the

building, or such movement of theground, as will impair the stability ofany part of another building

(2) In assessing whether a buildingcomplies with sub paragraph (1)regard shall be had to the imposed andwind loads to which it is likely to besubjected in the ordinary course of itsuse for the purpose for which it isintended.’

Structural design of walls forbuildings is covered by the mainBritish Standard Codes of practice forthe various materials:● steel to BS 5950-1 to 9(16)

● concrete to BS 8110-1 to 3(17)

● timber to BS 5268(18)

● masonry to BS 5628(19), ApprovedDocument 1/2(20) or BS 8103(21)

LoadsThere is a wide range of structuralconsiderations that may be required of a walling system, rangingfrom purely non-loadbearing claddingto the cellular masonry or concretewalling system which ultimatelycarries all the actions on a structure.The forces are illustrated by Figure1.2.

Dead loadsThe dead weight of any floors,partitions, ceilings, roofs andcladdings must be carried to theground (foundations) via a structuralframe or a loadbearing wall. Thedesign dead load at any point is thesum of all the individual dead loadsacting at that point from abovefactored up to allow for the variabilityof materials. The partial factor ofsafety is normally around 1.2–1.4 forunfavourable loads and 0.9 forfavourable loads. Eccentric loads

16 1 The basic functions of the vertical elements

Chapter 1.1 Strength and stability

Compressive

Shear (in plane)

Shear (in plane)

Lateral (flexural)

Shear (normal)

Figure 1.2Different loadings on a wall


Foundations,basements andexternal worksPerformance, diagnosis, maintenance,repair and the avoidance of defects


P M Trotman

BRE

Garston

Watford

WD25 9XX

iii

Contents

Preface vReadership vScope of the book vDesign criteria viSome important definitions viAcknowledgements vii

0 Introduction 1Records of failures and faults in buildings 2BRE Defects Database records 2BRE publications on foundations, basements and external works 4Changes in construction practice over the years – a historical note 6Summary of main changes in common practice since the 1950s 11Building user requirements in the third millenium 14

1 The basic features of sites 151.1 Geology and topography 171.2 Fill, contaminated land, methane and radon 431.3 Surface water drainage requirements, water tables and groundwater 571.4 Site microclimates; windbreaks etc 621.5 The effects of vegetation on the ground 70

2 Foundations 772.1 General points on foundations 792.2 Old brick and stone footings 1092.3 Concrete strips, pads etc 1122.4 Piles 121

3 Basements, cellars and underground buildings 1293.1 Structure 1323.2 Waterproofing 1383.3 Other aspects of performance 150

iv Contents

4 Public and other utilities 1574.1 Water supply 1584.2 Wastewater drainage 1624.3 Surface water drainage, soakaways and flood storage 1854.4 Other utilities 193

5 Walls, fencing and security devices 1955.1 Embankments and retaining walls 1965.2 Freestanding walls 2065.3 Fencing 2125.4 Exterior lighting and security devices 220

6 Hard and soft landscaping 2256.1 Pavings 2276.2 Trees, plants and grass 233

References and further reading 239

Index 249

This book is the fifth of the BREBuilding Elements books andcompletes the planned series whichwas begun in 1996. The first fourbooks are Roofs and roofing (firstpublished in 1996), Floors andflooring (1997), Walls, windows anddoors (1998) and Building services(2000).

ReadershipAs with the other books, Foundations,basements and external works isaddressed primarily to buildingsurveyors and other professionalsperforming similar functions; forexample, architects and builders whomaintain, repair, extend and renew thenational building stock. Surveyors andarchitects undertaking routine surveysof existing buildings may identify siteproblems or issues – in particularconcerning the behaviour offoundations – which need to bereferred to specialists for advice. The larger non-domestic buildingsbuilt in the second half of thetwentieth century, and perhaps evensome in the first half of the century –the standard textbooks of the timewere quite explicit – will most likelyhave had the benefit of adequatelydesigned foundations. In the case,though, of relatively small buildingssuch as houses, many decisionsconcerning ground and foundationswill have been made by buildingprofessionals who had little or notraining in geotechnical engineering.

Some topics, such as deteriorationin drainage installations, may often beamenable to straightforwardrectification, but other aspects, suchas the installation of dampproofing inbasements, may be outside the

experience of individual surveyors.Clients need to be advised on when tocall in other consultants to rectifyexisting problems. The book iscertainly not addressed to thegeotechnical engineer or thelandscape architect, though it will inall probability find application in theeducation field.

Scope of the bookThe descriptions and advice given inthis book concentrate on practicaldetails. But there also needs to besufficient discussion of principles toimpart understanding of the reason forcertain practices. In previous books inthis series, some of the informationwhich applied generally to the subjectmatter of the book was given inChapter 1, but here the topics, thoughall closely related to the site, are rathermore disparate, and the principleswhich govern practice will inconsequence be found dispersed inindividual chapters.

Included in foundations andbasements is all work below DPClevel, including strip foundations,piles, retaining walls to basements;but not including ground floor slabs orrafts, which were included in Floorsand flooring, and building serviceswithin the footprint of the building,which were included in Buildingservices.

Many points relating to the use ofparticular materials in close proximityto the ground, such as the durability ofbrick, block and concrete have beendealt with in Walls, windows anddoors to which reference can be made.

Large civil engineering structures such as port installations,bridges and tunnels, underground carparks and very large non-buildingstructures such as storage tanks areexcluded from the scope of this book.

Included in external works are allitems outside the building footprintbut inside the site boundary,encompassing wastewater and surfacewater drains, supply of utilities (eggas, electricity and cabled services),footpaths, and access for vehiclesincluding car parks and hard standingsto be found in the vicinity ofbuildings. Perimeter and freestandingboundary walls are also dealt with, asis security fencing and, in outlineonly, lighting; CCTV surveillancesystems, though, are normally left inthe hands of specialist consultants andcontractors, and are therefore notcovered in detail.

With such a broad scope, it will beapparent that only brief reference canbe made to most topics, and the texttherefore concentrates on thoseaspects with which BRE has beenmost heavily involved, whether inlaboratory research, site investigationor development of legislation.

In principle, all types of buildingsare included. However, it is inevitablethat the nature of foundationsbecomes very sophisticated in somebuilding types such as those which arevery tall, and these installations rarelylend themselves to simple guidancefor use by non-specialists. Indeed,there may well be no professional rolewhatsoever for them in this respect.However, even the relatively simplesystems used in the majority ofdomestic construction provideadequate potential for improvement.

v

Preface

Foundations, basements andexternal works is not a manual ofconstruction practice, nor does itprovide the reader with theinformation necessary to designfoundations, basements or externalworks. Both good and bad features ofthese elements are described, andsources of further information andadvice are offered. The drawings arenot working drawings but merelyshow either those aspects to which theparticular attention of readers needs tobe drawn, or simply provide typicaldetails to support text. The discussion,for the most part, is deliberatelyneutral on matters of style andaesthetics and is wary of suggestingthat there is ever a unique optimumsolution.

In a similar fashion to the otherbooks in this series which deal withother building elements, the presenttext concentrates on those aspectsrelating to the subject matter of thebook, in this case the site, and which,in the experience of BRE, lead to thegreatest number of problems orgreatest potential expense if carriedout unsatisfactorily. It follows thatthese problems will be picked up mostfrequently by maintenance surveyorsand others specifying and carrying outremedial work. Occasionally there isinformation relating to an item,perhaps a fault, which is infrequentlyencountered, and about which it maybe difficult to locate information.Although most of the informationrelates to older buildings, muchmaterial concerning observations byBRE investigators of new buildingsunder construction in the period from1985 to 1995 is also included.

The case studies provided in someof the chapters are selected from thefiles of the BRE Advisory Service,and the former Housing DefectsPrevention Unit, and represent themost frequent kinds of problems onwhich BRE has been consulted.

The standard headings within thechapters are repeated only where thereis a need to refer the reader to earlierstatements or where there issomething relevant to add to what hasgone before. An exception to thesequence of standard headings occursin Chapter 2.1 where the amount of

material to be described requires afurther breakdown into diagnosis,monitoring and remedial work.Chapter 3 also does not follow thestandard headings; it was found to bemore appropriate to deal separatelywith structure and waterproofing ofbasements which is reflected in thesub-chapter headings.

In the United Kingdom, there arethree different sets of buildingregulations: The Building Regulations1991 which apply to England andWales; The Building Standards(Scotland) Regulations 1990; and TheBuilding Regulations (NorthernIreland) 1994. There are manycommon provisions between the threesets, but there are also majordifferences. The book has beenwritten against the background of thebuilding regulations for England andWales, since, although there has beenan active Advisory Service forScotland and Northern Ireland, thehighest proportion of site inspectionshas been carried out in England andWales. The fact that the majority ofreferences to building regulations areto those for England and Wales shouldnot make the book inapplicable toScotland and Northern Ireland.

Although practically all topicsrelating to the construction ofbuildings are encompassed in theConstruction (Design andManagement) Regulations 1994, theramifications for each of the topicscovered in this book are quitedifferent. It is therefore not practicalto spell them out, beyond noting thatthere must be a Health and Safety Planand File for all construction workwhich should include information onhow to manage health and safetyissues after the installation iscompleted and throughout its life untildemolition(1).

Design criteriaWhile this book is mainly aboutexisting buildings and not specificallyabout the design of new buildings, ithas been necessary in severalcircumstances to give some designcriteria so that subsequentperformance of the completedbuilding may be assessed against whatwas required or intended.

Some important definitionsThe term ‘footprint’ has been used todescribe the area actually covered bythe building fabric. Note that this termis not synonymous with the termcurtilage, as used in legislation, whichin its normally accepted meaningincludes any ground forming a part ofthe enclosure within which thebuilding stands.

The term ‘surcharge’ as used in thisbook has two distinct meanings: apreloading of the ground (eg to induceconsolidation), and a condition inwhich water is held under pressurewithin a gravity drain, but which doesnot escape to cause flooding.

So far as water terms are concerned,there has been a significant change inusage since the 1980s. The term‘potable water’ to describe water of aquality suitable for drinking is now nolonger popular though it is stillcontained in current Standards, andthe words ‘drinking’ and ‘wholesome’to describe water are preferred.

The use of the term ‘foul water’ inrelation to sewage has fallen out ofofficial use, being superseded by theterm ‘wastewater’ which is often moreclosely defined as ‘greywater’ or‘blackwater’; ‘greywater’ iswastewater not containing faecalmatter or urine, ‘blackwater’ iswastewater that contains faecal matteror urine. However, for the purposes ofthis book, we tend to retain the termfoul water when referring togreywater and blackwater;wastewater, though, can includesurface water (eg run-off from carparks).

The term ‘aerobic’ indicatesconditions in which free oxygen ispresent, and ‘anaerobic’ in which it isnot present.

vi Preface

Since the book is to a considerableextent about the problems that canoccur in the below-ground fabric ofbuildings, two words, ‘fault’ and‘defect’, need precise definition. Faultdescribes a departure from goodpractice in design or execution ofdesign; it is used for any departurefrom requirements specified inbuilding regulations, BritishStandards and codes of practice, andthe published recommendations ofauthoritative organisations. A defect –a shortfall in performance – is theproduct of a fault, but while such aconsequence cannot always bepredicted with certainty, all faults havethe potential for leading to defects.The word failure has occasionallybeen used to signify more seriousdefects and catastrophes.

Where ‘investigator’ has been used,it covers a variety of roles including amember of BRE’s Advisory Service, aBRE researcher or a consultantworking under contract to BRE.

Particular terms relating to topicsunder discussion will be found in thevarious chapters which follow.

AcknowledgementsPhotographs which do not bear anattribution have been provided fromour own collections or from the BRE Photographic Archive, a unique collection dating from the early1920s.

To the following colleagues, andformer colleagues, who havesuggested material for this book orcommented on drafts, or both, we offerour thanks:

R B Bonshor, Lesley Bonshor, R Cox, Maggie Davidson, R M C Driscoll, Hilary Graves, J Griggs, M S T Lillywhite, Dr R Orsler, C R Scivyer, J Seller andE Suttie, all of the Building ResearchEstablishment.

We are particularly grateful toRichard Driscoll, who contributedmuch of Chapters 1.1 to 1.3, 1.5, andthe majority of Chapter 2; to Ron andLesley Bonshor who preparedhistorical and other material onfoundations and geotechnics, and oncracking; to Mike Lillywhite whoprepared some of the historical studieson foul drainage upon which we havedrawn; and to John Ramsay, Memberof The Landscape Institute, whocommented on Chapters 1.4 and 1.5from the point of view of thelandscape architect, and alsocontributed much of Chapter 6.2.

HWHPMTDecember 2001

Preface vii

Before the introduction of concretestrip foundations for masonry walls, itwas common practice to providemasonry footings wider than the wallabove in order to spread the imposedload on the soil. Where the groundwas of poor bearing capacity, withhigh water tables leading to boggyareas, there may even have beenbrushwood faggots or fascines placedbeneath the footings in the hope ofimproving bearing capacity.

As well as describing the lowercourses of corbelled brickwork, thischapter deals with masonry belowDPC level – particularly its durability.

Characteristic details

Basic descriptionIn Georgian and Victorian times thefoundations of small buildings tendedto be very shallow in depth, often nomore than half a dozen or so coursesof bricks. This brickwork belowground level was often stepped inorder to spread the load, as shown inFigure 2.24. Before the widespreadintroduction of concrete in footingstowards the end of the nineteenthcentury, unless the walls were foundedon rock, progressively wider coursesof bonded masonry were absolutelynecessary to spread loads overadequate widths of subsoil. It wascommon practice to use as manybricks as possible laid as headers, thatis to say, normal to the line of the wallso that the corbelling action wasmaximised. Where the wall was ofstone, the same principle would apply,with the maximum use of throughstones (Figure 2.25).

Local authority byelaws in the lastyears of the nineteenth centuryrequired the course above the concreteto be twice the width of the wall to becarried, with successive courses abovethat reducing by quarter brick widtheach side until the required thicknessof wall was achieved. In modernconstruction, this stepped profile hasbeen dispensed with as a simple massconcrete strip adequately distributesthe load.

Main performance requirements and defectsStrength and stabilityThere is an important differencebetween the behaviour of brickworkbuilt in lime mortar, and brickworkbuilt in cement mortar. Masonryfootings which used lime mortars aremuch more tolerant of movement thanare those built with cement mortars,the ductile nature of the lime mortarallowing the bricks to move slightlyrelative to each other when understress. In consequence, footings ofbuildings built before the 1930s,before lime mortars began to besuperseded by cement mortars, aremuch less likely to crack when thesoils on which they are founded shrinkor heave.

109

Chapter 2.2 Old brick and stone footings

Figure 2.24 Maximum use of brickheaders in footings gives the bestloadbearing qualities

Figure 2.25 Typical stone footings widelyused until the end of the nineteenth century

��

Firm ground

Where alterations are to be made toan old building, it is crucial to exposeand measure any masonry footings,and to carry out an assessment of theloadbearing capacity of the soil onwhich they are carried (Figure 2.26).

DurabilityIt should be noted that discussion ofthis topic in this chapter applies onlyto those materials used below groundlevel. Different considerations applyto materials used above ground level,and reference should be made to theappropriate parts of Walls, windowsand doors (101).

Only a few influences are known toreduce the normally indefinite life ofmaterials recommended forconstruction of foundations, the mostsignificant of these being sulfateattack and frost. Mortar for brick andblockwork can have very variabledurability. Breakdown may behastened by the use of unsuitable,poor quality or incorrectly preparedmaterials. The relevant codes giveextensive guidance on the preferredspecifications for givencircumstances. Local building controldepartments should have informationon the presence of aggressive soil orgroundwater in particular areas.Where any doubts exist, samplesshould be taken for analysis.

Washing out mortar from footingsor fines from the ground beneathfoundations can result from leakingwater mains or drains.

Clay bricksSo far as the bricks themselves areconcerned, there is a wide range ofperformance. Engineering bricks arehighly durable, the inherently goodresistance of these materials being dueprincipally to their low porosity. Thedeterioration of brick depends on thenature of the contamination to which itis exposed, and can occur as a result ofeither a chemical interaction with theceramic materials – leading to thedissolution of the glassy phase, whichin some cases can constitute as muchas 60% of the brick – or a physicalexpansion mechanism due to thecrystallisation of salts within the brickpores(77); this is potentially a moreserious problem (Figure 2.27). The

salts are transported by water to theinterior of the brick and can derivefrom the external environment or fromthe rehydration of the soluble phase ofthe brick. The most deleterious saltsare those that are readily hydratable(eg sulfates of sodium and calcium).

The site of crystallisation isdetermined by the dynamic balancebetween the rate of evaporation ofwater from the brick surface and therate of solute migration to the site ofcrystal growth. If the rate of solutemigration is faster then the rate ofevaporation, then crystallisationoccurs on the surface of the brick(efflorescence). Althoughefflorescence is unsightly it is notharmful. If the rate of solute migrationis slow, or if the salt is relativelyinsoluble, crystallisation will takeplace within the pores of the brick.This is usually referred to as sub-florescence or cryptoflorescence, andit can result in delamination, flakingand spalling(77).

A more detailed explanation of theeffect of crystallisation of soluble salts on clay bricks is to befound in Chapter 2.2 of Walls,windows and doors.

Calcium silicate bricksCalcium silicate bricks are resistant toattack by most sulfate salts in the soiland groundwater. The durability ofcalcium silicate bricks under saltcrystallisation attack can be attributedto a combination of their very lowsoluble salt content, and their lowporosity and coarse pore structure.However, calcium silicate bricks maybe attacked by high concentrations ofmagnesium and ammonium sulfate.They may also suffer severedeterioration if they are impregnatedby strong salt solutions, such ascalcium chloride or sodium chloride,and then subjected to frost(77).

110 2 Foundations

Figure 2.26 A narrow diameter hole hasbeen excavated in a confined spacealongside a one brick solid external wall toestablish the depth and width of whatturned out to be brick footings. Both depthand width had to be established by probingwith a long crowbar. The footings were onshrinkable clay, and were unusually deepfor an Edwardian house. Tree roots arevisible in the sides of the excavation

Figure 2.27 A variety of contaminantshas obviously been in contact with this 100 year old clay brickwork below DPClevel, but only minor damage has resulted.There is also evidence of previousalterations and some repointing


Building servicesPerformance, diagnosis, maintenance,repair and the avoidance of defects


P M Trotman

BRE

Garston

Watford

WD2 7JR

iii

Contents

Preface vReadership vScope of the book vSome important definitions viAcknowledgements vii

Some less common technical abbreviations viii

0 Introduction 1Records of failures and faults in buildings 1BRE Defects Database records 1BRE publications on building services 6Changes in construction practice over the years 6Summary of main changes in common practice since the 1950s 10

1 Building physics (services) 171.1 The building as a whole 181.2 Energy, heat transfer and thermal comfort 211.3 Condensation 291.4 Artificial lighting 34

2 Space heating and cooling 432.1 Solid fuel, open fires and stoves 462.2 Boilers and hot water radiator systems 582.3 Floor and ceiling heating, and electric storage radiators 772.4 Warm air systems and air conditioning 802.5 Geothermal and heat pumps 912.6 Solar energy 93

3 Ventilation and ducted services 993.1 Natural ventilation, through windows, trickle vents and airbricks 1013.2 Natural ventilation, passive stack 1193.3 Forced air mechanical systems 125

iv Contents

4 Piped services 1334.1 Cold water supply and distribution 1344.2 Domestic hot water services 1454.3 Sanitary fittings 1554.4 Above ground drainage 1674.5 Fire protection 1824.6 Gas, including storage of LPG 1894.7 Refuse disposal 192

5 Wired services 1975.1 Electricity 1985.2 Telephone, radio, TV and computer 2105.3 Security and fire detection systems 2165.4 Rooms for speech and sound reinforcement systems 221

6 Mechanical handling devices 2296.1 Passenger and goods lifts, escalators and dumb waiters 2306.2 Goods handling devices, conveyors and warehouse storage 240

References and further reading 245

Index 257

Readership Scope of the book

v

Preface

Some important definitions

vi Preface

Acknowledgements

Preface vii

17

Chapter 1 Building physics(services)

Figure 1.1Part of the plant room for a small office building. The rate of change of technologicaldevelopment is increasing, and plant rooms can become congested

18 1 Building physics (services)

Chapter 1.1 The building as a whole

Figure 1.2Simple protection from the weather, which might have sufficed in years gone by, is nolonger enough. When this substantial dwelling was built in the nineteenth century, the manychimneys now surviving indicate that the main rooms were heated by open fires; even so,some rooms were unheated. Used since the early 1920s as offices, the servicing systemshave needed to change out of all recognition

Integration of building servicesinto the overall design process

●

●

●

1.1 The building as a whole 19

��

��

��

��

��

��

��

��

��

��

��

��

��

��

��

��

��

��

Tanks and feeds Ductwork and trunking

Pipework

Sprinklers

Electrical supply

Radiators or convectors Boilers and air heating

��

Figure 1.3A schematic illustration adapted from Designing for production (31) showing some of theenvironmental services for a large building

Communications

Understandingdampness

Peter TrotmanChris Sanders

Harry Harrison

Und

erstanding

dam

pness

HW

Harrisonand PM

Trotman

BRE Bookshop, Building Research Establishment,Watford WD25 9XXTel: 01923 664761Fax: 01923 662477email: [email protected]

BR 466

About the authors

Peter Trotman served a student apprenticeship with Bristol Aero-Engines Ltd (now Rolls-Royce Ltd), qualifying as a mechanical and structural engineer. He was involved with theprovision of facilities for ground running of aero engines. Peter joined BRE in 1967 andspent five years in the Public Health Engineering Section, carrying out research into waterservices and drainage. He has served on BSI committees dealing with waterproofing ofbelow-ground structures and was a founder member of the Basement Development Group.In 1975 he joined the BRE Advisory Service, and became its Head in 1990, managingprogrammes of site investigations, lecturing to construction professionals and preparingBRE technical publications. More recently he has become Co-ordinator for the CIB,International Council for Research and Innovation in Building and Construction, CommissionW86 – Building Pathology.

Chris Sanders graduated in physics and meteorology in 1973 and worked for BRE inScotland for 29 years, initially on the risk of condensation and mould in houses and roofsand the assessment of the effects of moulds and mites on health. He carried out theanalysis of the condensation and mould data in four English House Condition Surveys. Hehas produced guidance documents giving advice on avoiding thermal bridging in housingand other buildings and carried out the thermal analysis of the Robust Details that wereproduced in association with Approved Document L1 of the English Building Regulations.He is Convenor of the CEN Working Groups developing European standards on moisture andclimatic data and chairman of the BSI committee revising BS 5250, the code of practice forcondensation in buildings. He was actively involved with the UK Climate Impacts Programmeand the BRE programme investigating the effects of climate change on future buildings anddeveloping cost effective measures for the repair of housing after flooding. Chris left BREin August 2003 to become the director of the newly established Centre for Research onIndoor Climate and Health at Glasgow Caledonian University.

H W (Harry) Harrison is an architect. After short periods with a large UK building contractorand a small architectural practice, he joined BRE. He retired 34 years later as Head ofConstruction Practice Division. At BRE he carried out research into many aspects of thedesign and specification of buildings and building components, especially ofweathertightness, accuracy and jointing. He has served on British and InternationalStandards Committees responsible for putting into practice the results of BRE and otherresearch, and for several years was the secretary of Commission W60, the PerformanceConcept in Building, of the International Council for Building Research, Studies andDocumentation (now the International Council for Research and Innovation in Building andConstruction). He was a founder member of the International Modular Group. In later yearshe became a specialist in building defects, and was responsible for the Housing DefectsPrevention Unit and the BRE Advisory Service. In the 1990 Queen’s Birthday Honours he wasappointed a Companion of the Imperial Service Order for services to building research.

constructing the future

Understanding dampnessEffects, causes, diagnosis and remedies

Peter Trotman, Chris Sanders and Harry Harrison

iii

ContentsA complete list of contents starts on page 212Preface

Readership viiScope of the book viiSome important definitions viiiAcknowledgements viii

1 IntroductionWhat is dampness? 1Types of dampness 1

Condensation 2Rain penetration 2Rising damp 3Construction moisture 3Leaking pipes 3Leaks at roofing features and abutments 3Spills 4Ground and surface water 4Contaminating salts 4Where is dampness apparent? 4

Records of dampness -related problems in buildings 5BRE Advisory Service records 5BRE Defects database records 6House Condition Surveys 6

Changes lifestyle and construction 10Changes in domestic lifestyles 10Changes in external walling practice 10Changes in floor construction practice 11Changes in roof construction practice 12

Changes in levels of risk 13BRE publications on dampness 13

2 Visible and hidden effects of dampnessHealth effects of mould and damp 15

Mould problems and health 15Diagnosis 18

Surveys 18Staining 18

Visible moisture 18Condensation 18Rain penetration 19Rising damp 19

Mould growth 20Surface moulds 20Algae, lichens and mosses 21

Toxic mould 21Remedies 21

Salts 23Diagnosis 23Remedies 23

Frost 23Diagnosis 23Remedies 25

Timber rot 25Types of fungi 25Occurrence of rot 25

Understanding dampness

iv

Remedies 26Metal corrosion 28

Parts of the building at risk 28Diagnosis 32Remedies 34

Hidden dampness 36Diagnosis 36

3 Measuring moistureInstruments for measuring moisture 39

Sampling 39Electricalresistance moisture meters 40Resistance gauges 42Microwave techniques 42Capacitance methods 43Physical sampling by independent cores 43Drilled samples 43Moisture contents at which action may be required 46

Laboratory tests for salts 47Procedure 47Typical salts contents 48

Instruments for measuring the humidity 48Thermohydrograph 48Electronic sensors 49Wet bulb and dry bulb thermometers 49Dewpoint sensors 49

4 CondensationWater vapour 51

Behaviour of water vapour in the air 51Production of water vapour within buildings 54

Effects of condensation 55Condensate on surfaces 55Mould growth 55

Design to control condensation 59Interstitial condensation 59

Effects of interstitial condensation 60Controlling interstitial condensation 61

Vapour control layers 62Construction 62Joints 62Performance 62

Hygroscopic materials 63Materials affected 63Reverse condensation 63

Incidence of condensation 64Case studies of surface condensation 64On walls 64On windows and doors 65In roofs 65

On floors 67Investigating and curing condensation 69

Measuring temperature and humidity by data-loggers 69Diagnosis 69

Case studies 72Condensation in a terraced bungalow with ceiling heating 72Water dripping from the ceiling in a fine-art store room 74Condensation in an infants’ school 76Condensation in steel-framed houses 78

v

5 Rain penetrationDriving rain 84

Wind-driven snow 85Driving Rain Index 85Protection given by overhangs 85

Rain penetration in walls 91The mechanisms 91Run-off 92Solid walls 95Masonry cavity walls 98Panelled walls 100Curtain walling 102Timber frame walls 104 Survey methods 105Diagnosis 105Remedies – external 105Remedies – internal 109Workmanship 110Damp-proof courses 111

Rain penetration at openings 116Windows 116The window-to-wall joint 116Doors and thresholds 119

Rain penetration in roofs 122Pitched tiled and slated roofs 122Incidence of defects 127Diagnosis 130Other pitched roofs 130Patent glazing 133Flat and low pitch asphalt and bituminous felt roofs 134Bays and porches 140Chimneys 140

Case studies 142Water dripping from a bakery roof 142Rain penetration at a medieval church 144Water ingress through a solid brick parapet 145Rain penetration in a Victorian stately home 146

6 Rising damp and groundwater movementThe theory 150

Saturated ground 150Soluble salts 150Rainfall splashing 151

Diagnosis 151Failures of existing DPCs 151Range of possibilities 152Using an electrical moisture meter 153Drilled samples 154

Rising damp in walls 155The problem 155Earth, clay and chalk walls 155Solid masonry walls 155Cavity masonry walls 156Materials for DPMs and DPCs 157New build 158Treatment 158The choice of DPCs 158Replastering as a solution 162Dry lining 162


vi

Monitoring of the behaviour of a replacement DPC 163Replastering following a new DPC 163Impervious linings 164

Floors 164The problem 164Characteristics of failure 165Sources of moisture 165Excluding rising damp 169Materials for DPMs 176Volatile organic compounds 177Waterproofness of floorings 179

Groundwater and basements 182The problem 182Level of protection required 183Waterstops 184Converting basements during rehabilitation 185

Curing dampness problems 186Drained cavity 187Mastic asphalt tanking 187Cementitious render or compound 188Self-adhesive membranes 188Liquid-applied membranes 188Ventilated dry lining 189Partition walls 189Basement ceiling level 189Door thresholds 190Door and window frames 190Fixing services 190Chimney breasts 190Built-in timbers 190

Case studies 191Dampness in internal walls in a converted stable block 191Severe damp and related problems in N0-fines houses 192Moisture in chipboard flooring over foamed polystyrene 194Dampness from leaking pipes in commercial properties 196Damp in a listed building 198Soluble salts in the Tower of London 200

7 More about dampnessConstruction water and drying out 203

Entrapped water 203Surface DPMs 205Flooding 205

Immediate action 205Inspections 207Remedies 207

Spills and leaks 209Contaminated materials: sources and treatment 210

Animal residues 210Chimneys 210Storage of salts 210

Complete list of contents 212References and further reading - a complete list 215

vii

PrefaceMany years ago, before the Building Research Station was founded, the British Medical Associationasked the Royal Institute of British Architects to investigate the causes of dampness in dwellinghouses to help them find the reasons for the prevalence of certain diseases. The RIBA committeefound that direct penetration of rain through walls and lack of a damp-proof course (DPC) accountedfor nearly two-thirds of all cases; condensation contributed only 2%. The causes may have sincechanged in relative importance with changes in construction techniques, such as cavity walls and thetendency for houses to be better heated. Unfortunately, though, dampness is a continuing source ofdistress to occupants. It is possibly a source or a contributor to illness, it encourages deterioration inthe building fabric, and it is involved in half of the investigations undertaken over the years by BRE.

As well as damp patches on walls, ceilings and floors, dampness can lead to blistering paint, bulgingplaster, rot in building timbers, mould on surfaces and fabrics, and sulfate attack on brickwork. It canalso lead to less visible problems, such as reduced effectiveness of thermal insulation or cracking inbrickwork as a result of corrosion of embedded metal components. Despite all the technical advicethat has been published in the past, there is still a significant set of problems. This book seeks toaddress them.

ReadershipThis book is aimed primarily at all professionals involved in the design, maintenance and managementof domestic, public, commercial and industrial properties; this includes surveyors, architects, buildersand facilities managers. It will also be useful to student members of these professions. Much of thetext and many of the illustrations will also be of relevance to householders and other users ofbuildings.

Scope of the bookThe emphasis of this book is on existing buildings with some coverage of the design of new build. Itlists the causes of dampness in buildings and explores the consequential effects of that dampness onthe fabric, the maintenance of protection against dampness, and the remedies which the detrimentalresults of dampness will call for.

It is illustrated with photographs of defects from the BRE Advisory Service collection and drawings ofconstruction elements that need careful design and execution. Case studies illustrate some of themore typical problems which have been investigated as well as some interesting but informative non-typical cases, although it must be recognised that it is rare to find two cases which are identical inevery detail.

Chapter 1 contains background information. Chapter 2 provides a visual indication of the mostcommon manifestations of dampness to be seen in buildings, tabulated according to buildingelement. When the appearance of the defect under investigation has been matched with theappropriate photograph, a key provides a link to later chapters which give explanations of thephysics, further information to confirm the diagnosis, and the remedies which might be specified toput right the defect.

Although this book is mainly about existing buildings, and not specifically about the design of newbuildings, it gives some design criteria so that subsequent performance of the completed buildingmay be assessed against what was either required or intended.


viii

Some important definitionsCondensation: the process whereby water is deposited from air containing water vapour whenits temperature drops to or below the dewpoint.

Dampness: used here to cover a wide variety of phenomena relating to the unwanted presenceof water or water vapour, whatever its cause.

Deliquescent substance: substance which becomes damp and finally liquifies on exposure to theatmosphere, owing to the low vapour pressure of its saturated solution.

Dewpoint temperature of the air: the temperature at which condensation of liquid water startswhen air is cooled, at constant vapour pressure.

Hygroscopic substance: usually applied to solids which tend to absorb moisture from theatmosphere without actually becoming liquified.

Psychrometric: Relating to the measurement of water vapour in the air, including the use of thewet and dry bulb hygrometer.

Rain penetration of walls and roofs: results from water entering the structure to such an extentthat the resulting dampness or dripping of water becomes a nuisance.

Relative humidity: the ratio, normally expressed as a percentage, of the actual amount of watervapour present to the amount that would be present if the air were saturated at the sametemperature.

Reverse condensation (old term: summer condensation): interstitial condensation that can occurwhen moisture within a wall is driven in by solar radiation on south-facing walls.

Rising damp: normally the upward transfer of moisture in a porous material due to capillaryaction.

Thermal bridge (old term: cold bridge): part of a structure of lower thermal resistance whichbridges adjacent parts of higher thermal resistance and which can result in localised coldsurfaces on which condensation, mould growth and/or pattern staining can occur.

Vapour control layer (VCL): usually a thin sheet material with a vapour resistance greater than200 MNs/g, used on the warm side of thermal insulation to restrict moisture which diffusesthrough the insulation from condensing on any colder outer surface.

AcknowledgementsUnless otherwise attributed, photographs have been provided from our own collections or fromthe BRE Photographic Archive, a unique collection dating from the early 1920s.

We offer our thanks to the following colleagues and former colleagues who have suggestedmaterial for this book or commented on drafts, or both:Phil CornishStephen GarvinColin HunterTony RobertsCharles StirlingTim Yates

PMTCHSHWHApril 2004

This chapter tells you how to assess the risk of specific designs in actuallocations in the UK using the driving rain index, and deals with rainpenetration in solid and cavity masonry walls, cavity wall insulation, claddingsystems, DPC detailing principles and well-tried details, rain penetration ofpitched and flat roofs, parapets and leaking windows.

83

Chapter 5Rain penetrationDriving rain and the driving rain indexRain penetration in wallsRain penetration at openingsRain penetration in roofs

Figure 5.1 A disfiguring deposit of carbonate fromrain penetrating the sloping brickwork parapet

Figure 5.2 Although much of this results fromcondensation, there is also some rain penetration

There are regional construction differences throughout the UK as a result oflocal experience and practice as well as available materials. In moreexposed locations, walls may be sand:cement rendered and slate or tile-hung in Cornwall and Scotland. Pitched roofs are given a second line ofdefence with a sarking material of felt or plastics. In Scotland, boarding isused as the sarking. Windows in Scotland are usually inset to giveprotection; other parts of the UK use a narrow sill with the window muchcloser to the line of the outer leaf.

DRIVING RAIN

In the Building Regulations, control of moisture is a functional requirementand the building must be designed to adequately resist such penetration –see Approved Document Part C and Part G.

An International Council for Research and Innovation in Building andConstruction (CIB) Working Commission on Rain Penetration meeting in the1950s adopted a definition of rain penetration:By rain penetration is meant that rainwater penetrates into a wall eitherthrough the surface of the wall, or due to leakage at windows or similarinstallations. It is not necessary that water penetrates so far that it may bediscernible on the inside of the wall. More information is in Rain PenetrationInvestigations - A summary of the findings of CIB Working Commission onRain Penetration - Oslo 1963.

Rain penetration in modern cavity walls tends to show as a well-definedroughly circular area on internal finishes. Sometimessurface salts will define the outer limits of suchwetting. If the wetting persists, most of the wall maybecome visibly damp. In older, solid wall buildings,wetting may not be visible because successivecoats of emulsion paint or vinyl wallpaper havemasked the effects. The extent of the dampness, orif dry the salts which define it, can be traced with amoisture meter.

Moisture can be deposited on external surfaces inseveral ways:❐ Gentle rain or drizzle normally falls vertically and

will accumulate on flat surfaces. Some splashing may wet adjacent surfaces.

❐ Driving rain, which is heavy rain blown by a strong wind on to horizontal and vertical surfaces. Water can also be blown uphill on sloping surfaces.

❐ Snowfall and wind-blown snowdrifts have little effect at the time but when the snow melts, it cancause severe wetting, particularly very fine snow blown into pitched roofs.

❐ Fog wets external surfaces but in small quantitiesand has little effect.

❐ Condensation can occur on outside surfaces in tropical climates, particularly with air-conditioned buildings. Storms in these climates are more likely to be a test of weathertightness.


84

Figure 5.3 Severe wetting from driving rain on anexposed wall

Documents

Performance, diagnosis, maintenance, repair and the avoidance … · 2014-05-08 · 3.4 Fully supported built-up bitumen felt and felt strip slates 127 3.5 Rigid sheets 132 3.6 Shingles