A PILOT GREEN ROOF RESEARCH PROJECT IN SINGAPORE

Puay Yok Tan and Angelia Sia

National Parks Board, Singapore Botanic Gardens, 1 Cluny Road, Singapore 259569, Singapore

Abstract In an effort to promote green roofs in Singapore, the National Parks Board and the Housing Development Board of Singapore jointly embarked on a pilot project to install a green roof on the existing roof of a multi-storey carpark in a public housing estate. Serving as a demonstration and research site, this is also the first significant green roof installation in Singapore. This paper describes four distinct green roof systems used in the installation, and the results of various studies conducted in conjunction with the project. The studies focused on the evaluation of suitable plants, the associated thermal benefits, and air quality changes arising from the installation. For the study on suitable plants, the estimated daily atmospheric water deficit experienced showed that plants experienced irregular periods of depleted water in the root zone, hence pointing to the need to use drought tolerant plants even under humid tropical conditions. The evaluation of environmental benefits compared conditions on the roof before and after the installation of the green roofs. The green roofs installed significantly reduced the amount of visible radiation recorded on the facades of residential apartment blocks directly facing the multi-storey carpark, thereby reducing glare and improving visual comfort of the occupants. The use of infrared thermal imagery and thermocouple temperature sensors showed significant differences in surface temperatures between greenery-covered, or exposed surfaces. Marginal differences in ambient air temperature were recorded. The evaluation of air quality changes, looking at concentrations of acidic gaseous pollutants and particulate matter, showed variable results. The acidic gaseous pollutants sulphur dioxide and oxides of nitrogen showed marginal reduction following installations. The mass concentration of particulate matter, PM10 and PM2.5 increased significantly after installation, but number concentration had decreased marginally. Introduction Singapore has an international reputation as tropical Garden City. Despite being one of the most densely populated and built-up nations on earth, the city has a pleasant garden-like ambience because of the greenery that envelops the city. However, faced with land constraints and a projected increase in population from the current 4.2 million to 5.5 million over the next 40 to 50 years, the need for increased land use intensification through high-density and high-rise developments will lead to increasing competition for land between greenery and infrastructure developments. Given the finite limitation of land available on the ground for greening, the logical solution to address the imbalance is to bring greenery onto built structures, either onto facades, rooftops, or onto high-rise balconies and decks of buildings. Greenery on built structures will not only help ameliorate our harsh tropical conditions; its presence helps to create conducive environment that facilitates social interaction, often on under-utilized places such as rooftops. To that end, as the custodian of the Garden City, the National Parks Board (NParks) of Singapore has embarked on an initiative to promote the use of greenery on built structures such as buildings and link-ways in the city. We coined this form of greenery “skyrise greenery”.

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Green roof technology that originates in temperate countries is an attractive but untested form of skyrise greenery in Singapore. Because of their lightweight nature, they can be potentially used to green up many existing roofs without the need to structurally retrofit them to cater to increased loading on the roofs. There is a large potential for this technology in Singapore because of the large number of high-rise buildings. When designed to create self-sustaining plant communities that do not need intensive maintenance, green roof technology is well-suited for roofs of many high-rise buildings that are not meant to be publicly accessible, or which will be difficult to access for regular maintenance. As the national public housing agency in Singapore, creating an attractive living environment for residents of housing estates through greenery provision is a key objective of the Housing Development Board (HDB). There is also an increasing focus in recent years to incorporate rooftop greenery on the roofs of newly constructed multi-storey carparks (MSCPs). However, there remains an existing pool of older MSCPs whose roofs that are not designed to handle the weight of an intensive form of rooftop gardens. The logical solution is to install green roofs designed to meet the structural loading limit of the roofs. HDB and NParks thus recently collaborated in a pilot project to install green roofs on the roof of an existing MSCP in a public housing estate. The objectives were to test the efficacy of green roof systems under local conditions, identify potential technical barriers, and evaluate the various environmental benefits arising from the installations. Additionally, the pilot project aim to also serve as a demonstration installation for various interested parties to observe the first large-scale green roof in Singapore, and to help generate a greater awareness of the technology in Singapore. Methodology (A) Site Description and Green Roof Systems Installed Four green roof systems were selected through a request for proposal exercise, which specified the installation of green roofs on the roof of a MSCP in Punggol public housing estate (Figure 1) according to a set of performance specifications. The MSCP has ten parking decks, with a roof of 4,017 sq m (approximately 43,229 sq ft) and is about 24 m about ground. In the design of the MSCP, the roof was segmented into four discrete sections, each of which was installed with a green roof system described below, with the green roof area covering about 75% of the roof area. Each green roof installation was required to meet the maximum structural loading limit of 1.5 kN/sq m. No irrigation system was provided, with the aim of the systems being totally reliant on rainfall, except during extended periods without rainfall for more than two weeks.

Figure 1. Multistorey carpark prior to installation of green roofs.

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The key components of the green roof systems installed are listed in Table 1. To protect certain proprietary information pertaining to the systems, specific characteristics of the components are

omitted. Table 1. Components of green roof systems installed in pilot project KEY FEATURES SYSTEM 1 SYSTEM 2 SYSTEM 3 SYSTEM 4 Brand ZinCo Extensive

Green Roof System

Daku Extensive Green Roof System

Non-proprietary gardenroof

Substrate Zincolit Plus Daku Roof Garden Substrate

Mixture of Seramis, Leca Chips and Compost

Gardenlite pumice, coco peat, fine sand and hydrogel

Substrate Depth 80 – 100 mm 50 – 80 mm 65 mm 75 mm Filter membrane ZinCo Filter Sheet

SF DAKU Stabilfilter Generic geotextile Polypropylene

woven filter Drainage-Water Reservoir Cells

ZinCo Floradrain FD 25 & ZinCo Floratec FS 50

Daku FSD 20 FlorDepot L35 Garden Hydrocell

Root Barrier ZinCo Root Barrier WSF 40 laid over ZinCo Protection and Separation Mat TSM 32

Nuraplan GR 15 (Root-cum-rot resistance waterproofing membrane)

Generic UV-stabilized polyethylene sheet

Geomembrane HDPE 100

Installer ZinCo Singapore (Pte) Ltd

Hitchins (FE) Marketing Pte Ltd

Ecoflora (S) Pte Ltd

Garden and Landscape Centre (Pte) Ltd

(B) Selection and Evaluation of Suitable Plants In the application of green roof technology to a tropical country like Singapore, the single and most challenging component is the living component, i.e. the choice of plants, as the other components of the green roofs, such as drainage and water reservoir elements, and growing substrate properties, are physical elements can be engineered to match known performance criteria, such as infiltration rate, water holding capacity, drainage rate, nutrient holding capacity, etc. On the other hand, suitable plants that can be used on green roofs in the tropics still remain largely unknown or unpublicized. Even though rainfall is abundant in Singapore, with an annual rainfall of around 2100 mm and an annual surplus of 280 mm after evaporation, rainfall distribution is nevertheless non-uniform. There are typical wet and dry months, with the former occurring during December and April, and the latter during February and July. The additional challenge encountered during the drier months is the limited water holding capacity of the shallow substrates used for the green roofs. Two key criteria for the selection of plants are therefore succulence or drought tolerance, and an ability to regenerate from seeds or underground organs upon return of rainfall. On the other hand, during the wetter months when there is continuous rainfall often over periods of 5 – 7 days, many succulent plants may not tolerate the constant moisture in the root zone. Therefore, plant selection is paradoxically challenged by a need to select for drought tolerant plants as well as those that can tolerate prolonged periods of moisture around the root zone. Substrate moisture will therefore be a key determinant of plant performance. To estimate the likely substrate moisture level experience by the plants, the daily atmospheric water deficit (which is

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defined as the difference between rainfall and evaporation) (1) was estimated for the period between July 2003 and June 2004, based on rainfall data and evaporation data from the Meteorological Services of Singapore. The plants tested on the green roofs were selected through a short-listing exercise conducted in conjunction with the participating installers (Table 2). A total of 43 taxa were tested, with approximately 42% of the plants introduced from outside Singapore, and the remaining 58% sourced from the local landscape industry.

Table 2. Plants tested in various green roof systems Agavacea Convolvulaceae Furcraea foetida ‘Mediopicta’ Ipomoea pes-caprae ssp. Brasiliensis

Aloaceae Crassulaceae Aloe vera Kelanchoe tomentosa Sedum acre Aizoceae Sedum aizoon Aptenia cordifolia Sedum kamtschatikum ‘Weihenstephaner Gold’ Carpobrutus edulis Sedum mexicanum Delosperma cooperi Sedum nussbaumerianum Delosperma lineare Sedum xrubrotinctum Sedum rupestre Aspleniaceae Sedum sarmentosum Asplenium nidus Sedum sexangulare Sedum sieboldii Araceae Sedum spectabile Acorus gramineus Sedum spurium ‘Purpurteppich’ Sempervivum tectorum Amaryllidaceae Zephyranthes candida Davalliaceae Zephyranthes rosea Nephrolepis exaltata Caprifoliaceae Dracaenacea Lonicera japonica Sanseveria trifasciata ‘Hahnii’ Sanseveria trifasciata ‘Golden Hahnii’ Commelinaceae Sanseveria trifasciata ‘Laurentii’ Callisia repens Tradescantia pallida ‘Purpurea’ Liliaceae Ophiopogon intermedius Compositae Wedelia biflora Meliaceae Aglaia odorata Convallariaceae Liriope muscari Moraceae Ficus pumila Palmae Portulaceae Chamaedorea seifrizii Portulaca grandiflora cultivars Rhapis humilis Rubiaceae Pandanaceae Ixora coccinea Pandanus amaryllifolius Murraya paniculata

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(C) Evaluation of Environmental Benefits The evaluation of environmental benefits arising from the installation of the green roofs focused on the changes in temperatures of the roof surfaces and ambient air, as well as the changes in the air quality on top of the MSCP. The evaluation was done through comparison of the data collected before and after the green roofs were installed. Pre-installation data collection was done between June to July 2003, and post-installation data collection was done between February to March 2004. The green roofs were installed between August to October 2003. Our partners from the National University of Singapore conducted this component of the project. Temperature changes and glare reduction The following temperatures were measured on the MSCP: (a) surface temperature of the roof before and after installation, (b) substrate temperature after installation (Figure 2), and (c) ambient air temperatures above the original roof and above the green roofs at heights of 300 mm and 1200 mm above the surfaces (Figure 3). Surface temperatures were measured by thermocouples placed in close contact with the various surfaces, and continuous data were recorded by dataloggers. Ambient air temperatures were recorded by mini HOBO data loggers with temperature sensors housed within white ventilated wooden boxes. In addition, during February and March 2004, infrared thermal images of the green roofs were captured using an infrared radiometer (Thermo Tracer TH7102WX, NEC, Japan).

Surface tem pera tu re o f the exposed roo f

Be fo re A fte r

Surface tem peratu re o f the so il

Su rface tem peratu re o f the roo f

Figure 2. Measurement of roof surface and substrate surface temperatures

Figure 3. Measurement of ambient air temperatures at heights of 300 mm and 1200 mm above

roof or green roof surfaces.

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As the MSCP is in close proximity with adjacent residential blocks, visible radiation reflected off the concrete surface of the original roof onto the facades of the residential blocks and windows could cause visual discomfort, or glare for residents. To evaluate if the green roofs could mitigate this, mini HOBO dataloggers with light sensors were hung at different building heights and parallel to the facades (facing the MSCP), to collect data on visible radiation levels at these positions (Figure 4). During the data collection periods, a HOBO Weather Station was also used to collect data on ambient air temperature, relative humidity, solar radiation, wind speed, wind direction and rainfall on the roof.

Figure 4. Light sensors hung parallel to building facades at various heights to record visible radiation levels.

Evaluation of air quality changes The evaluation of air quality changes focused on the concentrations of acidic gaseous pollutants and particulate matter (PM), through the use of various monitoring and sampling equipment placed on the roof before and after installation of the green roofs (Figure 5). Acidic gaseous pollutants were sampled using an annular denuder system (URG, Chapel Hill, NC, USA), whereas PM concentrations were studied using condensation particle counter (TSI Incorporated, St. Paul, MN, USA) and minivolume aerosol sampler (Airmetrics, Eugene, OR, USA). A Micro-Orifice Uniform Deposition Impactor (MSP Corporation, Minneapolis, MN, USA) was also used to study the size distribution of the PM. An Aethalometer (Magee Scientific Company, Berkeley, CA, USA) was used to measure black carbon mass concentration in the atmosphere.

Figure 5. Monitoring and sampling equipment to measure air quality changes.

MOULDI

Aethalometer

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Results and Discussion (A) Evaluation of Suitable Plants As previously described, water availability is likely to be the key determinant of successful plant growth for the majority of the plants tested. The daily atmospheric water deficit data in Figure 6 showed that even in the typically wetter months of April and December, there could be periods of up to nine consecutive days when the plants will likely experience depleted moisture in the root zone. Therefore, green roofs in the humid tropics do experience xeric conditions, and the suggestion that water availability is the most important factor affecting plant growth on green roofs based on the European experience (2) is likely to also apply to green roofs in the tropics. The most challenging period for the plants was during the three-week drought experienced during February 2004, when several locally sourced landscape plants such as Acorus gramineus, Asplenium nidus, Ipomoea pes-caprae ssp. Brasiliensis, Rhapis humilis, and Wedelia biflora, among others, showed complete wilting within the first two weeks of the drought. From visual comparison, succulent plants such as several Sedums and Delosperma lineare comparatively withstood the drought condition better. However, by the third week of drought, hand watering had to be done to prevent mass die back of the plants on the green roofs. Subsequent to this period, the majority of the green roof plants were able to tolerate the shorter periods of water deficit experienced. Based on visual observations, the plants that are still thriving on the roof as on Jan 2005 are listed in Table 3.

Table 3. Plants observed to be thriving on the green roofs as on Jan 2005. Agavacea Convallariaceae Furcraea foetida ‘Mediopicta’ Liriope muscari

Aloaceae Crassulaceae Aloe vera Kelanchoe tomentosa Sedum acre Aizoceae Sedum mexicanum Aptenia cordifolia Sedum nussbaumerianum Carpobrutus edulis Sedum sarmentosum Delosperma lineare Sedum sexangulare Amaryllidaceae Liliaceae Zephyranthes candida Ophiopogon intermedius Zephyranthes rosea Meliaceae Caprifoliaceae Aglaia odorata Lonicera japonica Pandanaceae Commelinaceae Pandanus amaryllifolius Callisia repens Tradescantia pallida ‘Purpurea’ Portulaceae Portulaca grandiflora cultivars Dracaenacea Sanseveria trifasciata ‘Hahnii’ Rubiaceae Sanseveria trifasciata ‘Golden Hahnii’ Ixora coccinea Sanseveria trifasciata ‘Laurentii’ Murraya paniculata

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Figure 6. Daily atmospheric water balance (mm) (difference between daily rainfall and evaporation) for the Punggol (north-eastern) region of Singapore. Daily evaporation is estimated from the average monthly evaporation data of 1991 – 1995 divided by number of days in the month. Daily rainfall is obtained from the Punggol Weather Station from Jul 2003 to Jun 2004. The shaded box on top of each chart indicates the likely periods when the substrate had depleted moisture, based on an evapotranspiration of 5 mm per day and a water-holding capacity of 30 L/m2 for an average green roof system. Sedums used in the pilot project are probably the first sedums used in open landscapes in Singapore. About half of the Sedums tested did not grow well under tropical humid conditions. These were possibly ill suited to the constant moisture around the root zone during periods of regular rainfall. With the exception of Sedum spectabile, which flowered freely, none of the Sedums were observed to flower. This is most likely due to the lack of distinct differences in seasonal photoperiod and small diurnal or seasonal temperature differences. The plants in Table 3 constitutes only a small fraction of plants that will eventually be found suitable for green roofs in the tropics. Indeed, the tropics is blessed with such a high biodiversity of plants that there is a large pool of potential plants to be tested in futures. For green roofs under partial shade in particular, many epiphytic plants that have evolved the Crassulacean Acid Metabolic (CAM) mode of photosynthesis will particularly be well-suited for green roofs, as CAM plants tend to have high water use efficiency and tolerate higher temperatures. (B) Temperature Changes and Glare Reduction The benefits of rooftop greenery in reducing surface temperatures of buildings have been evaluated in several studies focusing on the intensive form of rooftop greenery, or rooftop gardens (3, 4, 5). This component of this study was the first evaluation for the extensive form of rooftop greenery in Singapore. As most of the results will be presented in a manuscript for publication currently in preparation, only a summary of the key results are highlighted here. It is apparently from Figure 7 that green roof helps to reduce rooftop surfaces temperature. Greenery covered surfaces were between 15 – 20 oC lower than exposed concrete surfaces on the roof.

Figure 7. Digital and infra-red thermal images of a section of the green roofs. Small arrowheads point to greenery covered surfaces.

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This result was also corroborated in the measurement of the surface temperatures through the use of thermocouple. In one of the green roof system measured, the original roof surface temperature was up to 18 oC higher compared to when the roof was covered by the green roof (Figure 8). The diurnal variation in the temperature was also much reduced after the installation of the green roofs. The surface temperature of the growing substrate was also between 6 - 10 oC lower than the originally exposed roof. Ambient air temperatures during the day were between 1.7 - 3 oC lower compared to before the installation (data not shown). It should however, be pointed out that the surface temperature results are highly dependent on the substrate moisture. After a period without rainfall for 2-3 weeks, measurements made on 22 and 23 February 2004 showed that the substrate surface temperature can exceed the surface temperature of the original exposed roof. Especially in areas that tend to be sparsely covered by vegetation, the peak substrate surface temperature recorded was up to 73.4°C during daytime. Ambient air temperature, correspondingly, at 300 mm above the substrate surface can reach 40°C. These high temperatures are likely to represent extreme cases, and will be well mitigated when the amount of greenery cover in the green roofs progressively increases. Our measurements also showed that there were significant differences in the various temperatures recorded between the various green roof systems.

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Figure 8: Comparison of surface temperatures measured on one of the four green roof system. The surface measured were the upper roof surface temperature before green roof installation (Pre-installation roof temp), the upper roof surface temperature after green roofs have been installed (Post-installation roof temp), and the surface temperature of the growing substrate used for the green roofs (Substrate surface temp). Based on the surface temperature measurements, a concrete roof slab thickness of 250 mm, and a R value of 0.17 m2K/W), it was also estimated that the green roofs installed reduced heat flux through the roof by a maximum of 60%. The visible light level measurements from sensors attached on building facades also showed that there were significantly lower light levels sensed at the building facades. An example of the

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measurement is showed in Figure 9. The reduction in light level ranged from 12% to 56%. This can lead to improved visual comfort for residents looking out of the windows. A likely explanation for the reduction in lux level is the reduction in radiation reflected off the roof surface, as the measured reflected global radiation after installation of the green roofs had declined by 32% to 50% (data not shown).

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Figure 8. Lux level at the façade a building facing the MSCP, with readings recorded by sensors placed at low (low), medium (middle) and high (high) heights on the buildings. The colored lines show the levels after installation of green roofs. (B) Air Quality Changes One of the often-cited benefits of green roofs is the use of the plants to absorb or trap gaseous and particulate pollutants in the atmosphere, thereby improving air quality in urbanized areas. To our knowledge, no direct data collection has been done to evaluate this benefit of green roof in the tropics. This component of the study thus focused on evaluating if there was improvement in air quality arising from the green roofs through the direct measurement of air quality on the MSCP. As most of the results will be presented in another manuscript for publication, only key components of the results are highlighted here. Data collected showed that the air quality above the roof was directly influenced by local traffic emissions, especially from the adjacent expressway, and including those from the carpark itself. In the absence of long distance movement of transboundary pollutants from the neighbouring countries during the data collection periods, the pre-installation and post-installation comparison of air quality parameters can provide an indication impact of green roofs in this aspect. For the acidic gaseous pollutants, the level of sulphur dioxide was reduced by 37% after installation of the green roof, whereas the indicators of nitrogen dioxide level showed variable results, with nitrous acid level declining by 21% and nitric acid level increasing by 48% (Table 4). It should be pointed out that the levels of sulphur dioxide recorded are well below the national pollution level limit of 80 µg/m3 (annual average concentration).

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Table 4. Levels of acidic gaseous pollutants measured on the roof of MSCP before and after installation of green roofs

Pollutant Pre-installation level (µg/m3)

Post-installation level (µg/m3)

SO2 2.204 1.394 HONO 1.722 1.359 HNO3 0.047 0.070

The data collected on airborne particulate matter showed that the mass concentration for PM2.5 had increased by 16%, whereas PM10 had increased by 42% after installation of the green roofs. However, the increased level of PM10 (36.7 µg/m3) is still well within the national pollution level limit of 50 µg/m3 (annual average concentration). Overall, the mass concentration of particles with sizes 0.56 µm and above had increased significantly, whereas particles with sizes smaller than 0.56 µm (ultra-fine particles) had decreased by 24%. A likely explanation for the increase in the particles coarser than 0.56 µm is the re-suspension of particles from the gravel chips used around the edges of several of the green roofs, as well as from substrate not fully covered by the greenery (Figure 9). Figure 9. Gravel chips and exposed substrate as possible sources of coarse p The concentration of soot or black carbon mass materials had also declined athe green roofs (data not shown). Since both ultra-fine particles and soot are mfrom motorized vehicular emissions, this could be an indication that green roreduce the concentrations of pollutants from traffic emissions within the immthe green roofs. It is also noteworthy that the particle number concentratimatter had decline marginally by 6% after installation of green roofs. Thiimportant as the number of particles inhaled by human is often a better ineffects resulting from exposure to air pollutant than the amount of particles. Conclusions This is a significant project for the green roof movement in Singapore. Prcollected for this project showed that even in the humid tropics, green roofs conditions, and water stress arising from non-uniform or insufficient rainfall ithe most limiting factor for plant establishment and growth. Plant selection thuon plants are drought tolerant, or those that regenerate rapidly from seedsstructures such as rhizomes, swollen roots, bulbs, etc. and other storage orgarainfall. It is paradoxical that suitable drought tolerant plants that are evalua

Exposed substrate

Gravel chips

articulate matter.

fter installation of ainly generated

ofs can perhaps ediate vicinity of on of particulate s could also be dicator of health

imarily, the data experience xeric s likely to the be s needs to focus or underground ns upon return of ted also need to

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survive constant moisture around the root zone arising from periods of continuous rainfall. This aspect was not fully investigated in this project and should be part of further studies on plant selection for green roofs in the tropics. Measurement of temperatures showed that green roofs can be effective in reducing surface temperatures of rooftops, but the effects on ambient air temperature appears to marginal. Temperature reductions are also strongly dependent on substrate moisture, and as green roofs are thin, thermal capacity can be lower than that of a concrete roof. This can cause the substrate to be heated up rapidly when dry, resulting in higher temperatures. Green roofs also help to significantly reduce the reflection of visible light from an otherwise bright concrete roof, to the facades of buildings facing the roofs, helping to improve visual comfort of the residents. The air quality studies provided an indication that green roofs can help to reduce the level of atmospheric pollutants arising from traffic emissions in the vicinity of the roof. However, the use of crushed stones, gravel and exposed substrates on the green roofs can lead to an increase in the concentration of coarse particulate matter. More studies need to be conducted to further verify the efficacy of green roofs in improving air quality. Acknowledgements The authors like to thank Dr Wong Nyuk Hien (National University of Singapore), Dr R. Balasubramanian (National University of Singapore), and the four companies which participated in this project for their contributions to this project. The role of Housing and Development Board as the collaborator for this project is also acknowledged. This project was also possible because of funding from MND Innovation Fund. References 1. Blight GE. (1997) The "active" zone in unsaturated soil mechanics. First Geotechnical

Research Centre Lecture, 22 July 1997, Singapore. 2. Koehler M. (2003). Plant survival research and biodiversity: lessons from Europe.

Greening Rooftops for Sustainable Communities, 29-30 May 2003, Chicago. 3. Tan PY, Wong NH, Chen Y, Ong CL, Sia A. (2003). Thermal benefits of rooftop gardens

in Singapore. Greening Rooftops for Sustainable Communities, 29-30 May 2003, Chicago.

4. Wong NH, Wong VL, Lee SE, Cheong D, Lim GT, Ong CL, Sia A. (2002). The thermal effects of plants on buildings. Architectural Science Review, 45:1-12.

5. Wong NH, Chen Y, Ong CL, Sia A. (2003). Investigation of the thermal benefits of rooftop garden in the tropical environment. Building and Environment, 38: 261-270.

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