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”.
1
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.
2
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
Brand
Substrate
Substrate Depth
Filter membrane
Drainage-Water
Reservoir Cells
Root Barrier
Installer
SYSTEM 1
ZinCo Extensive
Green Roof
System
Zincolit Plus
SYSTEM 2
Daku Extensive
Green Roof
System
Daku Roof Garden
Substrate
SYSTEM 3
Non-proprietary
SYSTEM 4
gardenroof
Gardenlite pumice,
coco peat, fine
sand and hydrogel
50 – 80 mm
DAKU Stabilfilter
Mixture of
Seramis, Leca
Chips and
Compost
65 mm
Generic geotextile
80 – 100 mm
ZinCo Filter Sheet
SF
ZinCo Floradrain
FD 25 &
ZinCo Floratec FS
50
ZinCo Root Barrier
WSF 40 laid over
ZinCo Protection
and Separation
Mat TSM 32
ZinCo Singapore
(Pte) Ltd
Daku FSD 20
FlorDepot L35
Nuraplan GR 15
(Root-cum-rot
resistance
waterproofing
membrane)
Hitchins (FE)
Marketing Pte Ltd
Generic UVstabilized
polyethylene sheet
Geomembrane
HDPE 100
Ecoflora (S) Pte
Ltd
Garden and
Landscape Centre
(Pte) Ltd
75 mm
Polypropylene
woven filter
Garden Hydrocell
(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
3
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
Furcraea foetida ‘Mediopicta’
Convolvulaceae
Ipomoea pes-caprae ssp. Brasiliensis
Aloaceae
Aloe vera
Crassulaceae
Kelanchoe tomentosa
Sedum acre
Sedum aizoon
Sedum kamtschatikum ‘Weihenstephaner Gold’
Sedum mexicanum
Sedum nussbaumerianum
Sedum xrubrotinctum
Sedum rupestre
Sedum sarmentosum
Sedum sexangulare
Sedum sieboldii
Sedum spectabile
Sedum spurium ‘Purpurteppich’
Sempervivum tectorum
Aizoceae
Aptenia cordifolia
Carpobrutus edulis
Delosperma cooperi
Delosperma lineare
Aspleniaceae
Asplenium nidus
Araceae
Acorus gramineus
Amaryllidaceae
Zephyranthes candida
Zephyranthes rosea
Caprifoliaceae
Lonicera japonica
Commelinaceae
Callisia repens
Tradescantia pallida ‘Purpurea’
Compositae
Wedelia biflora
Convallariaceae
Liriope muscari
Palmae
Chamaedorea seifrizii
Rhapis humilis
Pandanaceae
Pandanus amaryllifolius
Davalliaceae
Nephrolepis exaltata
Dracaenacea
Sanseveria trifasciata ‘Hahnii’
Sanseveria trifasciata ‘Golden Hahnii’
Sanseveria trifasciata ‘Laurentii’
Liliaceae
Ophiopogon intermedius
Meliaceae
Aglaia odorata
Moraceae
Ficus pumila
Portulaceae
Portulaca grandiflora cultivars
Rubiaceae
Ixora coccinea
Murraya paniculata
4
(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).
S u r fa c e te m p e r a tu r e
o f th e e x p o s e d r o o f
S u r fa c e te m p e r a tu r e
o f th e s o il
S u r fa c e te m p e r a tu r e
o f th e r o o f
B e fo r e
A fte r
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.
5
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.
Aethalometer
MOULDI
Figure 5. Monitoring and sampling equipment to measure air quality changes.
6
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
Furcraea foetida ‘Mediopicta’
Convallariaceae
Liriope muscari
Aloaceae
Aloe vera
Crassulaceae
Kelanchoe tomentosa
Sedum acre
Sedum mexicanum
Sedum nussbaumerianum
Sedum sarmentosum
Sedum sexangulare
Aizoceae
Aptenia cordifolia
Carpobrutus edulis
Delosperma lineare
Amaryllidaceae
Zephyranthes candida
Zephyranthes rosea
Caprifoliaceae
Lonicera japonica
Commelinaceae
Callisia repens
Tradescantia pallida ‘Purpurea’
Dracaenacea
Sanseveria trifasciata ‘Hahnii’
Sanseveria trifasciata ‘Golden Hahnii’
Sanseveria trifasciata ‘Laurentii’
Liliaceae
Ophiopogon intermedius
Meliaceae
Aglaia odorata
Pandanaceae
Pandanus amaryllifolius
Portulaceae
Portulaca grandiflora cultivars
Rubiaceae
Ixora coccinea
Murraya paniculata
7
DAILY ATMOSPHERIC WATER DEFICIT (MM)
Jul 03
Aug 03
70.0
60.0
40.0
50.0
50.0
30.0
40.0
20.0
30.0
30.0
20.0
30.0
20.0
10.0
10.0
10.0
80.0
60.0
80.0
50.0
60.0
- 10.0
-10.0
Mar 04
70
35
60
120.0
30
100.0
25
Jun 04
50
40
29
50
40
20
15
30
10
20
5
30
20
10
-10
29
27
25
23
21
19
17
15
13
0
9
0
11
29
27
25
23
21
19
17
15
13
11
9
7
5
3
10
1
-10
27
May 04
40
-5
25
-8
140.0
0.0
23
21
19
17
15
13
-6
-20.0
160.0
0
-4
0.0
Apr 04
20.0
-2
20.0
0.0
- 20.0
2
0
20.0
10.0
40.0
4
40.0
30.0
0.0
60.0
9
6
40.0
10.0
80.0
11
8
100.0
7
20.0
Feb 04
120.0
70.0
30.0
0.0
-10.0
Jan 04
Dec 03
90.0
40.0
7
1
Nov 03
50.0
31
29
27
25
23
21
19
17
15
13
9
11
7
5
3
1
-10.0
-10.0
-10.0
5
0.0
0.0
3
0.0
5
10.0
40.0
20.0
3
40.0
70.0
50.0
60.0
1
50.0
Oct 03
Sep 03
-10
Indicates periods when plants likely experience depleted moisture in the growing medium
8
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.
9
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
o
C 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.
Surface temperatures (Degree C)
48.0
44.0
40.0
36.0
32.0
28.0
23:40
21:30
19:20
17:10
15:00
12:50
8:30
10:40
6:20
4:10
2:00
23:50
21:40
19:30
17:20
15:10
13:00
10:50
8:40
6:30
4:20
2:10
0:00
24.0
Time
Pre-installation roof temp
Post-installation roof temp
Substrate surface temp
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
10
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).
1000
Lux
800
600
400
200
22:40
21:00
19:20
17:40
16:00
14:20
12:40
9:20
11:00
7:40
6:00
4:20
2:40
1:00
23:20
21:40
20:00
18:20
16:40
15:00
13:20
11:40
8:20
10:00
6:40
5:00
3:20
1:40
0:00
0
Time
Lux (low) B
Lux (middle) B
Lux (high) B
Lux (low) A
Lux (middle) A
Lux (high) A
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 postinstallation 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).
11
Table 4. Levels of acidic gaseous pollutants measured on the roof of MSCP before and after
installation of green roofs
Pollutant
Pre-installation level
Post-installation level
(µg/m3)
(µg/m3)
2.204
1.394
SO2
1.722
1.359
HONO
0.047
0.070
HNO3
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).
Exposed
substrate
Gravel chips
Figure 9. Gravel chips and exposed substrate as possible sources of coarse particulate matter.
The concentration of soot or black carbon mass materials had also declined after installation of
the green roofs (data not shown). Since both ultra-fine particles and soot are mainly generated
from motorized vehicular emissions, this could be an indication that green roofs can perhaps
reduce the concentrations of pollutants from traffic emissions within the immediate vicinity of
the green roofs. It is also noteworthy that the particle number concentration of particulate
matter had decline marginally by 6% after installation of green roofs. This could also be
important as the number of particles inhaled by human is often a better indicator of health
effects resulting from exposure to air pollutant than the amount of particles.
Conclusions
This is a significant project for the green roof movement in Singapore. Primarily, the data
collected for this project showed that even in the humid tropics, green roofs experience xeric
conditions, and water stress arising from non-uniform or insufficient rainfall is likely to the be
the most limiting factor for plant establishment and growth. Plant selection thus needs to focus
on plants are drought tolerant, or those that regenerate rapidly from seeds or underground
structures such as rhizomes, swollen roots, bulbs, etc. and other storage organs upon return of
rainfall. It is paradoxical that suitable drought tolerant plants that are evaluated also need to
12
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.
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