A comprehensive study of green roof performance from environmental perspective

Abstract

Green roofs have been established for over 100 years and they have been become one of the key elements in urban area in the past few decades. Many scientific researches focus on its cooling performance, efficiency and survival rates of plants. This article provides an overview mainly from two aspects, the vegetation on the green roofs and its benefits towards the surrounding environments. Vegetation is the key elements in installing green roofs. Here also provides some factors in choosing suitable plants on rooftops, factors including species, drought tolerant, solar radiation tolerant, and cooling ability of plants. In addition, green roofs play a critical role in improving the urban environment by enriching the biodiversity, delaying the storm peak to the drainage system, diminishing the runoff quantity, purifying the air pollutants as well as the runoff quality.

Full text

Review Article
A comprehensive study of green roof performance
from environmental perspective
Li W.C.
⇑
, Yeung K.K.A.
Centre for Education in Environmental Sustainability and Department of Science and Environmental Studies, The Hong Kong Institute of Education,
Hong Kong Special Administrative Region
Received 27 February 2014; accepted 20 May 2014
Abstract
Green roofs have been established for over 100 years and they have been become one of the key elements in urban area in the past few
decades. Many scientific researches focus on its cooling performance, efficiency and survival rates of plants. This article provides an over-
view mainly from two aspects, the vegetation on the green roofs and its benefits toward the surrounding environments. Vegetation is the
key element in installing green roofs. It also provides some factors in choosing suitable plants on rooftops, factors including species that
are drought tolerant, solar radiation tolerant, and cooling ability of plants. In addition, green roofs play a critical role in improving the
urban environment by enriching the biodiversity, delaying the storm peak to the drainage system, diminishing the runoff quantity,
purifying the air pollutants as well as the runoff quality.
Ó 2014 The Gulf Organisation for Research and Development. Production and hosting by Elsevier B.V. All rights reserved.
Keywords: Sedum; CAM; Albedo effect; Biodiversity
Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00
2. Vegetation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00
2.1. Native, non-native and invasive plant . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00
2.2. Drought tolerant and solar radiation tolerant . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00
2.3. Albedo effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00
2.4. Growth substrate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00
2212-6090/$ - see front matter Ó 2014 The Gulf Organisation for Research and Development. Production and hosting by Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.ijsbe.2014.05.001
⇑
Corresponding author. Address: Centre for Education in Environmental Sustainability and Department of Science and Environmental Studies, The
Hong Kong Institute of Education, 10 Lo Ping Road, Tai Po, New Territories, Hong Kong Special Administrative Region. Tel.: +852 2948 8630; fax:
+852 2948 7676.
E-mail address: waichin@ied.edu.hk (W.C. Li).
Peer review under responsibility of The Gulf Organisation for Research and Development.
Production and hosting by Elsevier
International Journal of Sustainable Built Environment (2014) xxx, xxx–xxx
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Please cite this article in press as: Li, W.C., Yeung, K.K.A. A comprehensive study of green roof performance from environmental perspective. Inter-
national Journal of Sustainable Built Environment (2014), http://dx.doi.org/10.1016/j.ijsbe.2014.05.001

3. Environmental benefits. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00
3.1. Enriching biodiversity in urban area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00
3.2. Cooling performance on the building and surroundings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00
3.3. Managing runoff quantity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00
3.4. Prevent and reduce pollution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00
4. Cost and Barriers of installing green roofs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00
5. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00
1. Introduction
Green roofs can be traced back as far as the gardens of
Babylon and the Roman Empire, i.e. they grew trees on top
of buildings (Peck, 2002). During 19th and 20th century,
rooftops in major cities of the United States were greened
to replace the rising land costs of building parks in the
inner city (Herman, 2003). Nowadays, the world leader in
green roof technologies is Germany, where more than
10% of houses have installed green roofs (Ko
¨
hler, 2006).
Ko
¨
hler (2006) reported that the first wave of constructing
green roofs in Germany came at the end of 19th century.
It only covered less than 1% of roofs in Germany during
this boom. However, incentive programs launched from
1983 to 1996 which required the installation of extensive
green roofs for buildings in central part of the city and it
allowed reduction of the additional costs of inst allation
(Dunnett and Kingsbury, 2004). Nowadays, green roofs
are also widespread in other European countries, for
instance France and Switzerland. In addition, Portland
government organized a few incentive programs to encour-
age the installation of green roofs on buildings. In Canada,
Toronto also promoted wider construction of green roofs
with sustainable alternatives to meet the urban environ-
mental challenges (Banting et al., 2005). Green roofs are
usually built in the inner city. Green roofs in the United
Kingdom are also used in build-up areas, so that it can
replace the gardens or local parks at ground level
(Herman, 2003).
Generally, there are three types of green roofs: namel y
intensive green roo f, semi-intensive green roof and exten-
sive green roof. Different types of green roofs requir e differ-
ent vegetations, and thus require different depths for
growing medium (Banting et al., 2005). Researchers sug-
gested few characteristics of extensive green roof plants:
(1) they establish fast and reproduce efficiently; (2) they
are short in height and cushion-forming or mat- forming;
(3) their roots are shallow but spreading; and (4) their
leaves are succulent or able to store water (Snodgrass
and Snodgrass, 2006; Maclvor and Lundholm, 2011). Four
types of vegetation have these characteristics: namely
Moss-Sedum, Sedum-moss-herbaceous plants, Sedum-her-
baceous-grass plants and grass-herbaceous plants; these
types of vegetations require 2–20 cm depth of medium for
growing (Banting et al., 2005). Sedum species are the most
common choice of plant for extensive green roof because of
their unique characteristics: grow with relatively shallow
roots, able to store water, have crassulacean acid metabo-
lism (CAM) to reduce water loss (Van Woert et al., 2005;
Durhman et al., 2006; M aclvor and Lundholm, 2011).
Another four types of vegetations can be applied in semi-
intensive green roofs: grass-herbaceous plants, wild
shrubs-coppices, coppices and shrubs and coppices; these
types of vegetations require a deeper growing medium,
i.e. 12–100 cm ( Banting et al., 2005). Lastly, there are seven
types of vegetations which can be planted on intensive
green roofs: Lawn, low-lying shrubs and coppices, medium
height shrubs and coppices, tall shrubs and coppices, large
bushes and small trees, medium-size trees and large trees.
They require even deeper growing medium, i.e. 15–
200 cm (Banting et al., 2005). Extensive green roof is the
least expensive among the three types of green roofs in
terms of installation as well as maintenance, as it can be
self-retained. Since the installation of extensive green roofs
is easier and more flexible, most of the researches focused
on the harsh environment on extensive green roofs pro-
vided. This article aims at summarizing the existing litera-
ture on the performance of intensive and extensive green
roofs in subtropical maritime monsoon climate zone.
Selection of plants is one of the essential components in
resulting thermal benefits and storm water runoff, hence
the energy savings follow.
2. Vegetation
2.1. Native, non-native and invasive plant
There are debates about using native plants on green
roofs aro und the world (Currie and Bass, 2010). In Peck
(2008)’s the book of award winning green roof designs,
45% of the award winning green roofs used native plants.
Another book written by Cantor (2008) also reco rded
59% of the green roofs used native plants. It shows the sig-
nificance of using native plants on green roofs. Moreover,
non-profit organizations, including the Ladybird Johnson
Wildflower Center and the Peggy Notebaert Nature
Museum in the United States, governmental organizations,
namely the city of Toronto’s Green Roof Pilot Program,
and even commercial organizations, for example Rana
Creek in the United States, also promoted the use of native
plants on green roofs (Butler et al., 2012). Butler et al.
(2012) also summarized the common reasons for choosing
2 W.C. Li, K.K.A. Yeung / International Journal of Sustainable Built Environment xxx (2014) xxx–xxx
Please cite this article in press as: Li, W.C., Yeung, K.K.A. A comprehensive study of green roof performance from environmental perspective. Inter-
national Journal of Sustainable Built Environment (2014), http://dx.doi.org/10.1016/j.ijsbe.2014.05.001

native plants in ground-level. First, Environmental
Protection Agency (EPA) in the United States (2012)
claimed that native plants were already adapted to the local
conditions; once they are established, they do not need
watering, fertilizers or pesticides. Native plants can help
restore the healthy ecosystem by attracting various ani-
mals, birds and butterflies (EPA, 2012). Currie and Bass
(2010) also wrote that native plants have the potential to
replicate local native species communities as well as benefit
the ecology. In Alberta, Clark and MacArthur (2007) held
a research of a semi-intensive green roof, which had a
native mixed prairie community. They found that there
was more biomass, in particular spiders and various spe-
cies; they also found that the biodiversity in the semi-inten-
sive roof was greater than an non-native extensive green
roof (Clark and MacArthur, 2007).
Yet there are concerns about promoting native plants
planting on green roofs. First, Sam Benvie (mentioned in
Clark and MacArthur, 2007) suggested that native plant
community can be threatened by other rare species and
invasive species, so the cost of maintaining a native plant
community on a green roof can be increased and challeng-
ing. Moreover, Dunnett (2006) stated the concern of
whether the seeds of native plants from non-local source,
i.e. local nursery, can survive dur ing the establishment.
Dunnett (2006) suggested rather than using seeds from
other areas, using local plants as source of seeds.
Dunevitz Texler and Lane (2007) cited reasons not to plant
rare species or native plants because rare species or native
plants, which are already in fragile populations, would be
impacted by altering genes from similar plants. In addition
to rare species, they often had various habitat require-
ments, so the process of planting and establishing could
be unsuccessful in long term (Dunevitz Texler and Lane,
2007).
Opinions toward the use of native species on green roofs
remain mixed. Butler et al. (2012) had also summarized
different opinions toward their use. Quantitative data from
14 papers had been published regarding the rates of growth
and survival of native plants on green roofs under different
conditions. The data are summarized and shown in Table 1
(Butler et al., 2012). Nonetheless, unsuccessful establish-
ment of native plants will influence the green roof
performance, i.e. esthetics. Green roof performance wi ll
then influence the long-term acceptance by the public
(Maclvor and Lundholm, 2011).
2.2. Drought tolerant and solar radiation tolerant
As mentioned above, green roof performance will influ-
ence the long-term acceptance by the public (Maclvor and
Lundholm, 2011). Thus, choosing appropriate plants is
important. This section summarized different researches
about vegetation’s performance as drought tolerant and
solar radiation tolerant.
Sedum is often regarded as an ideal choice for planting
on green roofs because of its properties (Van Woert
et al., 2005). Sedum are succulents and regarded as crassu-
lacean acid metabolism (CAM) plants in which the stomata
opens in nighttime to allow carbon dioxide to enter and
closes in daytime to avoid water loss from transpiration
(Ting, 1985). Not only Sedum have such ability, but fami-
lies of Portulacaceae, Crassulaceae and Euphorbiaceae are
also CAM plants which can survive for a long period of
time without watering (Liu et al., 2012). The research con-
ducted by Farrell et al. (2012) showed that CAM plants,
Sedum pachyphyllum, Sedum spurium and Sedum clavatum
survived for about 113 days without watering, depending
on the soil types. S. spurium was recorded to have a lower
drought tolerance with only 19% of survival rate, under a
low water regime of watering every 3 weeks (Nagase and
Dunnett, 2010). Additionally, the report conducted by
Liu et al. (2012) indicated that the temperature reduction
effect increases with plant height: 10 cm < 15 cm < 35 cm.
Table 1
The survival rates of native and non-native plant species under different treatments in the United States and Canada (Butler et al., 2012).
Location Full Sun Shaded References
No Irrigation Irrigation No irrigation
Native Non-native Native Non-native Native Non-native
U.S. 100% 100% Bousselot et al. (2009)
U.S. 100% 100% Butler and Orians (2011)
U.S. 0% 31% Carter and Butler (2008)
U.S. 0% 100% Durhman et al. (2006)
U.S. 33% 100% 33% 67% Getter et al. (2009)
U.S. 13% 100% 78% 100% Licht and Lundholm (2006)
U.S. 14% n/a Martin and Hinckley (2007)
U.S. 22% 100% Monterusso et al. (2005)
U.S. 33% 100% Rowe et al. (2006)
U.S. 20% 100% 100% 100% Schroll et al. (2009)
Canada 100% n/a Lundholm et al. (2009)
Canada 93% n/a Maclvor and Lundholm (2011)
Canada 67% 79% Ngan (2010)
Canada 10% 75% 100% 100% Wolf and Lundholm (2008)
W.C. Li, K.K.A. Yeung / International Journal of Sustainable Built Environment xxx (2014) xxx–xxx 3
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It was proved that even plants with high drought tolerance
can help in regulating the temperature.
Besides drought tolerance, solar radiation tolerance is
also considered because most of the roofs are exposed to
high solar radiation. Some plants are not favorable for
strong sunlight while some are capable of withstanding it,
e.g. Parthenocissus quinquefolia requires a site receiving less
than three hours of direct sunlight (Fairfax County, 2007).
On the contrary, some roofs might be shaded by nearby
objects, for example buildings. According to the experi-
ment conducted by Getter and Rowe (2006), Sedum
kamtschaticum, S. spurium and Allium cernuum are good
candidates for shaded locations; while Sedum acre , Sedum
album and Talinum calycinum are suitable for both shaded
and sunny locations on green roof.
2.3. Albedo effect
It is well known that there is a negative correlation
between albedo effect and surface temperature: the greater
the albedo, the lower the surface temperature. Gaffin et al.
(2006) conducted a research comparing surface radiation
reflectivity (albedo) of white roofs and green roofs. White
paint recorded an albedo of 0.8 on average, but it is diffi-
cult to maintain high albedos on white surfaces without
regular washing. It recorded an albedo decrease of 0.15
in a year (Gaffin et al., 2006). On the contrary, green roofs
recorded an equivalent albedo of 0.7–0.85 (Gaffin et al.,
2006). Gaffin et al. (2006) also made a comparison of tem-
peratures of the subsurface (the conventional rooftop level)
and the green roof surface. It is indicated that the subsur-
face temperature was significantly low er than the green
roof surface temperature; it is because the green layer insu-
lated heat (Gaffin et al., 2006). This proves that green roofs
can reduce the thermal loading.
Moreover, albedo increases with higher peak cover and
biomass on the green roof. A planted module of Maclvor
and Lundholm (2011) reflected on average 0.22 of incom-
ing solar radiation whereas growing medium only reflected
0.17 as control in the entire study period, from May to
October. Lundholm et al. (2010) also reported that the
average albedo of a conventional rooftop over the same
period (from May to October) is only 0.066. By comparing
the tw o sets of data, it was found that the best performing
species from Maclvor and Lundholm (2011) increased the
albedo effect by 22.2% over the growing medium alone
and more than 200% of albedo effect over the conventional
rooftop.
Blanusa et al. (2013) demonstrated that plants provide a
cooling effect by transpiration of water through stomata
and direct shading, as mentioned above. Stachys had a
higher ability in regulating its own temperature and leaving
its leaves cool (Bl anusa et al., 2013). It had the lowest sur-
face temperature even with limited soil moisture and clos-
ing stomata. One of the experiments compared the leaves
with hairs trimmed indicating that hairs on the leaves of
Stachys reduced the amount of infra-red radiation from
leaf, thus making the leaves cooler. Such co oling mecha-
nism may be due to the light hair color or its reflectivity
of incoming irradiance, thus it provided higher albedo
and avoided direct heat stress. Nevertheless, available
moisture and wat er transpiring through Stachys’ leaves
strongly altered its air cooling ability (Blanusa et al., 2013).
Surface temperature was mainly related to solar radia-
tion reflectivity (albedo). Solar radiation reflectivity is
influenced by species richness and biomass variability,
where greater biomass led to greater solar radiation reflec-
tivity (Lundholm et al., 2010). Thus the thermal loadings in
the daytime are decreased; the discomfort underneath the
roof will be alleviated (Maclvor and Lundholm, 2011;
Blanusa et al., 2013).
2.4. Growth substrate
According to Schrader and Boning (2006), soil forma-
tion takes place throughout the establishment of green
roofs. They made a comparison of selected abiotic proper-
ties and collembolan densities between five old extensive
green roofs and five young extensive green roofs
(Schrader and Boning, 2006 ). Collembolans are typical pio-
neer microarthropods and transported by air during the
primary succession, the pioneer period (Dunger, 1989).
Schrader and Boning (2006) found that acidification and
increasing contents of organic carbon took place in old
green roofs. They concluded that the soil formation can
improve the existing collembolans and promote urban
biodiversity (Schrader and Boning, 2006).
Apart from improving urban biodiversity, soil is another
important factor of cooling down the roof temperature as it
holds water and heat (Getter et al., 2009; Maclvor and
Lundholm, 2011). Growing medium which is more reflec-
tive can increase the overall albedo of the green roof, in
turn raising the overall cooling ability of the roof
(Maclvor and Lundholm, 2011). A shallower substrate held
less moisture content (Getter et al., 2009). Getter et al.
(2009) found that a 4 cm substrate de pth held less moisture
content than 7 or 10 cm depths, but the depths of 7 and
10 cm substrate are statistically the same. The depth of
growth substrate controls the water retent ion, hence the
runoff quantity and the runoff peaks.
3. Environmental benefits
3.1. Enriching biodiversity in urban area
Green roofs in urban and suburban areas act as green
corridor, which are the stepping stones for wildlife to enter
the nearby habitats (Kim, 2004). They can connect the
fragmented habitats with each other so as to promote the
urban biodiversity (Kim, 2004). A total number of 30 spe-
cies or even more of organisms were observed in the green
roof (Fountain and Hopkin, 2004; Schrader and Boning,
2006). Species like Isotoma viridis and Parisotoma notabilis
were observed and they are classified as cosmopolitan
4 W.C. Li, K.K.A. Yeung / International Journal of Sustainable Built Environment xxx (2014) xxx–xxx
Please cite this article in press as: Li, W.C., Yeung, K.K.A. A comprehensive study of green roof performance from environmental perspective. Inter-
national Journal of Sustainable Built Environment (2014), http://dx.doi.org/10.1016/j.ijsbe.2014.05.001

pioneer in urban soils (Dunger et al., 2004; Fountain and
Hopkin, 2004). The distributions of organisms in soil were
diverged from young and old roofs (Schrader and Boning,
2006). Schrader and Boning (2006) revealed that there are
three factors contributing to the biodiversity in the green
roof. First is the type of growing substrate; second is the
process of soil formation during the maturation of sub-
strate; and the last is the increasing biological activity as
well as increasing organic matter from dead leaves or
organisms. Nonetheless, it is suggested that green roof
could not be a justification for destroying the natural nor
replace the nature.
3.2. Cooling performance on the building and surroundings
The cooling performance of the green roof depends on
the plant specie s chosen (Maclvor and Lundholm, 2 011;
Blanusa et al., 2013). Green roofs cool down the tempera-
ture because of the direct coverage of plants and the open-
ing of stomata that allows transpiration during daytime
(Santamouris, 2012 ). The textures of leaf surface and
albedo effect also take place. The vegetation stores the heat
and cools down the air (Santamouris, 2012). The daily
maximum temperature on the vegetated rooftops was
reduced and dampens diurnal temperature fluctuations.
Researches in US indicated that vegetated rooftops
decreased the peak temperature from 0.5 K to 3.5 K; along
with dropping of temperature, the albedo increased from
0.05 up to 0.61 (Santamouris, 2012). Susca et al. (2011)
compared the albedos of the white-painted roof and green
roof, its influ ence toward the surface temperatures, and the
energy consumption for controlling the indoor temperature
below the green roofs. The white and green roofs substi -
tuted the black-painted roof and reduced the energy con-
sumption. Moreover, a green roof rather than a white
roof can further reduce the energy saving from 40% to
110%.
3.3. Managing runoff quantity
First of all, the definition of water retention means the
water storage capacity of a green roof. Green roof charac-
teristics including the growing medium and the drainage
layer influence the water retention capacity as well as the
runoff dy namics (Banting et al., 2005). In between different
types of vegetation in extensive green roofs, their water
retention ability varied from 40% to 60% of total rainfall.
Water retention for semi-intensive and intensive green
roofs depends on area coverage (Banting et al., 2005).
The size of rain event as well as the rain intensity affects
the water retention. Green roofs retained all small rain
events that were less than 10 mm. The retention of green
roofs differed from 88% to 26% when the rain events were
12 mm. Such retention was depended on the substrate and
the type of drainage (Simmons et al., 2008). The peak dis-
charge of small storms from vegetated roof was lower than
that from conventional roof; however, such effect was
reduced for larger storms. On a vegetated roof, 57% of
peaks were delayed up to 10 min comparing with a conven-
tional roof (Carter and Rasmussen, 2006). According to
DeNardo et al. (2005), the rainfall intensity reduced from
4.3 mm/h to average green roof runoff rate of 2.4 mm/h.
Therefore, green roofs reduced the peak intensities.
Age of green roof also affects the storm water dischar ge
(Getter et al., 2007). By comparing the organic matter con-
tent and physical properties of soil after five years of time,
the organic matter content was increased from 2% to 4%
and the pore space was also increased from 41% to 82%.
Along with these two factors, the water holding capacity
also increased from 17% to 67% (Getter et al., 2007). How-
ever, the vegetation plays a minor role in water retention
when comparing with the substrate. Van Woert et al.
(2005) proved that roofs with media alone retained 50.4%
of rainfall while vegetated roofs retained 60.6%. On the
contrary, Maclvor and Lundholm (2011) showed that some
plant species can evapo-transpire more water, so they cre-
ate more space for water storage capacity of media. In
addition, warmer seasons lead to higher evapo-transpira-
tion, thus the water storage regenerates faster. There were
seasonal variations toward the runoff reduction
(Bengtsson et al., 2005 ). During September to February,
the runoff reduction was 34% while during the period
between March and August, the runoff reduction was
67%. The slope has impacts on water retention too. The
lower the slope, the higher the water retention on the green
roofs (Villarreal and Bengtsson, 2005).
3.4. Prevent and reduce pollution
Green roofs act as a sink for nitrogen, lead and zinc
(Gregoire and Clausen, 2011); it is also the source of phos-
phorus (Ko
¨
hler et al., 2002; Berndtsson et al., 2009;
Gregoire and Clausen, 2011). On the thin soil of extensive
green roofs which does not affect the concentrations of
heavy metals in runoff water, i.e. the concentrations of
heavy metals in runoff wat er were the same as that in pre-
cipitation (Ko
¨
hler et al., 2002). Nonetheless, the green roof
retained over 65% of the zinc from precipitation (Gregoire
and Clausen, 2011). In addition, Gregoire and Clausen
(2011) found that more than 90% of the copper and zinc
concentrations from green roof runoff were in the dissolved
form. Moreover, taking account into the reduced runoff
volume, the amount of water reduction affected the reduc-
tion of nitrogen in runoff (Ko
¨
hler et al., 2002). Similarly,
the green roofs reduced the loads of pollutants due to run-
off reduction (Ko
¨
hler et al., 2002; Gregoire and Clausen,
2011).
As mentioned above, green roofs were the source of
phosphorus (Ko
¨
hler et al., 2002; Gregoire and Clausen,
2011) and copper (Gregoire and Clausen, 2011). Green
roof fertilization contributes to the increase in phosphorus
(Ko
¨
hler et al., 2002; Bern dtsson et al., 2009; Gregoire and
Clausen, 2011). Besides the concentration of phosphorus,
Gregoire and Clausen (2011) also found that the
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concentration of copper was from Harrell’s fertilizer, which
contained water soluble copper, used on the green roof.
Green roofs can also reduce the effects of acid rain by
raising the pH value from 5 to 6 in rain water to over 7
to 8 in runoff water. Plants can also absorb air pollutants
such as carbon dioxide and generate oxygen. In addition,
Yang et al. (2008) demonstrated that green roofs in
Chicago reduced the air pollution through the uptake of
ozone by plants. In addition, the uptake of NO
2
,PM
10
and SO
2
by plant was 27%, 14% and 7% respectively.
The maximum average uptake is in May while the
minimum average uptake is in February.
4. Cost and Barriers of installing green roofs
Environmental Protection Agency (2009) compared the
costs of extensive green roofs and intensive green roofs
installation. The costs of constructing green roofs
depended on the components, including the growing med-
ium, type of roofing membrane, quantity of plants and
drainage system. As indicated in the report conducted by
Environmental Protection Agency (2009), it was estimated
that the initial costs were varied from US$ 10 per square
foot for a simple extensive roof to US$ 270 per square
meter for an intensive roof. Maintenance costs for either
extensive or intensive green roofs range from US$ 8 to
US$ 11 per square meter. The maintenance costs of exten-
sive green roofs drop when plants cover the roof entirely,
whereas such costs for intensive green roofs remain con-
stant. Architectural Services Department (2006) in Hong
Kong also conducted a study on green roof application
in Hong Kong. It is indicated that the costs vary depending
on the sources of mate rials, either international or local.
Local supplier’s claimed using reputable imported product
cost from US$ 90 to US$ 130 per square meter; while they
claimed using local materials would cost from US$ 50 to
US$ 80 per square meter. Maintenance costs of local inten-
sive green roof range from US$ 1 to US$ 6 per square
meter each year; while that of extensive green roof ranges
from US$ 0.1 to US$ 0.3 per square meter each year.
Table 2 shows a list of barriers of installing extensive
green roofs on existing buildings. According to the survey
done by Zhang et al. (2012), “Lack of promotion from
the government and social communities among the public
and private sectors”, “Lack of incentive from the govern-
ment toward the owners of the existing buildi ngs”,
“Increase of maintenance cost” and “Technical difficulty
during the design and construction process” were the major
barriers during the stage of planning and designing. It is
suggested that the government should play the leading role
in the stage of planning and designing for implementation
of extensive green roof syst ems. During the stage of
construction and operation as well as management stages,
barriers including “Increase of maintenance cost” and
“Technical difficulty during the design and construction
process” were more essential.
5. Conclusion
The installations of green roofs have been promoted
worldwide, especially in European countries and Unit ed
States. Extensive green roofs are often the target of scien-
tific research since it costs less than intensive green roof.
In addition, its weight adding to the building is less than
intensive green roof; hence extensive green roofs are more
common. Nevertheless, extensive green roofs face harsh
climate, for instance high solar radiation, limited precipita-
tion and shallow growing substrate; therefore it limits the
choices of plants . These factors become obstacles in
constructing extensive green roof; whereas comprehensive
watering system can be installed on intensive green roofs.
Therefore, water efficiency is not the major problem for
intensive green roofs. Different types of green roof require
different nature of plants; nonetheless, three common crite-
ria of selecting plants using on extensive green roofs are
their drought tolerance, albedo ability and native or non-
native. Typical plant species used on it is Sedum because
of its feature of CAM which helps it to survive during
harsh climate. Due to the harsh climate faced by the exten-
sive green roofs, research usually focuses on the survival
Table 2
Summary of optional barriers for applying extensive green roof systems for existing buildings.
Barriers References
 Increase of maintenance cost Peck and Callaghan (1999) and Ngan (2004)
 Increase of design and construction cost Ngan (2004)
 Lack of incentive from the government toward developers Getter and Rowe (2006)
 Lack of incentive from the government toward owners of the existing buildings Peck and Callaghan (1999) and Getter and Rowe
(2006)
 Technical difficulty during the design and construction process Peck and Callaghan (1999) and Getter and Rowe
(2006)
 The old age of existing building Townshend (2007)
 The weak affordability of extensive roof to withstand wind load Peck and Callaghan (1999) Townshend (2007)
 Weak structural loading for applying extensive green roof system Townshend (2007)
 Poor utilities arrangement Townshend (2007)
 Lack of awareness on extensive green roof system in public and private sectors Hui (2006)
 Lack of promotion from the government and social communities among the public and
private sectors
Townshend (2007)
6 W.C. Li, K.K.A. Yeung / International Journal of Sustainable Built Environment xxx (2014) xxx–xxx
Please cite this article in press as: Li, W.C., Yeung, K.K.A. A comprehensive study of green roof performance from environmental perspective. Inter-
national Journal of Sustainable Built Environment (2014), http://dx.doi.org/10.1016/j.ijsbe.2014.05.001

rates of plants. The survival rates of plants directly influ-
ence the esthetic of the g reen roofs; hence influence the
acceptance of the general public. Other scopes of green
roof studies are including the temperature reduction caused
by green roofs, runoff quantity control as well as reduction
of pollution. A resear ch recent ly showed that broad leaves
performed better than Sedum on rooftop, i.e., cooling
efficiency. There are also arguments about whether native
species or non-native species should be introduced for
green roofs. Native plants can provide homes and food
for the native anima ls; however, one research claimed that
non-native plants also provide same function for the native
animals. Nevertheless, it is uncertain whether non-native
species become invasive or not; thus using native plants is
still the first priority.
From soil formation on green roofs, it improves the
urban biodiversity, for underground animals in the growth
substrate, for the inserts in the canopy. Green roofs not
only clean the air, but also the runoff. Plants on the roof-
tops can purify the air; plants and soil can purify the runoff
as well as delay the storm peak. Green roofs act as a sink
for nitrogen, lead and zinc from precipitation, but it also
increases the concentration of phosphorus, which came
from fertilizer used on green roofs. After reviewing
research from mainly environmental perspectives, from
the installation of green roofs, to its benefits to env iron-
ment, with respect to urban area, it has been concluded
that green roofs are good for rebuilding green areas in
urban area; however, it should not be an excuse to destroy
the outskirt green belts as green roofs cannot replace the
role of natural habitat.
Acknowledgments
The author would like to thank Mr. Chan Yung Hau for
the manuscript preparation and improvement. Financial
support from the Depart ment of Science and Environ men-
tal Studies of The Hong Kong Institute of Education is
gratefully acknowledged.
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