and Costs of Green Roofs
Haibo Feng and Kasun N. Hewage
Individual Benefits of Green Roofs 308
Energy Reduction in Heating and
Membrane Longevity 308
Acoustic Insulation 308
Aesthetic Benefits 309
LEED Certification Bonus 309
Public Benefits of Green Roofs 310
Reduction of Stormwater Runoff 310
Improvement of Air Quality 310
Mitigation of Urban Heat Island
Increment of Urban Biodiversity 311
Life Cycle Cost of Green Roofs 311
Initial Cost 311
Operation and Maintenance (O&M)
Disposal Cost 312
Green Roof Cost Benefit Assessment 313
Net Present Value and Payback
Scale of Implementation 314
Green Roof Policy Initiatives 315
Green roofs are well-suited for urban areas, as they provide excellent value
for money at both individual and public levels in comparison with other cur-
rently available green or gray infrastructure. However, the high initial invest-
ment required for green roofs acts as a barrier to their market penetration. In
general, individual benefits of green roof include reduction in energy use for
heating and cooling, membrane longevity, acoustic insulation, aesthetic bene-
fits, and LEED certification bonus (Berardi, 2016; Clark et al., 2008; Nurmi
et al., 2013; USGBC, 2015; Bianchini and Hewage, 2012). Public benefits
include reduction of stormwater runoff, improvement of air quality, mitiga-
tion of urban heat island effect, and increment of urban biodiversity, etc.
(Driscoll et al., 2015; Connelly and Hodgson, 2008; Rosenzweig et al., 2006;
Brenneisen, 2006). The costs in green roofs involve their initial construction,
operations, maintenance, demolition, and disposal (Bianchini and Hewage,
Nature Based Strategies for Urban and Building Sustainability.
©2018 Elsevier Inc. All rights reserved.
2012). The objective of this chapter is to present the economic benefits of a
green roof in terms of its public and individual benefits, summarize the total
costs of a green roof throughout its lifecycle, and estimate the payback
period based on the benefits and costs.
INDIVIDUAL BENEFITS OF GREEN ROOFS
Energy Reduction in Heating and Cooling
Green roofs reduce energy consumption in space heating through shading,
evapotranspiration, insulation, increase in thermal mass, and reduction of
heat loss through radiation. Green roofs can also be more efficient in pre-
venting heat loss in the winter compared with conventional roofs (Liu and
Baskaran, 2003; Berardi, 2016). The reduction in energy bills is usually the
most convincing factor for building owners to install green roofs. For exam-
ple, an experiment conducted in Ottawa found that a 6-inch extensive green
roof reduced heat gains by 95%, and heat losses by 26% compared to a con-
ventional roof (Liu, 2002). Another study on a two-story building was con-
ducted by Florida Solar Energy Center. Its findings revealed that 18% of
energy used for space cooling was saved by a green roof compared with the
conventional roof, and 44% was saved when the plants were more estab-
lished (Sonne and Parker, 2006). The economic benefit of reduction in space
conditioning demand has been quantified by a previous study, which demon-
strated that a green roof can save $0.180.68 m
in cooling, and 0.22 m
in heating annually (Bianchini and Hewage, 2012).
Green roof technology increases the lifespan of a building’s roof by protect-
ing against diurnal fluctuations, UV radiation, and thermal stress. Studies
have revealed that the lifetime of roofing membrane can be easily lengthened
up to 4050 years by green roofs (Clark et al., 2008), while a conventional
roof’s lifespan ranges from 10 to 30 years (Oberndorfer et al., 2007). The
cost of replacing a conventional roof at the end of its lifespan is estimated at
around $160 m
(Bianchini and Hewage, 2012). The benefit of installing a
green roof is the cost of installing a conventional roof 20 years in the future,
which is at $160 m
Green roofs improve the soundproofing of a building, and reduce the sound
reflection by increasing absorption (Azkorra et al., 2015). For buildings
located near very strong sources of noise such as night clubs, highways, or
flight paths, the sound insulation created by green roofs can be especially
308 SECTION | IV Nature Based Strategies: Social, Economic and Environmental
useful. There are no reliable estimates in the literature about the economic
value of the sound insulation benefit of green roofs. A commonly used tech-
nique to improve noise insulation is to apply an extra layer of plasterboard
into the ceiling. The noise insulation benefits acquired due to green roofs are
similar or higher than that gained by such an additional ceiling element,
since green roofs have more than one layer (Connelly and Hodgson, 2008).
Material and installation costs are approximately $29 m
plasterboard. Therefore, the noise insulation benefit of green roofs is also
estimated to be around $29 m
in air noise zones (Nurmi et al., 2013).
Aesthetics are the most intangible benefit, generally left out in cost-benefit
analyses due to the difficulty in valuing aesthetics in monetary terms. An
individual’s willingness to pay a higher price can be used as a method to
attribute a monetary value to qualitative characteristics such as aesthetics.
Commission for Architecture and the Built Environment in London states
that the price of buildings or houses will increase by 6% if there is a park
nearby, and by 8% if the building has a direct view of the park. Green roofs,
especially if spread over a larger area, has a similar function as a local park.
Accordingly, 2%5% and 5%8% of property value increments for exten-
sive and intensive green roof respectively have been assumed (Bianchini and
Hewage, 2012). The extensive green roof may raise property value from
$2.6 to $8.3 m
, while intensive green roofs may increase property value
from $8.3 to $43.2 m
. Besides the aesthetic benefits, green roofs can also
provide recreational spaces in urban areas if they are designed for public use
similar to parks.
LEED Certification Bonus
LEED certified buildings are gaining in popularity because of their lower
operating costs, better employee performance (in commercial and industrial
buildings), improved public relations, better health standards, as well as other
community benefits (CaGBC, 2014). The most attractive aspect for owners
is that it increases access to capital. It is estimated that the return-on-interest
of LEED certified buildings improved by 19.2% on average for green retrofit
projects in existing buildings, and 9.9% on average for new green construc-
tion projects (USGBC, 2015). Under the Canada Green Building Council
LEED program, buildings with green roof installations gain one point for
stormwater management, and one point for reducing heat island effect if the
roof covers at least 50% of the building. Another benefit of green roof tech-
nology is that the vegetation and soil media of green roof can be used as a
filter for the storm runoff, so that the water from the green roof system can
be used to irrigate other landscaping features without pretreatment (LEED
Economic Benefits and Costs of Green Roofs Chapter | 4.5 309
Canada, 2009). Under the LEED scheme, this may warrant an additional
point for water efficient landscaping. The ability to reduce energy demand
for cooling and heating, and increased energy efficiency may also garner
additional points for optimized energy performance. Furthermore, potential
points can be gained for reduced site disturbance, protection or restoration of
open space, and innovation in design.
PUBLIC BENEFITS OF GREEN ROOFS
Reduction of Stormwater Runoff
Green roofs can impact the stormwater retention capacity of buildings. Most
importantly, with the presence of green roofs, the rainwater that falls onto
the roof surfaces flows into the sewers at a slower rate, as green roofs are
able to retain water. Depending on regional climate, green roofs can lower
the sewer system capacity requirement, by holding as much as 50%95%
of annual rainfall precipitation (Driscoll et al., 2015; Beecham and
Razzaghmanesh, 2015). An investigation by the city of Portland revealed
that $30 m
is needed to manage the stormwater falling on impervi-
ous areas that do not absorb rainwater (City of Portland, 2008). Based on the
retention performance of green roofs listed above, green roofs will be able to
create $1528 m
savings per year by reducing the public infrastructure
Improvement of Air Quality
Green roofs are recognized as an air quality control technology. The vegeta-
tion reduces air pollution by actively absorbing many pollutants, and by pas-
sively filtering and directing airflows. It was estimated that eight metric tons
of unclarified air pollutants can be removed per year by 109 ha of green
roofs in Toronto, Canada (Currie and Bass, 2010). Another study conducted
in Chicago estimated that the annual mass of air pollutants which can be
removed by 19.8 ha of green roofs amounts to 1675 kg (Yang et al., 2008).
The cost estimate for the air quality benefit of a green roof is calculated
by considering the negative effects of pollutant on health, environment,
infrastructure, and climate change. The cost would be significantly higher in
urban environments, due to the effect on a larger number of people. In North
America, the NO
emissions tax is $3375 ton
(Clark et al., 2008). In
Europe, the SO
cost in a populated area is $2500 ton
, and $500 ton
cost (Nurmi et al., 2013). Based on the results from Yang et al. (2008)
and Clark et al. (2008), the benefits from the improvement of air quality
would be around $0.03 m
annually assuming all the air pollutants removed
by green roof are NO
310 SECTION | IV Nature Based Strategies: Social, Economic and Environmental
Mitigation of Urban Heat Island Effect
In urban environments, vegetation has often been replaced by impervious
and dark surfaces. Dark surfaces reflect less solar radiation and absorb more
energy. Due to the lack of vegetation and the presence of dark surfaces, the
urban heat island effect is created. A simulation study in New York showed
that the average roof temperature can be reduced by as much as 0.8Cif
50% of the roof area is covered with vegetation (Rosenzweig et al., 2006).
In Venice, the field observation and simulations results showed that the tem-
perature of a green permeable surface could be 4C lower than the existing
paved roof (Peron et al., 2015). It was also estimated that the urban heat
island effect can be reduced by 12 degrees Celsius if 6% of Toronto was
covered with green vegetation (Peak, 2004). Another report on the
Mediterranean region shows that 10%14% of the electrical energy con-
sumed in cooling residential buildings can be saved by green roofs (Zinzi
and Agnoli, 2012). Green roof performance in reducing the urban heat island
effect varies in different locations, due to the conditions in the surrounding
environment, and changes in building density.
Increment of Urban Biodiversity
Green roofs can help to increase local biodiversity by providing habitats for
different animal species such as birds and insects within a city. A study
conducted in Switzerland found that 79 beetles and 40 spider species were
supported by a single green roof, of which 20 species were endangered
(Brenneisen, 2006). Another study conducted in England on green roofs
which mimic conditions found in derelict sites discovered that these sites are
favored by black redstart, a rare species of bird in the United Kingdom
(Grant and Lane, 2006).
However, creation of a habitat for animals is treated only as a bonus
compared with other quantifiable benefits. It is not easy to quantify the
increase in biodiversity and estimate the corresponding costs and benefits
using a common methodology. While it is difficult to directly quantify the
economic benefits of habitat increase due to green roofs, the resulting envi-
ronmental benefits may be translatable to economic terms based on environ-
LIFE CYCLE COST OF GREEN ROOFS
There is a significant price variation among green roofs due to factors such
as type and size, locations of green roofs, and country. The current cost in
British Columbia, Canada for a standard extensive green roof varies from
$130 to $165 m
, and the cost of a standard intensive green roof starts from
Economic Benefits and Costs of Green Roofs Chapter | 4.5 311
(Bianchini and Hewage, 2011). Many factors such as labor and
equipment costs affect the installation price. In Singapore, a green roof price
ranges from $40 to $65 m
depending on the type of green roof and struc-
ture of the foundation (Wong et al., 2003). In China, the average price of a
green roof investigated from three provinces is between $48 and $76 m
(Jia and Wang, 2011; Liu and Hong, 2012). In a mature market like
Germany, the average green roof costs range from $15 to $45 m
lower green roof prices in Germany are a result of ongoing research and
development as well as market penetration spanning two decades. In newer
markets, no economies of scale exist and competition is scarce. Labor is also
more expensive because of the lack of experience and the tendency to use
custom design systems. One way to reduce the initial cost of green roofs is
to adopt the low-cost techniques developed by mature markets. The cost of
green roof generally decreases by 33%50% once the industry has estab-
lished itself (Toronto and Region Conservation, 2007).
Operation and Maintenance (O&M) Cost
Economic and environmental benefits of green roofs rely on their perfor-
mance. Therefore, O&M of vegetative roofs are critical in securing their pos-
itive impacts. The maintenance cost also depends on the size of green roofs,
the characteristics of the building, the complexity of the green roof system,
the type of vegetation, as well as the market O&M price. It is estimated that
annual O&M cost of green roofs in the United States is between $0.7 and
(Bianchini and Hewage, 2012).
There are different disposal options for green roofs at the end of life.
Materials can be landfilled, reused, or recycled. Water retention layer, drain-
age layer, and root barrier layers of green roof can be recycled again at the
end of the lifespan. However, many cities do not have the necessary facilities
for the recycling process. Landfill costs depend on many factors such as
technology, location, size of the facility, and available landfill capacity in a
A study indicated that the operations and maintenance cost in landfilling
is on average $56 per ton waste disposed without considering the energy
recovery option (Chang and Wang, 1995). Another report compiled in
Europe did a complete analysis on the green roof disposal cost, including
inert material landfill, sanitary landfill, and incineration with energy recov-
ery. The disposal cost for an entire green roof is estimated at $1120 ton
)(Peri et al., 2012). Bianchini and Hewage (2012) illustrated
that the cost to dispose green roof materials is in the range between $0.03
and $0.2 m
312 SECTION | IV Nature Based Strategies: Social, Economic and Environmental
GREEN ROOF COST BENEFIT ASSESSMENT
Net Present Value and Payback Period
In order to assess the total benefits and costs of green roof, the values
need to be converted into a net present value (NPV) by the means of dis-
counting. The lifespan of a green roof has been estimated as about 40
years minimum and 55 years maximum (Mahdiyar et al., 2016). In this
analysis, 40 years is used to conduct the assessment. Based on the study
from Gollier and Weitzman (2010), 3% of the discount factor was applied
to this analysis. Based on the benefits and costs of green roofs introduced
above, Table 1 summarized all the economic inputs for the analysis and
NPVs as output.
There is a wide range in terms of the values in Table 1, especially the
aesthetic benefits and stormwater runoff reduction benefits, and the life-
cycle costs. One of the reasons is due to the different systems of green
roofs. For example, extensive green roofs have shallow soil roofs with
simple growing plants, and are usually not accessible. Therefore they have
a lower lifecycle cost.
On the other hand, intensive green roofs are similar to a ground level
garden with a deep growing medium and artificial irrigation (Kosareo and
Ries, 2007). Therefore, the initial cost and O&M cost are higher. At the
same time, intensive green roofs have higher benefits in stormwater run-
off deduction due to its deep growing medium, and better aesthetic values
because it acts like a garden. Another reason is the cost and technique
variances between different markets. In the mature market like Germany,
the costs are much lower than the new markets in Asia and North
America, and the benefits generated from green roofs are more than the
new markets because of its mature techniques and great popularity. Some
other reasons are sizes of green roofs, weather conditions, and building
As shown in Table 1, the total NPV of individual benefits in 40 years is
between $135.9 and $195.8 m
, and the total NPV of public benefits in 40
years is between $478.7 and $751.7 m
. Based on the result, it is obvious
that the public benefits are over three times greater than the individual bene-
fits, even though two of the public benefits are not counted in the calculation
due to the unavailable data.
If the total NPV of lifecycle costs for green roofs in 40 years is close to
, which is at the lower side of the range ($42.3978.8 m
), it will
only take 13 years of the individual benefits to balance the cost of green
roofs. If the public benefits are considered, the payback period will be
reduced to 3 years. If the total NPV of lifecycle cost for green roofs in
40 years is close to $978.8 m
, this cost could still be paid back in its life-
time by the individual benefits and public benefits together.
Economic Benefits and Costs of Green Roofs Chapter | 4.5 313
Scale of Implementation
As shown in Table 1, the values created by the mitigation of urban heat
island effect and increment of urban diversity are not available in this analy-
sis, because the value would be very small if only one or a few green roofs
were installed. However, the benefits of green roof will increase
TABLE 1 Economic Data Input and NPV Output ($ m
) for the Cost
Lifespan (year) \ 40 \
Discount rate (%) 3 \ \
Reduction of energy 0.40.9 Annual 15.735
Use in heating and
Membrane longevity 160 At year 20 88.6
Acoustic insulation 29 One time 29
Aesthetic benefits 2.643.2 One time 2.643.2
n/a n/a n/a
Total NPV 135.9195.8
15 - 28 Annual 477.5750.6
Improvement of air
0.03 Annual 1.18
Mitigation of urban
heat island effect
n/a n/a n/a
Increment of urban
n/a n/a n/a
Total NPV 478.7751.7
Initial cost 15540 One time 15540
0.713.5 Annual 27.3438.7
Disposal cost 0.030.2 At year 40 0.010.06
Total NPV 42.3978.8
314 SECTION | IV Nature Based Strategies: Social, Economic and Environmental
tremendously if implemented at a larger scale. Intangible benefits such as
aesthetic appeal of green roofs and increased urban biodiversity can be
gained with large scale of implementation (Niu et al., 2010; Nurmi et al.,
2013) . The costs of green roofs will also be reduced with a higher imple-
mentation rate. Large scale of implementation would also reduce the volume
of stormwater entering local waterways, which will lead to lower water tem-
peratures, less in-stream scouring, and better water quality (Spengen, 2010).
Green Roof Policy Initiatives
Based on the analysis above, the public benefits of green roofs are over three
time larger than the private benefits. Therefore, municipal authorities should
play a key role in promoting green roofs in urban areas and residential neigh-
borhoods through policy and regulatory measures.
In Toronto, Green roofs are required on all new institutional, commercial,
and multiunit residential developments. The incentive offered for green roof
is $75 m
up to an upper limit of $100,000 (City of Toronto, 2016). In New
York, green roof tax abatement is implemented, so that each square foot of
green roof can get a rebate of $5.23, up to $200,000 per project (NYC,
2014). In Singapore, the National Parks Board aims to increase greenery pro-
vision by funding up to 50% of the installation cost of rooftop greenery
(National Parks, 2011). In Tokyo, it is mandatory for a new building to cover
25% of roof with greenery (Growing Green Guide, 2013).
In Munich, all building roofs with a surface area larger than 100 m
should be landscaped. This policy was implemented around 20 years ago,
and it makes the green roof a recognized construction standard in Munich
(IGRA, 2011). As a world leader in green roof development, Germany’s
experience shows that it is necessary to introduce a green roof policy rather
than rely solely on the goodwill of building owners (Ngan, 2004).
Green roofs have personal and social benefits. The cost benefit assessment
showed that the lifecycle costs of green roofs can be retrieved in most of the
markets around the world. The payback periods in the mature markets and
markets with average initial costs are shorter than the lifespan of green roofs.
With a larger implementation scale, the social benefits of green roofs will be
increased tremendously. Governments should play a key role in promoting
the green roof construction by providing incentives to transfer the social ben-
efits into private investors, such as tax abatement, direct cash rebate, low
interest loans, etc. These incentives will also expand the public benefits, and
lower the lifecycle cost of green roofs.
Economic Benefits and Costs of Green Roofs Chapter | 4.5 315
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