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Report on the Environmental Benefits and Costs of Green Roof Technology for the City of Toronto

Authors:
Report on the
Environmental Benefits and Costs of
Green Roof Technology for the City of Toronto
Prepared For
City of Toronto and
Ontario Centres of Excellence
Earth and Environmental Technologies (OCE-ETech)
Prepared By
Ryerson University
Professors:
Dr. Doug Banting
Professor Hitesh Doshi
Dr. James Li
Dr. Paul Missios
Students:
Angela Au
Beth Anne Currie
Michael Verrati
October 31, 2005
Project Contact:
Hitesh Doshi
Dept. of Architectural Science, Ryerson University
350 Victoria Street, Toronto, Ontario , M5B 2K3
Phone: 416 979 5000 x6502
E-mail: hdoshi@ryerson.ca
Report on the Environmental Benefits and Costs of Green Roof Technology for the City of Toronto
Prepared by Ryerson University i
Executive Summary
Report on the Environmental Benefits and Costs of
Green Roof Technology
for the City of Toronto
Toronto has been at the forefront of organized green roof activity over the last several years.
In early 1990’s volunteers under the Rooftop Garden Resource Group (RGRG) started to
promote green roof development in the city. This has been taken over by Toronto-based
Green Roofs for Healthy Cities, a not for profit organization, which carries out world-wide
education on green roofs.
The City of Toronto has been an active participant in studying wider use of green roofs as a
sustainable alternative to meet many of the urban environmental challenges. In the past few
years the City has shown leadership in promoting green roofs. In order to inform its actions,
the City in partnership with OCE-ETech and the Federation of Canadian Municipalities,
engaged a team of Ryerson researchers to develop further understanding of: the types of
available green roof technology, the measurable benefits of green roofs to the city’s
environment, potential monetary savings to the municipality through use of green roofs, and
minimum thresholds of green roofs that could be used for part of any incentives or programs.
This report presents the findings on the municipal level benefits of implementing green roof
technology in the City of Toronto. Beyond this report, which addresses the immediate needs
of the City of Toronto in assisting them to formulate appropriate programs and policies, the
Ryerson team has been charged by OCE-ETech to develop a generic technological solution
that can be used to predict the costs and benefits related to green roofs. This work is ongoing
and is not reported here.
For this report, the Ryerson team conducted an extensive literature review to determine the
benefits related to green roofs and, in particular, the quantification of these benefits. It also
collected information on the types of buildings in Toronto and their geographic distribution.
The information collected was modeled as a GIS database and used for aggregating the
benefits on a city-wide basis. The Ryerson team also developed a method to compute the
monetary value of the benefits. A survey of the existing green roof technologies and
standards was carried out to inform the development of minimum requirements for green
roofs.
The findings of the work are presented in the following sections of the report: Section 1,
about the study, provides historical background related to this work; Section 2, survey of
research related to green roofs, provides the findings of the literature review on benefits of
green roofs; Section 3, survey of types of green roofs and their standards, provides
information on the different green roof technologies currently available and the performance
standards pertaining to green roofs; Section 4, green roof benefits and costs for the City of
Toronto, provides the details of the quantification of benefits of city-wide implementation of
Report on the Environmental Benefits and Costs of Green Roof Technology for the City of Toronto
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green roofs. The report ends with a summary and recommendation, in Section 5, which
provides the recommendations, the minimum thresholds and guidance for further work.
Of the many benefits of green roofs reported in the study, the ones that had the most
quantifiable monetary value based on currently available research data are: benefit from
stormwater flow reduction including impact on combined sewer overflow (CSO),
improvement in air quality, reduction in direct energy use, and reduction in urban heat island
effect.
The literature review indicated other benefits that could not be quantified in this report. These
benefits included: aesthetic improvement of urban landscape, increase in property values,
benefits resulting from green roofs used as amenity spaces, use of green roof for food
production, and increased biodiversity. Further work is needed to quantify these benefits.
The benefits on a city-wide basis were calculated based on the assumption that 100% of
available green roof area be used. The available green roof area included flat roofs on
buildings with more than 350 sq. m. of roof area, and assuming at least 75% of the roof area
would be greened. The total available green roof area city-wide was determined to be 5,000
hectares (50 million sq. m.).
The benefits were determined as initial cost saving related to capital costs or an amount of
annually recurring cost saving. These are shown in the following charts and table.
Initial Savings
Air Quality, $0, 0%
Building Energy,
$68,700,000, 22%
Urban Heat Island,
$79,800,000, 25% Stormwater,
$118,000,000, 38%
Combined Sewer
Overflow (CSO) ,
$46,600,000, 15%
Report on the Environmental Benefits and Costs of Green Roof Technology for the City of Toronto
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Annual Savings
Urban Heat Island,
$12,320,000, 33%
Building Energy,
$21,560,000, 58%
Air Quality,
$2,500,000, 7%
Stormwater, $0, 0%
Combined Sewer
Overflow (CSO) ,
$750,000, 2%
Category of benefit Initial cost saving Annual cost saving
Stormwater $118,000,000
Combined Sewer Overflow (CSO) $46,600,000 $750,000
Air Quality $2,500,000
Building Energy $68,700,000 $21,560,000
Urban Heat Island $79,800,000 $12,320,000
Total $313,100,000 $37,130,000
The report also presents the minimum considerations for the type of green roof to achieve the
stated benefits. The key considerations include that: the roof system be of the type known as
an extensive roof system, that it cover a significant portion of the roof, have a maximum
runoff coefficient of 50%, and have at least a 150 mm. depth where structural loads permit.
Green roofs with less depth could be used on roofs where structural loading does not permit
the 150 mm. depth.
The benefits quantified in this report show that there is a case for development of public
programs and the promotion of green roofs. The City of Toronto may wish to use this
information to embark on consideration of programs that will give further impetus to the
construction of green roofs.
Report on the Environmental Benefits and Costs of Green Roof Technology for the City of Toronto
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Green roof technology is an emerging technology and many questions need further
exploration. Although this study has made several advances in predicting benefits of green
roofs, and it has provided information for the City of Toronto to move further on programs
and policies pertaining to green roofs, there are several areas that will require further work.
Questions remain to be answered regarding the uncertainty of the benefits, impact of less than
100% green roof coverage, impact of building specific constraints, the quantification of
program costs leading to a complete cost benefit analysis, quantification of other social
benefits and consideration of the effect of alternative technologies that may be able to
perform one or more of the functions of a green roof. These questions are important and will
need to be considered in further studies. Some of these will be explored in the continuing
work by the Ryerson team. In the meantime, policy decisions regarding green roofs will need
to consider the potential impact of these questions.
Report on the Environmental Benefits and Costs of Green Roof Technology for the City of Toronto
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Table of Contents
Executive Summary i
1.0 About the study 1
1.1 Study objectives.......................................................................................................1
1.2 Context and background of green roof related activity in Toronto..........................2
1.2.1 Green Roof Demonstration Project..............................................................2
1.2.2 Sustainable Technologies Consortium York University Roof Monitoring
Program........................................................................................................3
1.2.3 City of Toronto, FCM and OCE-ETech partnership....................................4
1.3 Other FCM sponsored studies of GRT ....................................................................5
1.3.1 City of Winnipeg study................................................................................5
1.3.2 City of Waterloo study.................................................................................5
2.0 Survey of research related to green roofs 7
2.1 Potential green roof benefits....................................................................................7
2.2 Energy budgets of individual buildings ...................................................................8
2.3 Urban heat island ...................................................................................................12
2.4 Stormwater management implications...................................................................14
2.4.1 Combined sewer systems...........................................................................15
2.4.2 Sanitary sewers ..........................................................................................15
2.4.3 Wastewater treatment systems...................................................................16
2.4.4 Storm sewer systems..................................................................................16
2.4.5 Control measures for sanitary sewer systems ............................................16
2.4.5 Control measures for stormwater...............................................................17
2.5 Air quality impacts.................................................................................................23
2.6 Green amenity space..............................................................................................24
2.7 Habitat preservation...............................................................................................24
2.8 Property values.......................................................................................................27
2.9 Derivation of economic benefit from green roofs..................................................27
2.9.1 Methodology..............................................................................................27
2.9.2 Time period................................................................................................28
2.9.3 Discount rate..............................................................................................28
2.9.4 Installation and maintenance costs.............................................................28
2.9.5 Economies of scale ....................................................................................29
2.9.6 Administration costs ..................................................................................29
2.9.7 Energy cost savings....................................................................................29
2.9.8 Urban heat island .......................................................................................30
2.9.9 Stormwater flow reduction ........................................................................30
2.9.10 Air pollution and greenhouse gas effects...................................................30
Report on the Environmental Benefits and Costs of Green Roof Technology for the City of Toronto
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2.9.11 Food production.........................................................................................31
2.9.12 Aesthetic benefits.......................................................................................31
2.9.13 Job creation................................................................................................31
2.9.14 Cost-benefit ratios and life-cycle cost assessments ...................................31
2.10 Summary of green roof research on costs and benefits..........................................32
3.0 Survey of types of green roofs and their standards 34
3.1 Green roofs described............................................................................................34
3.2 Currently available green roof technology.............................................................34
3.2.1 Complete systems ......................................................................................35
3.2.2 Modular systems........................................................................................36
3.2.3 Precultivated vegetation blankets...............................................................38
3.3 Survey of green roof system standards and performance requirements.................39
3.3.1 FLL guidelines...........................................................................................39
3.3.2 Green roof requirements ............................................................................43
4.0 Green roof benefits and costs for the City of Toronto 46
4.1 Description of approach.........................................................................................46
4.1.1 Identification of benefits............................................................................46
4.1.2 Quantification of impacts...........................................................................46
4.1.3 Monetary valuation of benefits ..................................................................46
4.1.4 City of Toronto specific determination of benefits use of building
inventory data.............................................................................................47
4.2 Methodology and results........................................................................................48
4.2.1 Use of geographic information system (GIS) ............................................48
4.2.2 Costs of GRT .............................................................................................49
4.2.3 Stormwater.................................................................................................50
4.2.4 Combined sewers.......................................................................................51
4.2.5 Air quality..................................................................................................54
4.2.6 Building energy and the urban heat island.................................................55
5. Summary and recommendations 59
5.1 Other benefits.........................................................................................................60
5.2 Green roofs on sloped surfaces..............................................................................60
5.3 Recommendation for types of green roofs.............................................................60
5.4 Next steps...............................................................................................................62
Appendices
Appendix A - References
Appendix B - List of GIS data and sources
Appendix C - Data for stormwater calculations
Appendix D - GIS maps
Report on the Environmental Benefits and Costs of Green Roof Technology for the City of Toronto
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List of Tables and Figures
Figure 2.1 Methodology to analyze the impact of cool roofs and cool pavements on energy
use. [After: Akbari et al. (2001)] ...................................................................................... 9
Table 2.1 Summary of key findings from literature review related to heat transfer, energy
use and green roofs ......................................................................................................... 10
Table 2.1 Summary of key findings from literature review related to heat transfer, energy
use and green roofs (continued)...................................................................................... 11
Figure 2.2 Generalized cross-section of a typical urban heat island [ After Oke (1987)]....... 12
Table 2.2 April 2003 hydrologic retention for the WCC green roof in Goldsboro, NC [ after
Jennings et al. (2003) ].................................................................................................... 18
Figure 2.3 Relationship between the peak flow and runoff on green roof [ after Jennings et
al. (2003)]........................................................................................................................ 19
Table 2.3 Green roof runoff volume reduction for 2003 and 2004 monitoring seasons (TRCA
2005)............................................................................................................................... 21
Table 2.4 Peak flow reductions for a range of event sizes (TRCA 2005) .............................. 21
Table 2.5 Comparison of concentrations for selected parameters from the control roof and
the garden (TRCA 2005).................................................................................................21
Table 2.6 Summary of key findings from a literature review related to stormwater and green
roofs ................................................................................................................................ 22
Table 2.7 Summary of key findings from literature review related to air quality and green
roofs ................................................................................................................................ 26
Figure 3.1 A typical Sopranature green roof assembly on conventional roof (Adapted from
Soprema Inc.).................................................................................................................. 36
Figure 3.2 Photograph showing Green Roof Block System (Adapted from St. Louis
Metalworks Company).................................................................................................... 37
Figure 3.3 Photograph showing GreenGrid System (Adapted from Western Solutions Inc.) 37
Figure 3.4 Photograph showing ELT system (Adapted from Elevated Landscape
Technologies).................................................................................................................. 38
Figure 3.5 Photograph showing Xero Flor System installation and cross section (Adapted
from Xero Flor Canada).................................................................................................. 39
Table 3.1 Growing medium depth required for various types of vegetation on different types
of green roofs and Annual average water retention as percentage of rainfall for selected
types of green roofs......................................................................................................... 42
Table 3.2 Green roof requirements in selected European jurisdictions.................................. 45
Table 4.1 Available areas for green roof implementation...................................................... 49
Table 4.2 Input and calibrated data for the SUDS model....................................................... 53
Table 4.3 Analysis of CSO scenarios using the SUDS model................................................ 53
Table 4.4 Impact on air quality from grass roofs Reductions in contaminants and monetary
impact as shown.............................................................................................................. 55
Table 4.5 Direct Energy savings from green roof implementation......................................... 56
Table 4.6 Indirect energy savings from green roof implementation (Impact of reduction in
urban heat island effect in Toronto)................................................................................ 58
Report on the Environmental Benefits and Costs of Green Roof Technology for the City of Toronto
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Table 5.1 Summary of municipal level environmental benefits of green roof implementation
in the City of Toronto (Assuming green roof coverage of approximately 5,000 hectares
......................................................................................................................................... 59
Table C1 Unit response functions for runoff calculations......................................................C1
Table C2 Comparison of unit costs of best management practices and estimation of cost
saving of green roofs.......................................................................................................C2
Prepared by Ryerson University Page 1
Report on the
Environmental Benefits and Costs of
Green Roof Technology for the
City of Toronto
1.0 About the study
1.1 Study objectives
This study is part of a project undertaken by Ryerson University through funding provided by
the Ontario Centres of Excellence-Earth and Environmental Technologies (OCE-ETech) and
matched by the City of Toronto as the major partner. Other partners include Trow Associates
and 401 Richmond. This document reports on the first part of the project and deals with the
municipal level benefits of Green Roof Technology.
Individual building owners are driving construction of green roofs; the City of Toronto is
investigating the development of programs to promote green roofs and the required standards
for their implementation.
In order to develop appropriate actions, the City of Toronto identified a need to determine the
social and environmental benefits of green roofs on a city-wide level. The City of Toronto
needs to have an understanding of:
the types of green roof technology;
the measureable benefits of green roofs to the city’s environment;
potential monetary savings to the municipality;
minimum threshold points for the City of Toronto to provide incentives to make
significant cost savings.
This study conducted by Ryerson University and as reported here will be used to inform the
City of Toronto in developing programs to promote the use of green roof technology. The
social and environmental benefits of this technology are of primary importance. It is expected
that the information in this report will assist the City of Toronto to formulate the appropriate
types of government programs or incentives to encourage private investment in green roofs
and thus reap the social benefits. The public costs of these programs or incentives are as yet
to be determined and not part of this project.
Beyond this study for the City of Toronto, the overall objective of the project is to build on
the knowledge gained in order to formulate a model and useable technology that will allow
individual building owners and other municipalities to measure the benefits of green roof
technology.
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1.2 Context and background of green roof related activity in Toronto
Green roof activities have been ongoing in Toronto for almost a decade. Promotion of green
roofs in Toronto can be traced back to a small number of dedicated volunteers under the
umbrella of the Rooftop Garden Resource Group. Their activity was enhanced with the
involvement of an association that is today known as Green Roofs for Healthy Cities
(GRHC).
The City of Toronto's formal involvement in green roofs is rooted in the recommendations of
the Environmental Plan (2001). The Plan was the first to formally identify the need for a
strategy to encourage green roofs and rooftop gardens. The Natural Environment policy
within the City’s new Official Plan further supports green roofs calling for “the development
of innovative green spaces such as green roofs, and designs that will reduce the urban heat
island effect.”
Another place where green roofs have found a potential is in the Wet Weather Flow
Management Master Plan for the City of Toronto completed in 2000. It examined ways to
improve the water quality of local rivers and Lake Ontario by strengthening mechanisms to
prevent and reduce stormwater runoff. Green roofs may appear in future stormwater planning
policies that discuss best management practices; however, the policies have not been drafted
at this point.
The following subsections deal with specific activities that have been undertaken to promote
green roofs in the City of Toronto.
1.2.1 Green Roof Demonstration Project
In Fall 2000, Green Roofs for Healthy Cities, the Toronto Atmospheric Fund, the Federal
Government and the City of Toronto partnered to initiate a Green Roof Demonstration
Project.
Two demonstration green roofs were constructed as part of the project:
eight plots covering more than 300 square metres on the podium roof of Toronto’s
City Hall building;
a 465 square metre green roof on the Eastview Neighbourhood Community Centre.
For the first two years, the City of Toronto and Green Roofs for Healthy Cities managed the
demonstration roofs jointly. After that period, the City assumed the management of the
project.
The objectives of this million dollar, three-year project, were to find solutions to overcome
technical, financial and information barriers to the widespread adoption of green roof
infrastructure in the marketplace by:
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generating reliable technical data on green roof performance in areas such as energy
efficiency, stormwater retention, the extension of roof membrane life span and plant
survival in the Toronto climatic context;
conducting research on city-wide cooling benefits of green roofs in the summer and
the potential spin-off from greenhouse gas reduction, smog reduction and energy
efficiency gains from reducing cooling loads in buildings;
evaluating the costs and benefits of future public-private investments in green roofs;
increasing awareness of the benefits of green roof technology by giving professionals
the opportunity to visit a working demonstration site with multiple applications.
The City Hall podium green roof has been used to study the different plants that can be used
for green roofs in Toronto.
The Eastview Neighbourhood Community Centre green roof is an extensive green roof built
beside a regular membrane finished flat roof. It has been extensively instrumented, and
results from the measurements have been published. The results to date have been
encouraging.
1.2.2 Sustainable Technologies Consortium York University Roof Monitoring Program
The Sustainable Technologies Consortium was formed in order to address the growing need
for research to support the implementation of technologies promoting sustainable
development. The Consortium is a public partnership between the Toronto and Region
Conservation Authority (TRCA), Seneca College, the University of Guelph and Ryerson
University. The multi-disciplinary nature of the Consortium's members was intended to
reflect the nature of sustainable technology research, which integrates various disciplines and
research interests. The mandate of the consortium is two-fold:
to pursue scientifically defensible research in sustainable development;
to quantify the potential benefits of technologies relating to stormwater management,
water and energy conservation, and air pollution.
The impetus for the Sustainable Technologies Consortium can be traced to the International
Joint Commission, which in 1987 identified Toronto as one of 42 regions of concern
bordering the shorelines of the Great Lakes. It developed a Remedial Action Plan to restore
polluted drainage networks and water bodies located in the city or along the shorelines of
Lake Ontario. Some of the goals and actions recommended by the Remedial Action Plan can
be achieved using green roof technology.
In 2003, a research site was established on the York University Computer Science Building.
A green roof was designed during building construction and has been monitored by the
TRCA. Unlike the Eastview Neighbourhood Community Centre building which was a retrofit
the York University green roof was installed over a new building.
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Measurements of climate, soil and runoff have been taken to quantify the stormwater quality
and quantity benefit of the green roof. The monitoring devices have been linked to a single
logger and network server that statistically calculates and communicates measured data via
the internet. The internet connection also provides real-time measurements of activities (e.g.
rainfall) that can be accessed from anywhere in the world.
Initial results from the monitoring of the effects of green roof technology on stormwater
runoff control have been positive.
In addition to the monitoring, a hydrological modeling analysis of Highland Creek was
undertaken using the monitored data. This data has been used in this study.
1.2.3 City of Toronto, FCM and OCE-ETech partnership
The encouraging results from the green roofs on the Eastview Neighbourhood Community
Centre, Toronto City Hall and the York University Computer Science Building have provided
positive impetus for ongoing promotion of green roofs. City planners started to examine the
possibility of developing programs to promote green roofs. City of Toronto staff explored the
possibility of seeking funding from Federation of Canadian Municipalities (FCM) to carry
out research that would inform the development of programs and policies to promote green
roofs.
The City of Toronto was successful in procuring funds from FCM for further studies to
examine the municipal level social and environmental costs and benefits of green roofs. FCM
has been the national voice of municipal government since 1901 and is dedicated to
improving the quality of life in all communities by promoting strong, effective, and
accountable municipal government. Recently, the Government of Canada endowed the FCM
with $250 million to establish the Green Municipal Funds and support municipal government
action to cut pollution, reduce greenhouse gas emissions, and improve quality of life.
The City of Toronto approached OCE-ETech, who organized a green roof think tank in
November, 2003. OCE-ETech is a division of the Ontario Centres of Excellence and helps
Ontario organizations grow by finding solutions for their innovation challenges. It engages
clients and academic partners in various market driven strategic clusters of activities,
including sustainable infrastructure and energy solutions. Research into the municipal level
costs and benefits of green roofs was identified as an area of research.
Based on the City of Toronto's interest and FCM support, OCE-ETech put out an expression
of interest (EOI) in the winter 2004. Teams from Ryerson and other universities participated
in the EOI. In spring, 2004 a team from Ryerson and two teams from other universities were
shortlisted to submit a detailed proposal. Ryerson submitted a proposal with the City of
Toronto as the major partner. In Fall, 2004 Ryerson was selected to carry out the project
related to the costs and benefits of green roofs.
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As indicated earlier the project has multiple parts. The first part, which resulted in this report,
was to examine the social and environmental benefits of green roofs at the municipal level
for the City of Toronto.
1.3 Other FCM sponsored studies of GRT
In the past, FCM has supported work specific to green roof technology at two other
municipalities: the City of Waterloo and the City of Winnipeg. The following sections
provide a brief description of these studies.
1.3.1 City of Winnipeg study
Funded by the FCM grant, the City of Winnipeg explored the feasibility of developing a
green roof strategy for flat-topped buildings in its downtown area. Such a strategy could help
alleviate stormwater management problems in Winnipeg’s downtown. The City felt that a
green roof strategy could be incorporated into the Combined Sewer Overflow (CSO) control
model to reduce runoff effects and provide other environmental benefits.
The Assiniboine district was the focus of this study due to its high concentration of flat-
topped buildings. The area is also the most prone to overflows from the combined sewer
system. Recent aerial photographs and visual inspections indicated that an area of 218,773
square metres (almost 20% of the total area of the district) could be used for green roof
development.
Control-system models were created to simulate rainfall and runoff during a typical year.
Various scenarios were examined to determine whether a green roof strategy could reduce not
only the number of overflows in a year, but also their volume and the volume of wastewater
going to the water pollution control centre.
In this study plant species were also evaluated for their carbon-fixation and sequestering
potential.
Data collected during the stormwater modelling process indicated that the number of
overflows could be reduced by 16%, if 100% of the potential roof space in the district were
used. The volume of the overflow could also be reduced by approximately 48%, which in
turn would cut the volume of flow to the water pollution control centres.
In terms of the carbon fixation it was found that if 100% of the potential green roof space
were developed, 24.5 tonnes of carbon would be fixed (removed) annually.
1.3.2 City of Waterloo study
As part of its Environment First Policy, the City of Waterloo developed an Environmental
Strategic Plan, which was adopted by Council in 2002. Green roofs fit into the Environmental
Strategic Plan in all important areas.
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In 2003, the City of Waterloo received a grant of $25,000 from FCM for a "Green Roofs
Feasibility Study." As a condition of the grant, a green roof demonstration site was to be
constructed on a city-owned building.
A multidisciplinary steering committee was formed to guide the study. It included 12
professionals from various agencies, levels of government and community interests. Totten
Sims Hubicki, Enermodal Engineering and Elevated Landscape Technologies were retained
to complete the above study on behalf of the City of Waterloo.
The purpose of the Green Roofs Feasibility Study was to identify a city-wide green roofs
implementation plan for municipally owned buildings in the City of Waterloo, including
identification of potential costs and associated maintenance. It would also, through the
selection process, identify a preferred location for a green roof demonstration site in the City
of Waterloo. The function of the demonstration site would be to raise public and industry
awareness and to provide an educational forum to display the benefits of green roofs. A
business plan for the green roof demonstration site, including an analysis of performance,
benefits, and costs, was to be generated as part of the feasibility study.
The study, which was completed in February, 2005, identified a mechanism for selecting a
site for implementing green roof technology. The green roof has recently been constructed on
the City Hall building.
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2.0 Survey of research related to green roofs
Municipal programs and policy development related to green roofs need to be informed by
supporting research on the costs and benefits of green roofs. The green roofing industry in
North America is not as mature as in some European jurisdictions, where a number of social
and environmental benefits have been attributed to green roofs.
The purpose of this section is to document the results of a review of published literature
pertaining to the social and environmental costs and benefits of green roof technology. The
immediate objective of the review is to clarify the issues of green roof costs and benefits for a
municipality, so that the methodology can be refined to reflect the most current state of
knowledge about the performance of green roofs.
To begin this section we list the benefits that have been attributed to green roofs. In
subsequent sections these are explored in terms of the evidence from reported research in the
public domain. The derivation of actual economic benefit is then addressed and the
implications summarized.
2.1 Potential green roof benefits
Municipalities considering policies for green roofs will need to examine the tangible and
intangible benefits and costs associated with green roofs on a community-wide basis. What is
needed is an approach that is comprehensive and realistic in determining the costs and
benefits across the spectrum of circumstances and potential opportunities that may arise from
installing green roofs.
Impacts of green roof that have been commonly cited are as follows:
effects on energy budgets of individual buildings;
effects on the urban heat island;
effects on stormwater management strategies;
effects on urban air quality;
repercussions for urban amenities, such as food production, aesthetics, recreation;
urban agriculture, noise reduction, real estate, therapeutics, open space;
effects on waste management from increase in roof material “life cycle”;
promotion of horticulture/landscaping,
promotion of biodiversity and wild life protection;
promotion of health and well-being
The following review provides an exhaustive profile of existing publications from the
scientific literature and also addresses the findings of agencies currently managing green
roofs and jurisdictions in which green roof-advocacy policies are currently in place. It should
be recognized that the highest priority is reserved for research presented in peer-reviewed
Report on the Environmental Benefits and Costs of Green Roof Technology for the City of Toronto
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research publications. A great deal of green roof research has been undertaken in Germany;
the results of many of these studies were originally published in German. Therefore, in some
instances, citations identify reviews by others who have examined the results of the original
German studies.
2.2 Energy budgets of individual buildings
Green roofs have been investigated for their effects on building energy costs. The insulating
effects of added materials seem likely to reduce the penetration of summer heat and the
escape of interior heat in winter and there is some scientific evidence to support these
notions. There is possibly an even greater benefit in the summer due to the cooling created by
the evapotranspiration effect from plants and the evaporation of retained moisture from the
soil. Since different climatic conditions and architectural standards present distinctive energy
transfer opportunities, research results should be interpreted in terms of where the study was
undertaken and how relevant it is to the Canadian environment. Similarly, the conversion of
energy savings into cost savings must recognize Canadian market conditions.
In some of the earliest reported research, measurements in Berlin conducted in 1984 revealed
not only reduction in maximum surface temperature but also temperature amplitudes reduced
by half due to green roof installation (Kohler et al., 2002).
Akbari et al. (1999, 2001) investigated means of reducing building energy in mid-latitude
cities as one of several means for reducing the urban heat island (UHI) effect and documented
the enhanced air conditioning demands (up to 10%) brought about by the UHI. This elevated
load generally occurs in the late afternoon hours, corresponding to the peak summer electric
utility load. Akbari also demonstrated that the afternoon electric utility load for southern
California increases by more than 2% per degree Celsius increase in air temperature. Also
noteworthy, was the determination that ozone concentration in the Los Angeles basin was
positively correlated with air temperature, increasing at a rate of 5% per degree Celsius
(Akbari et al., 1990; Sailor, 1995). By making roofs cooler, designers can reduce the amount
of absorbed solar energy, and consequently reduce the amount of heat conduction into
buildings. This reduces daytime net energy inputs (Akbari and Konopacki, 20041; Akbari et
al., 2001; Konopacki et al., 1997) and the demand for air conditioning.
Del Barrio (1998) explored the thermal behaviour of green roofs through mathematical
analysis. The main conclusion of this study is that green roofs effectively act as thermal
insulators. Eumorfopoulu (1998) also carried out calculations to examine the thermal
behaviour of a planted roof and concluded that green roofs can contribute to the thermal
performance of buildings. This study further showed that of the total solar radiation absorbed
by the planted roof, 27% is reflected, while the plants and the soil absorb 60%, and 13% is
transmitted into the soils. Evidently, with a green roof the insulation value is in both the
plants and the layer of substrates (Eumorfopoulu, 1998). Patterson (1998) also showed that
1 This study will be referred to as the LBL study
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green roofs prevented temperature extremes and the insulation value of the soil on the
structure lowered the cooling energy costs.
Figure 2.1
Methodology to analyze the impact of cool roofs
and cool pavements on energy use. [After: Akbari et al. (2001)]
Recently, some quantitative data were obtained through field measurements, experimental
and computational methods. Onmura et al. (2001) conducted a field measurement on a
planted roof in Japan. The evaporative cooling effect of a rooftop lawn garden showed a 50%
reduction in heat flux in the rooms below the garden. This research also revealed a reduction
in surface temperature from 60 to 30oC during the day. The importance of evaporation in
reducing the heat flux was quantitatively simulated in a series of wind tunnel experiments.
Niachou et al. 2001 conducted a measurement of surface and air temperature on a planted
roof. The work was further complemented by a mathematical approach through which
thermal properties of green roofs and energy savings were determined. Reviews by Wong et
al. (2003) and Kohler et al. (2002), have shown that under a green roof, indoor temperatures
were found to be at least 3 to 4oC lower than outside temperatures of 25 to 30oC.
In the only Canadian study, Liu and Baskaran (2003) report that field research in Ottawa has
revealed that the energy required for space conditioning due to the heat flow through the
green roof was reduced by more than 75%. The study focussed on controlled conditions
featuring a reference roof and a green roof of equal dimensions; the experimental roof surface
area was 72 m2 (800 ft2) with the green roof on one half and the reference roof on the other
half. An energy reduction from 6.0 to 7.5 kWh/day for cooling was demonstrated (Liu and
Baskaran, 2003; Bass and Baskaran, 2003).
Alcazar and Bass, (2005) have very recently reported that the installation of a green roof in
Madrid reduced total energy consumption by 1% with 0.5% reduction in the heating season
and a 6% reduction in the cooling season.
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2.3 Urban heat island
The air in urban areas is typically warmer than that in surrounding undeveloped areas. This
concept has been recognized in publications since early in the Industrial Revolution (Howard,
1818, cited in Landsberg, 1981). Over the years, concern for the catastrophic effects on
human health has prompted the development of strategies for reducing the urban heat island
effect. These strategies have included reducing heat radiation and other emissions, expanding
vegetated spaces, and most recently the implementation of cool roofs (Akbari et al., 1999;
2001; 2003) and green roofs (Kohler et al., 2002; Wong et al., 2003a, 2003b; Bass et al.,
2002).
The most frequently observed and documented climatic effect of urbanization is the increase
in surface and air temperatures over the urban area, as compared to the rural surroundings.
Oke (1995) simply defines an urban heat island (UHI) as the ‘characteristic warmth’ of a
town or city. This warmth is a consequence of human modification of the surface and
atmospheric properties that accompany urban development. This phenomenon is given its
‘island’ designation due to the isotherm patterns of near-surface air temperature which
resemble the contours of an island rising above the cooler conditions that surround it. This
analogy is further illustrated in Figure 2, which shows a schematic representation of near-
surface temperature for a large city, traversing from countryside to the city centre. A typical
‘cliff’ rises steeply near the rural/suburban boundary, followed by a ‘plateau’ over much of
the suburban area, and then a ‘peak’ over the city centre (Oke, 1987, 1995). The maximum
difference in the urban peak temperature and the background rural temperature defines the
urban heat island intensity. Over large metropolitan areas, there may be several plateaus and
peaks in the surface temperature. Cooler patches coincide with open areas where vegetation
or water are found.
Figure 2.2
Generalized cross-section of a typical urban heat island [ After Oke (1987)]
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Many observations of the urban heat island over small and large cities have been reviewed by
Landsberg (1981) and Oke (1987, 1995). The intensity of an urban heat island depends on
many factors, such as the size of city and its energy consumption, geographical location,
absence of green space, month or season, time of day, and synoptic weather conditions.
Oke (1987) recognized that the urban heat island is especially related to the high urban
densities and configurations of buildings in downtown areas. He demonstrated that buildings
can create ‘canyons,’ which substantially reduce the amount of sky view available for long
wave radiation heat loss at night. Other factors contributing to the intensity of the heat island
effect include: containment of heat by pollutants in the urban atmosphere, daytime heat
storage due to the thermal properties of urban surface materials, emission of heat (from
buildings, transportation, and industrial operations), decreased evaporation due to the
removal of vegetation and the hard surface cover in the city which prevent rainwater
percolation into the soil. The absence of vegetation and the nature of this hard surface cover
can be addressed by green roof treatments. It is impermeable urban surfaces (buildings,
roadways, sidewalks, patios, parking lots etc), and an absence of soil and vegetation that
results in rapid shedding of water from rainfall and snowmelt. In the presence of stored
moisture, energy is naturally used to evaporate water (as in rural and open areas). This
sensible heat used to evaporate water creates a cooling effect, thereby reducing the
temperature of the surroundings. In cities, the absence of such stored moisture, due to the
increase of impervious surfaces, results in an elevation of surface temperature, which in turn
increases the air temperature due to radiative heat transfer.
Through better understanding of the general causes and associated problems of the urban heat
island, specific strategies for reversing the effect have been gaining acceptance by
municipalities. These include designs to exploit natural sources of cooler air from the
surrounding countryside and adjacent water bodies, parks within the city, air circulation
created by urban structures themselves, and evaporative cooling from vegetation or other
sources of water in the city (Landsberg, 1981; Chandler, 1976). Designs to reduce the heating
of surfaces are also seen as especially useful in overcoming the urban heat island effect. The
benefits of tree planting programs in metropolitan areas have been significant in cooling the
air, as well as adding to the aesthetics, and reducing greenhouse gas (CO2) contributions
(Parker, 1982; Landsberg, 1981; Oke, 1987). However, the demand for space in cities inhibits
expansion of forested areas.
Green roofs present the opportunity to expand the presence of vegetated surfaces by replacing
impermeable surfaces in urban areas, providing for a reduction in peak summer urban heat
island temperatures.
Rosenfeld et al. (1998) addressed strategies to cool urban areas by reducing the heat island
effect and smog in Los Angeles. By focusing on the energy demand of buildings, they
developed a model that showed Los Angeles could be cooled up to 3°C by reroofing and
repaving using "cool" (high reflectance) materials, and by planting shade trees around
buildings. However, Sailor (1995) had argued that in the urban environment, the lack of
vegetation, which controls evapotranspiration, is the most significant factor contributing to
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the urban heat island. Therefore green roof technology offers the possibility of much greater
impact on the urban heat island effect than reflective roofs alone.
Quantifying the mitigation of the UHI has proved to be difficult (Kohler, 2003). Bass et al.
(2002) attempted to mathematically model the effect of green roofs on the UHI in Toronto.
Using a mesoscale model and the natural and urban surface parameters, low level air
temperatures were simulated for a 48 hour period in June, 2001. The simulation assumed
50% green roof coverage and showed a reduction of 1°C in low level urban temperatures.
The simulation was repeated with the addition of irrigated green roofs. Irrigated green roofs
produced a cooling of 2°C and extended the 1°C over a larger geographic area. However, as
successfully as the model operated, model assumptions, case study choices and input data of
unknown quality created unexpectedly low reductions (Bass et al. 2002). It should be noted
that UHI is of major concern in summer months. It is not deemed to be of much concern in
the winter months in northern climates.
2.4 Stormwater management implications
Rainfall and snowmelt in urban areas are typically more problematic than in rural
environments. Under natural conditions, precipitation is impeded from running off by
vegetation, ground-surface retention and subsurface storage. The retained rainwater will
contribute to the soil moisture and ground water replenishment. Urban landscapes are
dominated by impervious surfaces, such as concrete sidewalks, building walls and roofs, and
paved parking lots and roads. These collect the flow and direct it into storm gutters, sewers
and engineered channels (collectively called the urban drainage system). Urban runoff
eventually reaches receiving waters as sudden uncontrolled surges. Many surface
contaminants are picked up in the passage of this runoff and are carried with this torrent of
stormwater. Common contaminants include suspended solids, heavy metals, chlorides, oils
and grease, and other pollutants that arise from the use of roadways and from other surfaces
the water has passed over.
There are two basic categories of interrelated problems concerning urban runoff and
wastewater from areas served by drainage systems: quantity management and quality
management. Quantity management problems arise from upstream and downstream flooding,
associated with overloaded sewer systems, and from erosion of conveyance channels
downstream in the drainage basin. Untreated overflows to receiving waters from combined
storm and sanitary sewer systems result in water quality management problems. Sanitary
overflows usually contain high concentrations of organic compounds, bacteria and nutrients,
which cause short and long-term quality problems to receiving waters. On the other hand,
storm overflows often contain a considerable amount of trace metals and a high concentration
of suspended solids, which may have long-term impacts on receiving waters as pollutants
slowly release from deposited sediments. The following sections describe quantity and
quality problems associated with each type of drainage system.
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2.4.1 Combined sewer systems
Currently, the principal problems residual to existing combined sewer systems are
deterioration of receiving water quality associated with combined sewer overflows during
high runoff conditions, sewer backup, and downstream flooding.
Combined sewer overflows result from the limited capacity of interceptors to carry the large
volumes of intermittent storm runoff for treatment. Since the design capacities of interceptors
are usually limited to 2.5 to 3.5 times dry-weather flow, it is likely that excess combined
sewer discharges will be spilled to receiving waters even during moderate rainfalls. For
instance, with a customary interceptor capacity of 2.5 times dry-weather flow in Toronto, an
average of 12 overflows per month has been observed (Hogarth, 1977).
Pollutant characteristics of combined sewer overflows are comparable to those of raw sewage
with high concentrations of biochemical oxygen demand (BOD), suspended solids and
coliform organisms. The high concentrations of pollutants in combined sewer overflows arise
primarily from two sources. The first is associated with a process commonly called the ``first
flush effect,'' in which solids deposited during dry-weather periods of low flow wash out by
scouring during the initial stages of storm runoff. According to studies (Camp, 1963), as
much as 30% of dry-weather solids may be contained in the overflows, even though only 3 to
6% of dry-weather flow volume may be lost in overflows. The second is related to the
pollutant characteristics of stormwater runoff, which often contains a variety of pollutants
such as nutrients and trace metals.
Localized upstream flooding problems associated with combined sewer systems are worse
than the roadways flooding associated with storm sewer systems because of the backup of
combined sanitary and storm flow to building drains. Sewer backup is due to obstructed flow
or inadequate capacity at the downstream end of the system, and sometimes to hydraulic
instability inside the sewer which causes pressurized flow to move upstream in the system. In
contrast, downstream flooding in drainage basins is usually due to the limited capacity of
receiving channels.
Increased erosion due to high runoff flow rates at downstream receiving channels, occurs
frequently after urban development. Land development alters the hydrologic characteristics of
catchments, resulting in increased runoff volumes, runoff velocities, and peak discharge rates.
All these changes cause a greater rate of channel erosion downstream in the development.
2.4.2 Sanitary sewers
Quantity problems of sanitary sewer systems are primarily due to extraneous flows and
infiltration/inflow during and after storm events, resulting in hydraulic overloading of both
collection systems and treatment plants. Water enters sanitary sewers as infiltration through
cracked pipes and defective joints, and as inflow through cross connections, faulty manholes,
and submerged manhole covers. Extraneous flows due to improper house connections and
illegal drains are also responsible for excess flow in sanitary sewers.
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Quality problems associated with sanitary sewer systems are usually related to overflows.
Although all sanitary flows are designed to be treated at treatment plants, overflow points are
often built into the sewer systems to prevent overloading the plants. The overflow may be
diverted to storm sewers or directly into receiving waters. As a result, the water quality of the
receiving waters may be seriously impaired similar to the overflow situation in combined
sewers.
2.4.3 Wastewater treatment systems
There are approximately 400 wastewater treatment plants in Ontario. They are mostly
secondary treatment plants with phosphorus removal. Generally, organic and solids removal
at these plants is about 85-90% under normal operation conditions. Problems of wastewater
treatment systems are primarily associated with shock loadings, bypasses, and overloading
due to wet weather. Other associated problems are related to odour and sludge management.
2.4.4 Storm sewer systems
Separated storm sewers are usually designed for storms with return periods of two to five
years. As a result, sewer capacities are exceeded quite frequently. In addition to inadequate
sewer capacity, the gradually-varied nature of storm flow and/or hydraulic instability in
sewers (such as localized hydraulic jumps or waves) can also induce upstream and
downstream flooding. As in combined sewer systems, increased runoff after urban
development can cause greater rates of channel degradation downstream in drainage basins.
Over the past two decades at least, it has been realized that direct discharge of storm flows to
receiving waters can cause significant deterioration of the receiving water quality (Lightfoot,
1989); in contrast, point sources such as treatment plant discharges are usually adequately
regulated. As a result, the attention to storm sewer problems has been focused on their water
quality impact. Although the main sources of pollution of stormwater runoff are from
atmospheric deposition and washoff of accumulated pollutants on the land surface, it is
common for illegal connections of sanitary sewers and/or industrial waste flows to be partly
responsible for the contamination of storm water.
2.4.5 Control measures for sanitary sewer systems
Sanitary sewers are sized to convey peak and minimum wastewater flows without the
deposition of suspended solids. These sewers are designed to flow by gravity between one-
half and full depth. Collecting sewers gather flows from individual buildings and transport
them to an interceptor or main sewers. Maintenance holes (previously called manholes) and
other transition structures are usually built at every change in pipe size, grade and alignment.
Grades should be designed so that the criteria for maximum and minimum flow velocities are
satisfied. Pumping stations are used to equalize loadings and raise the hydraulic head so
wastewater can flow through wastewater treatment systems by gravity. Theoretically,
wastewater treatment plants should be able to handle the designed wastewater flows and no
sanitary bypasses or overflows are permitted. In practice, sanitary overflow points are built to
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spill excess wastewater to receiving water to prevent overloading of wastewater treatment
plants. However, wastewater treatment operators must inform local public health units if
there is a sanitary sewer overflow.
Control measures for sanitary sewer system are usually aimed at reducing extraneous flows
and rainfall-derived infiltration/inflow into the sewers. Regulations should be enforced to
prevent runoff from entering sanitary sewers and the direct connection of foundation drains to
sanitary systems. To reduce infiltration to sanitary sewers, inspection and repair of faulty
joints and leaks are required, as is good quality control during sewer construction. For
overloaded sanitary sewer systems, construction of relief sewers or tunnels parallel to the
existing lines may be needed to divert flows to alternative outlets.
2.4.5 Control measures for stormwater
Stormwater best management practices (BMPs) have provided a number of tools to decrease
the quantity and improve the quality of stormwater runoff at the source, along the drainage
system and at the outlet. These include such devices as downspout disconnection, stormwater
gardens, rain barrels, infiltration trenches, stormwater exfiltration/filtration systems, sand
filters, bio-retention areas, wet and dry detention ponds, and constructed wetlands. However
most "Downstream Outlet" best management practices require a significant amount of land to
host them, which is not generally available in downtown urban environments. The
opportunity for green roofs to act as source level viable stormwater management devices is
logical, since flat rooftops recreate the open space, previously at ground level, that has
otherwise been eliminated for vegetation (Jennings et al. 2003).
Unlike some other BMPs, green roofs may be able to offer controls and improvements in
both the quantity and quality of stormwater runoff. Graham and Kim (2003) conducted a
study in Vancouver, BC which showed that suitably designed green roofs have great potential
benefit in terms of protecting stream health and reducing flood risk to urban areas. The
modeling results for a 50-year watershed retrofit scenario also show that green roof re-
development on existing buildings could help to restore watershed health over time. Not only
are green roofs able to filter contaminants out of rainwater that has flowed across the roof
surface (Dramstad et al., 1996), but they can also degrade contaminants, either by direct plant
uptake, or by binding them within the growing medium itself (Johnston and Newton, 1996).
Numerous studies have demonstrated quantitatively that a properly installed and maintained
green roof will absorb water and release it slowly over a period of time, as opposed to a
conventional roof where stormwater is immediately discharged. Typical extensive green
roofs, depending on the substrate depth, can retain 60 to 100% of the stormwater they receive
(Thompson, 1998). In addition, according to the ZinCo planning guide (1998), living roofs
are normally able to retain 70 to 90% of the stormwater that falls on them during the summer
months, depending on the frequency of rain and drying rates. In winter months, green roofs
are predicted to retain 40 - 50% of the stormwater. These data are subject to variation based
on variations in climatic conditions. The amount retained also depends on numerous factors
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such as the volume and intensity of rainfall, the amount of time since the previous rainfall
event, and the depth and saturation level of the existing substrate (Monterusso, 2003).
Several studies conducted in Germany have shown that a green roof with a substrate depth of
2 to 4 cm with a vegetation mix of mosses and sedum can retain 40 to 45% of the annual
rainfall that falls on it (Liesecke, 1998). By increasing the depth of the substrate to 10 to 15
cm and changing the vegetation to a mixture of sedum, grasses, and herbs, green roofs can
retain up to 60% of stormwater on an annual basis (Liesecke, 1993). Liesecke also indicated
that there were noticeable differences between retention in warm weather and in cool
weather. In warm weather, shallow substrate depth can retain 11% more stormwater than it
can during cold weather (Liesecke, 1993). For deeper substrates, the effect was even more
pronounced (20% more in warmer weather).
Liptan et al, (2003) demonstrated similar findings. Within a 15-month monitoring period,
they found that precipitation retention was approximately 69% of the total. However,
between December and March the rainfall retention was 59%, while from April to November,
rainfall retention was 92%.
Research conducted by Jennings et al. (2003) in North Carolina showed that a green roof can
retain up to 100% of the precipitation that falls on it in warm weather. However, the
percentage retained for each storm decreased when there had not been an adequate amount of
time between each storm event. As shown in Table 1, the percentage retained for each storm
decreased with each respective rain event. The percentage of the stormwater retained dropped
from 75% to 32%. According to the experimental results, Jennings et al. concluded that the
capability of green roof retention is highly dependent on the volume and intensity of rainfall.
Table 2.2
April 2003 hydrologic retention for the WCC green roof in Goldsboro, NC [ after
Jennings et al. (2003) ]
Storm Event Rainfall (in) Green roof
Runoff (in) Retained (in) % Retained
7 April 2003 0.89 0.22 0.67 75
8-9 April 2003 1.02 0.57 0.45 44
9-11 April 2003 1.63 1.11 0.52 32
Rowe et al. (2003) found a similar result during their experiments. Their results showed that
on average green roofs can retain 61% of total rainfall. During light rain events (<2mm
daily), their green roof retained up to approximately 98% of rainfall, whereas the same green
roof was capable of retaining only 50% of the heavy rain events (when rainfall >6mm).
As Jennings et al. (2003) concluded, the water holding capacity of the substrate was found to
depend on the volume and intensity of the rainfall. Further, both Jennings et al. (2003) and
Rowe et al. (2003) found that their green roof was able to reduce the peak flow and the time
to peak (by 2 to 4.5 hours) when compared to a standard conventional roof (Figure 2.3). Liu
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(2003) also found a stormwater runoff delay on green roofs. During a light rain (19mm in 6.5
hours), the green roof delayed the discharge of stormwater for 95 minutes.
Figure 2.3
Relationship between the peak flow and runoff
on green roof [ after Jennings et al. (2003)]
Several studies have shown that, in most cases, increasing roof slope does not necessarily
increase runoff volume. Liesecke (1999) conducted studies on a green roof with 8.7% slope
and found that the annual retention rates ranged from 55% to 65%, and were considered
comparable to 2% slope roofs. Research that was done by Rowe et al. (2003) also indicated
that retention percentages were unaffected by green roof slope. Schade (2000) had also
reported similar findings that on green roofs with slopes ranging from 2% to 58% there were
constant water retention rates.
Green roofs not only reduce the quantity of runoff from roofs but can also filter contaminants
from rainwater. According to the United States Environmental Protection Agency (USEPA)
(2003), “the most recent National Water Quality Inventory reports that runoff from urbanized
areas is the leading source of water quality impairments to surveyed estuaries and the third-
largest source of impairments to surveyed lakes”. Most of the stormwater runoff enters water
bodies directly without any treatment. Other problems are also associated with regular surface
runoff, such as higher surface water temperatures due to the water travelling across hot,
impervious surfaces like roofs, roads and parking lots (USEPA, 2003).
The substrate on green roofs has the ability to retain particulate matter in the stormwater and
to reduce the quantity of runoff and, as a result the total mass of pollutants that flow off the
roof. Thus, the stormwater runoff quality as well as the receiving surface water quality can be
improved. Large numbers of studies have been conducted in Germany and Switzerland
regarding green roof runoff quality. Dramstad et al. (1996) demonstrated that the physical and
chemical properties of the growing substrate, as well as the green vegetative cover help to
control the nitrogen, phosphorus, and contaminants generated by industrial activities, which
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exit the roof surface. In some cases these substances can be taken up and broken down by the
plants themselves (Johnston, 1996), but most of the time heavy metals and nutrients that exist
in stormwater are bound in the green roof growing substrate instead of being discharged in
the runoff. Johnston and Newton (1993) also concluded that over 95% of cadmium, copper
and lead and 16% of zinc can be removed from the stormwater runoff through binding and
uptake in the growing substrate.
The Toronto and Region Conservation Authority is monitoring stormwater performance of a
green roof at York University (TRCA 2005). The objective of the study is to evaluate the
effectiveness of green roof in reducing the quantity and improving the quality of stormwater
runoff in Toronto’s Remedial Action Plan (RA) Area of Concern (AOC). The research site is
located on the Computer Science and Engineering building on the campus of York University
in the North West part of Toronto. The project consists of two roofs: one with a Sopranature
green roof by Soprema and another non green roof with shingles. Both roof surfaces have a
10% slope. The shingled roof is 131 sq. m. while the Soprema Green Roof (SGF) is
241 sq. m. The SGF consists of a 140 m substrate and is vegetated with wildflowers. The
substrate is composed of crushed volcanic rock, compost, blonde peat, cooked clay and
washed sand. It is designed to be light weight, retain rainwater, and reduce compaction. An
irrigation system is installed on the roof and is operated automatically by soil moisture
sensors.
Rainfall volume, water runoff quantity and quality from both surfaces, ambient air
temperature, relative humidity, soil temperature, and soil moisture, have been monitored
continuously since April 2003. Tables 2.3 and 2.4 summarize the effect of the green roof on
the runoff volume and peak flow reductions in 2003 and 2004. It is noted that the green roof
provided significant reductions in runoff volume and peak flows. On average, the runoff
volume could be reduced by almost 65% while peak flow could be reduced by almost 98% of
most of the rainfall less than 30 mm. Water quality analysis was conducted for 23 events and
it was found that the green roof could improve water quality benefits such as suspended
solids, copper and Polycyclic Aromatic Hydrocarbons (PAHs). Table 2.5 summarizes the
results on water quality
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Table 2.3
Green roof runoff volume reduction for 2003 and 2004 monitoring seasons
(TRCA 2005)
Measured outflow per unit area
(L/m2) Difference of
inflow vs outflow volume
in %
Year Total rainfall
(mm)
Garden Control Garden
2003 663.8 304.8 675.8 54.1
2004 443.1 108.1 388.7 75.6
Table 2.4
Peak flow reductions for a range of event sizes (TRCA 2005)
Rainfall event category Average difference in peak flow control vs garden
in %
20-29 mm 85.1
30-39 mm 68.2
?40 mm 50.3
Table 2.5
Comparison of concentrations for selected parameters
from the control roof and the garden (TRCA 2005)
Flow-weighted mean concentrations Parameter Guideline Control Garden Loading difference
control vs garden
in %
Suspended solids (mg/L) - 6.34 2.33 82.0
Total phosphorus (mg/L) 0.03 0.078 0.577 -276.0
Total Kjeldahl Nitrogen (mg/L) 3.2 0.711 1.61 -15.6
Copper (µg/L) 5 111 42.9 79.8
Zinc (µg/L) 20 10.8 8.2 60.5
Escherichia Coli (#/100 mL) 100 549 662 34.3
PAH; Phenanthrene (ng/L) 30 191.3 31.6 89.8
PAH: Fluoranthene (ng/L) 0.8 275.7 30.7 93.1
Note: Guidelines listed are Provincial Water Quality Objectives (PWQO) where available.
For parameters with no PWQO, the Canadian Water Quality Guideline is used.
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2.5 Air quality impacts
Declining air quality is an ongoing problem in cities globally, and solutions are being
proposed. Some have been acted upon, ranging from local initiatives to global accords.
Among these are both restriction of point-source emissions and restoration of biological
systems that reduce airborne contaminants. In cities there is strong interest in measuring and
dealing with air pollution levels since air contaminants are intensified due to the density of
human activity, including use of fossil fuels, the presence of the urban heat island and the
absence of natural biological controls. Inter-regional transport and global warming concerns
serve only to heighten the issue, as the magnitude and frequency of smog alerts and summer
heat waves increase (MacIver and Urquizo 1999). Evidence suggests that green roofs provide
one opportunity to reduce local air pollution levels by lowering extreme summer
temperatures, trapping particulates and capturing gases.
Akbari et al. (2001) and Kats (2003) discuss cool roofs and green roofs in terms of their
potential indirect effect of reducing CO2 emissions from power plants due to a reduction in
the demand for summertime peak-period cooling.
It is well known that smog forms when nitrogen oxides (NOx) react with volatile organic
compounds, a process that is accelerated by higher ambient temperatures. In the report by
Rosenfeld, et al. (1998), which looked at strategies to cool urban areas and reduce the heat
island effect and smog in Los Angeles, it was noted that on a typical summer day in Los
Angeles, 1350 tons of NOx and 1500 tons of volatile organic compounds (VOCs) react to
form ground level ozone. By calculating the small NOx savings from avoiding air
conditioning electricity use and combining it with the NOx avoided by cooling Los Angeles
up to 3 degrees, these researchers estimated that a 10% reduction in smog is equivalent to
reducing precursors by about 25%, that is, reducing NOx releases by 350 tons per day. Los
Angeles has a smog offset trading mark that trades NOx at $US3,000 per ton. To convert this
to c/kWh of peak power they multiplied it by 0.5kg/MWH to get .15c/kWh. Hence, the 350
tons/day of avoided “equivalent” NOx is then worth about $US1,000,000 per day to Los
Angeles. The researchers then converted this saving to a yearly value, to find, on average, the
100 smog days experienced might provide a $US100 million per year saving to a city as large
as Los Angeles.
Yok and Sia (2005), in their report on a pilot green roof project in Singapore, noted air
quality improvements due to reduction of sulphur dioxide by 37% and nitrous acid by 21%.
However, nitric acid increased by 48% and particulates (PM 2.5 and 10) also increased,
possibly from re-suspended chips related to gravel ballast and bare spots on the green roof,
though the particle number concentration decreased by 6% on the green roof.
Johnson and Newton (1996) estimate in urban forestry studies that 2,000 m2 of unmowed
grass on a roof could remove as much as 4,000 kg of particulates from the surrounding air by
trapping it on its foliage.
Report on the Environmental Benefits and Costs of Green Roof Technology for the City of Toronto
Prepared by Ryerson University Page 24
Several researchers report that vegetation benefits air quality by trapping particulates and
dissolving or sequestering gaseous pollutants, particularly carbon dioxide, through the
stomata of their leaves (Nowak and Crane, 1998). Their research has predicted rates of
entrapment and mitigation, given seasonality, daylight hours, and species, etc., and their
model is currently being studied in Toronto (Currie, 2005).
2.6 Green amenity space
Some researchers believe that the need for meaningful contact with nature may be as
important as people’s need for interpersonal relationships (Kaplan, 1993). Moreover,
impediments to meaningful contact with nature can be seen “as a contributing factor to rising
levels of stress and general dissatisfaction within our modern society” (Zubevich, 2004).
Many urban buil