An example: the City of Toronto

Hitesh Doshi, Professor, Ryerson University, Department of Architectural Science


Doug Banting, Professor, Ryerson University, Geography Department


James Li, Professor, Ryerson University, Civil Engineering Department


Paul Missios, Professor, Ryerson University, Economics Department


Angela Au, Research Assistant, Ryerson University


Beth Anne Currie, Research Assistant, Ryerson University (Presently with Urbanspace Property Group)


Michael Verrati, Research Assistant, Ryerson University

Abstract

A study to determine the environmental benefits and costs of green roof technology for the City of Toronto has been recently completed. The study is currently undergoing review. This presentation will discuss the approach that was taken to determine the benefits of green roof on a city scale and the preliminary findings. 
The purpose of this study was to determine the environmental costs and benefits of green roof at the municipal level. Such an exercise requires the compilation of very specific information from many diverse sources. The approach involved the following:
identification of environmental benefits at municipal level,
quantifying the impact of green roof for each of the benefits,
monetizing the benefit,
applying the benefits on the citywide basis based on actual distribution of buildings.

 
Identification of benefits

A literature review on this subject assisted in narrowing down the quantifiable benefits of green roof at the municipal level. These were related to reduction in water flowing into the stormwater system, the CSO system, improvements n air quality, mitigation of urban heat island effect and the reduction in energy consumption due to reduce space heating and cooling needs.

Quantification of impacts

Once the benefits were identified it was necessary to quantify the impacts of green roofs to each of these benefits. For the purpose of this study the impacts were quantified based on research that has been reported to date. In as much as possible this study relied on local research. For instance the impact of green roofs on stormwater has been modeled for a local watershed (the Markham branch of Highland Creek). This study relied on the results of the work on local watershed to quantify the impact of stormwater for the rest of the City of Toronto. Another example is the impact of green roof on air quality. For this, the study used the work done on the impact of air quality in downtown Toronto.

Monetization of benefits

Once the impacts were quantified in terms of their respective benefit e.g. stormwater benefits were measured as reduction in water flow, they needed to be monetized so that an economic value could be developed for each of these benefits. Some of the work stated earlier built into it the monetary considerations of each of the benefit. For others this study had to develop functions to transfer the benefit into monetary terms. Again this study relied on local data such as from recently completed study on green roofs by the City of Waterloo for this information.

City of Toronto specific determination of benefits – use of building inventory data

Finally this study was required to determine benefits taking into account the land use in Toronto. This was achieved using a GIS database. This study was based on aggregation of results based on building distribution and land use within each watershed. Consideration was given to determine the impact of different levels of green roofing. However the final calculations were based on 100% greening of eligible roofs. 
It is expected that as this project continues a method can be developed to allow different scenarios to be constructed. 
The following assumptions had to be made about the eligible buildings for green roof applications in this study:
Green roofs are considered on roofs with relatively low slope i.e. "flat" roofs with slopes less than 2%. It is possible to install green roofs on sloped roof greater than 2%. Many low-rise residential buildings, which constitute large percentage of total available roof area, have sloped roofs. However, application of green roofs on sloped surfaces is not very common and the benefits that apply to applications on “flat” roofs do not necessarily apply to sloped surfaces. The reported research on the benefits of green roofing is relevant for construction practices used in “flat” roofs and cannot easily be extrapolated to green roofs over sloped surfaces. For this reason at this time on low sloped or “flat” roofs are considered as eligible roofs for greening.
Green roofs will be installed on building that has a roof area of at least 350 sq. m. On buildings with low slope roofs the roof surfaces are often used for positioning equipment for heating, cooling and ventilation purposes. It was determined based on empirical evidence that on average the roof would have to be at least 350 sq. m. before any significant free area would be available for greening.
Greenery over underground parking garages or similarly non-conditioned enclosed spaces at grade level is excluded from consideration as green roofs in calculating the benefits in this study. There are three reasons for this assumption. Firstly there was no easy way to identify and measure the amount of such spaces in the GIS database. Grade level greening shown on GIS database do not distinguish between areas over spaces and areas over ground. Secondly the greening of such spaces at grade level is often covered by requirements related to site plan and development or the need by owners to maintain certain aesthetic appeal of their property. This situation would not require a separate incentive or policy for more widespread use. Thirdly the benefits resulting from such application of greenery would 
Where installed on a building green roofs will occupy an area of at least 75% of the roof footprint. The benefits in this study are estimated based on the use of extensive green roof system with a certain minimum amount of coverage. Use of intensive green roofs or greening on roofs using planters or greenhouses will result in benefits that are highly dependent on the nature of design and layout of such systems. The benefits of using these systems in terms of stormwater control and energy usage will be lower than that for a typical extensive roof. This assumption will therefore provide an upper limit of the social and environmental benefit at the municipal level.
The total area available for installation of green roofs was calculated as shown in Table 1.

Tabele 1 Available areas for green roof implementation

Hitesh Doshi
Ryerson University Toronto
350 Victoria Street
Toronto, ON, Canada M5B 2K3
hdoshi@ryerson.ch