December 5, 2005

Toronto Roundtable pursues plan to increase green roofs on city buildings

A recent public meeting convened by Toronto's Roundtable on the Environment gave interested parties an opportunity to comment on "Making Green Roofs Happen," a discussion paper describing ways to encourage the use of rooftops in the city to grow vegetation.

The special meeting coincided with the release of a study titled "Environmental Benefits and Costs of Green Roof Technology for the City of Toronto." The results of the study were presented, together with an overview of how other cities encourage green roofs, proposed standards for green roofs and options for implementation of a green roof strategy in Toronto in 2006. The Roundtable is expected to make recommendations to Toronto city council in the near future for implementation of such a strategy.

The study was undertaken by the city of Toronto in partnership with Ontario Centres of Excellence - Earth and Environmental Technologies. Funding for the work came from the Federation of Canadian Municipalities Green Municipal Funds.

The environmental benefits cited for establishing green roofs in urban settings include:

*less stormwater runoff, with a projected reduction in stormwater flow of 12 million cubic metres (m3) per year.

*reduced energy consumption, with estimated citywide savings of $22 million from reduced energy for cooling (equivalent to 4.15 kilowatt-hours per square metre (KWh/m2) per year). The cost avoided due to reduced demand at peak times was estimated at $68 million.

*a reduction in the urban heat island effect. The study found that widespread greening of Toronto roofs would reduce local ambient temperature from 0.5 to 2 degrees Celsius and yield $12 million in citywide savings from reduced energy for cooling (equivalent to 2.37 kWh/m2 per year). The cost avoided due to reduced demand at peak times would be $80 million.

Improved air quality would be an added benefit, with lower levels of carbon monoxide, nitrogen oxides, ozone, particulate matter, sulfur dioxide and carbon dioxide emissions.

Other benefits would include infrastructure savings worth $79 million, erosion control measures savings worth $25 million, pollution control cost avoidance worth $13 million, and three additional "beach open" days per year worth $700,000.

The benefits were calculated based on a number of assumptions about green roofs. The study team determined that there are approximately 5,000 hectares or 50 million m2 of roof area eligible for green roofs in Toronto. Other working assumptions addressed size (greater than 350 m2), coverage (at least 75% of the roof area) and installation (over heated spaces). Excluded from consideration were roofs over underground parking garages and other non-conditioned enclosed spaces at grade level.

The "Making Green Roofs Happen" discussion paper presented more than a dozen recommendations and suggestions for promoting the implementation of green roofs in Toronto, starting with a proposed definition. For the purposes of promoting, encouraging, or requiring green roofs, a green roof should be defined as a system where a vegetated area becomes part of the roof. Green roofs should include the following components: vegetation, growing medium, filter layer, drainage layer, root resistance layer (if necessary), and waterproof membrane. They should also meet criteria outlined in a separate section of the paper.

Among other things, the paper recommends that:

*the city adopt a policy to install green roofs on all new municipal buildings where the roof profile so permits and, where feasible, on existing buildings when roofs are due to be replaced.

*efforts be made to work with the Toronto Atmospheric Fund (TAF) and the Better Buildings Program (BBP) to determine the feasibility of incorporating green roofs in provision of TAF or BBP loans.

*consideration be given to reducing water rates for properties with green roofs.

*Toronto and Region Conservation Authority be requested to provide advice on plant materials to encourage biodiversity.

*Toronto and Region Conservation Authority be requested to provide advice on plant materials to encourage biodiversity; and

*information be collected from green roof owners and installers about the costs of installation and maintenance, and about benefits such as energy savings. This would allow the city to provide examples of costs and benefits to building owners looking for such information. A list of green roofs in Toronto should also be made available to the public.

The "Making Green Roofs Happen" discussion paper or the "Environmental Benefits and Costs of Green Roof Technology" report may be requested by calling 416/395-7352 or viewed on-line at

In related activities, a University of Toronto researcher has delivered what is claimed to be the first-ever analysis of the ability of green roofs to keep buildings warm in winter.

"Everyone studies how green roofs operate in warm conditions," said Brad Bass of the U of T Centre for Environment. "No one else has looked at winter design."

Bass analyzed a test roof built in Ottawa by Karen Liu of the National Research Council's Institute for Research in Construction, to offer the first conclusive data that winter green roofs can help reduce heat loss and energy consumption during cold months.

The winter green roof uses evergreens - juniper shrubs - and a thicker soil base than typical leafy green roofs, which generally provide passive benefits to the environment by reducing the need for air conditioning on hot days. The winter roof was installed on both a standard test house and an energy-efficient winterized house. Bass used environmental systems performance software to chart the indoor temperature fluctuations in both buildings.

"The results for the winterized house were good, and the results for the regular house were dramatic," he reported. "The assessment opens up designers to considering winter roofs as part of a year-round energy efficiency strategy."

The winter green roof project was funded by Environment Canada, the National Research Council, the University of Toronto, the Office of Energy Research and Development (Natural Resources Canada) and the Climate Change Action Fund. More information is available from Brad Bass at the U of T Centre for Environment, 416/978-6285, E-mail

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