Densely populated urban areas are often dominated by impervious surfaces and lack green space, contributing to common—and costly—environmental problems such as higher temperatures, stormwater management challenges, and decreased water quality.
To combat this, Toronto turned to an underused resource: the city’s nearly 500 million square meters of rooftop. The city government passed legislation to mandate and support cooling, absorbent green roofs. By replacing dark, impervious roof surfaces with soil and vegetation, green roofs retain stormwater, improve air quality, lower ambient temperatures, reduce building energy use, and create attractive and useful outdoor amenities.
Green roofs have the potential to mitigate Toronto’s specific environmental risks, including extreme temperature variations, air pollution, and energy insecurity. The city is also prone to flooding; in 2013, a single flood damaged 450,000 homes and left 750,000 people without power.
“We have cold winters, but we also have very hot summers, and given that we’re in an intense urban environment, temperatures are exacerbated by the urban heat island effect.”—Shayna Stott, environmental planner, Toronto Planning Division
Toronto became the first city in North America to mandate and govern the construction of green roofs on new development in 2009. The Green Roof Bylaw (which includes a Green Roof Construction Standard) and the parallel Eco-Roof Incentive Program are responsible for over 111,400 square meters of new green space, an estimated reduction in citywide temperature, and the widespread promotion of green roofs.
The bylaw was developed over several years, beginning in 2005, when Toronto commissioned Ryerson University to study the impact of greening all of the city’s available large rooftop space. The study provided evidence to support the efficacy of green roofs and prompted the city to launch a pilot cost-share program to incentivize green roof construction on private buildings.
In 2009, the city adopted the Toronto Green Roof Bylaw, which took effect in 2010 for most building types, and in 2012 for new industrial development. The bylaw and associated Green Roof Construction Standard incorporated feedback from numerous engagement sessions that included both industry and community stakeholders.
The bylaw established a graduated green roof requirement, ranging from 20 to 60 percent coverage, for all commercial, institutional, and residential buildings with a minimum gross floor area of 2,000 square meters. The bylaw’s Construction Standard established minimum requirements and mandatory provisions in 14 categories, including green roof assembly; waterproofing and drainage; vegetation selection and performance; and occupant health and safety.
Consideration of the local planning context resulted in exemptions for residential developments less than six stories tall. Industrial building owners also have the option to choose between a 10 percent green roof or a 100 percent “cool roof” with other stormwater retention measures sufficient to capture 50 percent of annual rainfall. “Cool roofs” use highly reflective materials to promote thermal emittance and reflect solar radiation, thereby lowering the roof surface temperature and, by extension, cool the building and surrounding area.
Developers can use the Green Roof Screening Tool to quickly determine if and how the bylaw will affect their projects. For building owners who cannot or choose not to meet the green roof requirements, the bylaw offers a cash-in-lieu alternative; however, only about 5 percent of developers choose this option. These payments fund the Eco-Roof Incentive Program.
The Eco-Roof Incentive Program, also established in 2009, provides funds to retrofit existing roofs and is open to schools, nonprofit buildings, and buildings measuring less than 2,000 square meters. The program’s success is due in part to its dedicated funding stream and its substantial grants; eligible green roof projects can receive $100 per square meter, up to $100,000, and cool roof projects can receive $2 to $5 per square meter up to $50,000.
“Implementation has gone fairly smoothly. We attribute that to the early industry consultation that occurred, leadership within the development industry to build capacity, and development of a requirement that was tailored to work well within the local existing regulatory structures that development faces.”—Shayna Stott, environmental planner, Toronto Planning Division
The impact of the Green Roof Bylaw and Eco-Roof Incentive in Toronto is significant. As of 2019, approximately 620 new green roof permits have been required under the bylaw, totaling over 501,000 square meters of new green roof area. The incentive program has provided funding for over 70 green roofs and 336 cool roofs since 2010.
The greening of just 5 percent of the city’s area through green roofs lowered citywide temperature by an estimated 1.5° to 2°C, with a greater temperature reduction in high-density areas and with a direct 4° to 5°C roof surface cooling effect. Each year, green roofs retain 12,300 cubic meters of stormwater runoff (equivalent of 50 Olympic-sized swimming pools), reduce polluting sewer overflows to allow for three extra beach days per year, and prevent 220 metric tons of greenhouse gas emissions.
Green roofs can provide significant benefits to building owners. For example:
- By mitigating extreme temperature fluctuations, green roofs can extend the life of a roof by about 20 years.
- In Toronto, buildings with green roofs save an average of 1,000 megawatt-hours annually, primarily from reduced air-conditioning demand.
- Green roofs can turn unused or underused space into an attractive outdoor amenity.
Since implementation, Toronto has made minor policy adjustments to the bylaw and incentive program. For example, after learning that the cost of the structural assessments was a barrier for smaller developments, Toronto implemented an additional structural assessment grant.
Today, green roofs are a rapidly growing industry, creating 125 full-time jobs in Toronto related to manufacturing, design, installation, and maintenance. Toronto is widely recognized as a leader and authority for its successful green roof program.
Sources and Interviewees
- Banting, Doug, et al. Report on the Environmental Benefits and Costs of Green Roof Technology for the City of Toronto. 2005. https://web.toronto.ca/wp-content/uploads/2017/08/8f39-Report-on-the-Environmental-Benefits-and-Costs-of-Green-Roof-Technology-for-the-City-of-Toronto-Full-Report.pdf.
- Stott, Shayna. Environmental Planner, City Planning Division, City of Toronto (Personal interview, 2019).
- “About Green Roofs,” Green Roofs for Healthy Cities–North America, accessed July 10, 2019, https://www.greenroofs.org/about-green-roofs/.
- “Toronto’s Resilience Challenge,” 100 Resilient Cities, accessed July 10, 2019, https://www.100resilientcities.org/cities/toronto/.
- Benfield, Kaid. What Cities Can Learn from Toronto’s Green Roof Policy. 2012. https://www.citylab.com/design/2012/04/what-cities-can-learn-torontos-green-roof-policy/1846/.
- “What is a ‘Cool Roof’?” CertainTeed Corporation, accessed July 10, 2019, https://www.certainteed.com/residential-roofing-commercial-roofing/what-cool-roof-0/.
- Will Koblensky, “How Toronto Became a Hub for Green Roofs,” Torontoist, September 21, 2016, https://torontoist.com/2016/09/how-toronto-became-a-hub-for-green-roofs/.
- “City of Toronto’s Eco-Roof Incentive Program and Green Roof Bylaw,” C40, accessed July 10, 2019, https://www.c40.org/case_studies/city-of-toronto-s-eco-roof-incentive-program-and-green-roof-bylaw.
- “The Business Case for a Green Roof,” Aquicore, accessed July 10, 2019, https://aquicore.com/blog/the-business-case-for-a-green-roof/.