Every year, cities are challenged by stormwater flooding. Urbanization has resulted in more hard surfaces such as roads, parking lots and buildings, where water cannot be absorbed. As more greenspaces are paved over for development, our sewer system becomes increasingly strained, especially during extreme rainfall events. During exceptionally heavy rain events, sewer systems can become overloaded with stormwater causing overflows. These sewer overflows cause flooding and often result in sewage entering local waterways. Green spaces like parks, gardens, and green roofs act as a sponge to soak up rain, absorbing it before it can enter the sewer system.
Toronto’s Green Roof Bylaw
In 2009, Toronto introduced the Green Roof Bylaw to require and govern the construction of green roofs for new development or additions greater than 2,000 square metres in gross floor area. Green roofs function as miniature ecosystems, offering a range of environmental benefits, with stormwater management being a primary advantage.
When it rains, green roofs act as natural sponges. The vegetation intercepts rainwater, allowing it to be slowly absorbed by the underlying soil layer. This captured water is then either used by the plants or retained in the soil for extended periods. This significantly reduces the amount of stormwater runoff that can overwhelm sewer systems during heavy downpours.
Studies by the University of Toronto have shown that green roofs—depending on factors like plant type, soil depth, and irrigation schedule—retain between 30 and 60 percent of rainwater, significantly reducing the burden on sewers. The study found that a green roof with a 15 cm soil depth planted with drought-resistant succulents could retain up to 75 percent of rainwater compared to a conventional roof. This captured water is slowly released over time and prevents sudden surges that overwhelm the system.
Toronto’s Eco-Roof Incentive Program
The City of Toronto’s Eco-Roof Incentive Program was developed to drive the installation of green roofs and cool roofs. Rebates of up to $100,000 are available to property owners who install green roofs on their buildings. To date, the program has supported over 627 eco-roof projects, creating a combined green roof and cool roof area exceeding 1.14 million square metres.
Beyond the impressive numbers, the City estimates that green roofs funded by the program divert over 18.9 million litres of stormwater annually, significantly reducing the risk of flooding. Additionally, these eco-roofs are estimated to have contributed to a reduction in energy consumption by over 2,300-megawatt hours and avoided over 434 tonnes of greenhouse gas emissions.
This year, the City of Toronto is co-hosting the CitiesAlive conference from November 6-9, 2024. CitesAlive 2024 will explore the role of green roofs and green infrastructure in advancing climate mitigation and adaptation, addressing housing affordability, social and economic equity, and moving towards the decarbonization of landscapes. Find out more at: CitiesAlive.
To learn more or to apply, visit Eco-Roof Incentive Program – City of Toronto. Together, we can build a resilient, more sustainable city.
