A major source of stormwater runoff in urban areas comes from rooftops. A green roof is a rooftop surface with plants and a soil medium that helps to capture rainwater from storms before it can enter the stormwater system. Green roofs can also absorb air toxins, provide habitat and food, cool the building and save energy, and extend the roof lifespan by two to three times.
A number of municipalities have enacted bylaws to reduce the impacts of rooftop runoff, including downspout disconnect bylaws, rain barrel programs, and green roofs. The City of Toronto has a Green Roof Bylaw that requires the installation of a green roof on most new buildings and some additions, and mandates standards for construction and maintenance of green roofs.
Read more at: https://unflood-ontario.dreamhosters.com/green-roofs/