All new residential, commercial and institutional buildings with more than 2,000 sqaure meters of floorspace in the City of Toronto will soon require a vegetated area, known as a green roof, on a portion of the rooftop. Toronto says it is the first municipality to pass a mandatory green roof bylaw, effective February 2010. 

Green roofs offer environmental benefits such as reducing storm water runoff, cutting energy consumption, lowering the temperature in urban areas, providing opportunities for local food production and adding to the esthetics of large buildings. A study commissioned by the city in 2005 says it will also save money by reducing expenditures on infrastructure for storm water runoff, erosion control measures, pollution control and energy costs.

City Council passed the bylaw based largely on the study by Ryerson University, which says the widespread greening of Toronto’s roofs could reduce local temperatures from 0.5 to two degrees C. The study said green roofs would result in improved air quality, and more energy savings from better solar reflectivity and insulation.

Green roofs include a waterproof membrane, layers for root resistance, drainage and a filter, a growing medium and vegetation. Intensive or active green roofs have a deep growing medium and can be used for recreational spaces. Toronto’s Manulife Centre has a green roof that was built over a parking garage about 25 years ago, and now has trees that are three stories high.

An extensive green roof has a shallower growing medium and the landscape is designed to be more self-sustaining, says the city. They are lighter than intensive systems and require less structural support and not as much maintenance. An example of this kind of roof is the Mountain Equipment Co-op building in Toronto.

Modular systems, which are ideal for existing roofs, are grown off-site in large trays or containers, and placed on the roof. They come in a variety of designs. There are also systems that are grown on farms like sod turf, then rolled up and transported to the rooftop, where they are rolled out.

Green Roofs for Healthy Cities says that Chicago leads North America in the number of green roofs in place, but that “the mandatory bylaw in Toronto may change that, resulting in approximately 50 to 75 new projects annually.”   Realty Times – June 25, 2009