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In 2009 the City of Toronto became the first North American city to require and regulate green roofs on new developments. Buildings that have a minimum Gross Floor Area of 2,000m2 require at least 20% coverage of available roof space. The percentage increases with the size. In Germany they have been using green roofs for over 100 years and Wikipedia reports that over 100,000,000 square feet of new green roofs are being constructed each year. The Rockefeller Centre in NY has had a green roof on the 7th floor since 1936. The Ford Motor company has a plant with a green roof covering 450,000 square feet! This week Microsoft announced that they will be putting up 164,000 square feet of a green roof on the Silicon Valley campus. In Peterborough Ontario the Canoe museum will have quite the roof.
Green roofs are growing. Loyd Alter reports that green roofs are changing architecture and planning. You can find more examples at Greenroofs.com. They have a database of green roof projects listing 35,281,358 ft² (3,277,816 m²) of green roof space.
Why go green up top? Haven Kiers at UC Davis Arboretum and Public Garden has a good list with the economic benefits including: increased energy efficiency, reduced cooling costs, prolonged membrane durability,marketing opportunities and fire prevention. Environment benefits include: Environmental benefits include improved stormwater management, creation and preservation of habitat/biodiversity, improved air quality, urban heat island effect reduction, and improved liveability/aesthetics.
There are two basic types of green roofs. The "intensive roof" has soil greater than 6 inches (15.24 centimeters) and hosts plants, shrubs, and small trees. This requires regular maintenance. The other type or "extensive roof" has soil under 6 inches/15 centimeters and features herbs, grasses, succulent plants. This approach requires little (potentially no) maintenance. However, there are new approaches that allow for more variety including "comprehensive green roofs" that have low weight and maintenance but a wider variety of plants.
Click to watch a video overview of green roofs here: