An aerial view of most urban areas shows swathes of asphalt, black tar and gravel-ballasted rooftops. Heat radiates off of the dark roofs, and water rushes over the hard, hopefully impermeable surfaces. Yet, there is a new trend that breaks up the monotony of common roofs: green rooftops. Long popular in Europe, green rooftops have begun to appeal to homeowners, businesses and even cities as an attractive way to promote environmentalism while solving the problems of conventional roofs. Green roofs supplement traditional vegetation without disrupting urban infrastructure -- they take a neglected space and make it useful. Green roofs last longer than conventional roofs, reduce energy costs with natural insulation, create peaceful retreats for people and animals, and absorb storm water, potentially lessening the need for complex and expensive drainage systems. On a wider scale, green roofs improve air quality and help reduce the Urban Heat Island Effect, a condition in which city and suburban developments absorb and trap heat. Anyone who has walked across a scalding parking lot on a hot, summer day has felt one effect of an Urban Heat Island.
The layers of a green roof must, like any roof, accommodate drainage and protect the building from the elements with a waterproof membrane. But they also must create a growing area and potentially provide support, irrigation and root protection barriers while staying as light as possible. Two types of green roof exist: intensive and extensive. Intensive green roofs are essentially elevated parks. They can sustain shrubs, trees, walkways and benches with their complex structural support, irrigation, drainage and root protection layers. The foot or more of growing medium needed for an intensive green roof creates a load of 80-150 pounds (36-68 kilograms) per square foot. Extensive green roofs are relatively light at 15-50 pounds (7-23 kilograms) per square foot. They support hearty native ground cover that requires little maintenance. Extensive green roofs usually exist solely for their environmental benefits and don't function as accessible rooftop gardens.