A green roof is basically a layer of vegetation grown on the roof of a building. Green roofs can be installed on lots of different kinds of buildings, whether they are private homes or large industrial skyscrapers. In addition to their aesthetic benefits, green roofs also help regulate temperature. Unlike the typical black/gray roof that can is about 90 degrees Fahrenheit in the summer, the surface temperature of a green roof is usually much cooler than the air. Green roofs reduce the amount of energy a building uses, because they act as an insulator. Even though a green roof is typically more expensive and time consuming to install, the cost is offset by the energy use savings and reduced storm water management costs that a green roof provides.

http://www.epa.gov/heatisland/mitigation/greenroofs.htm