A green roof is a kind of building roof that is covered in vegetation. There are two kinds of green roof, extensive and intensive, categorized by their intended use and the nature of planting. The intensive green roof is named as such because it requires intensive maintenance and care. Intensive green roofs are more similar to garden plantings, involve soil depths suitable for a wide range of vegetation, and are often intended for occupation by roof users.

Extensive green roofs are usually accessible only for maintenance, and use planting strategies that require limited care and upkeep. Both kinds of roof reduce energy demand on buildings by providing insulation and absorbing heat from solar gain, and both can be used to augment evaporative cooling strategies. Green roofs fit into the cool roof concept.

http://www.holcimfoundation.org/T1304/Green_roof.htm