A green roof is any kind of vegetation that is intentionally grown on the roof of a building or structure. Usually it takes the form of a rooftop garden. A green roof can be fairly simple and involve only ground cover plants that are self-sustaining, or it can include irrigation systems, trees, and space for people.
Poster of Peggy Notebaert Nature Museum extensive green roof (photo credit: © Conservation Design Forum
Searle Chemistry Laboratory's extensive green roof (bottom right corner)