There are rooftop gardens and green roofs now available as well. There is a bit of a difference between these two. A green roof is for roofs that do not have a large load bearing capacity. They are thinner in layer than rooftop gardens can be, and are often formed of plants that can last during both heavy rain events and long periods of dry weather. Rooftop gardens can have much thicker soil layers and may even have trees and shrubbery in them. They are places for people to hang out and play on.

Both green roofs and rooftop gardens decrease heat absorption/loss (depending on the season), decrease runoff from flood events, and clean the air naturally. They also are a bit more aesthetically pleasing than tar-paper, tiled, and shingled roofs often are.

The first site is what someone wants to see happen to the skyline of New York City.
www.gardenvisit.com/landscape_archtecture/landscape_debate/new_york_roof_gardens

The other two are about a roof garden and a green roof to show some of the differences.
realestateiae.wordpress.com/2010/05/24/roof-gardens

daily.swarthmore.edu/2010/10/06/green-roofs