EcoGeek has a lovely article on Green Roofs.
Green roofs have a layer of living plants on top of the structure and the waterproofing elements.
There are two types of green roofs:
- Intensive green roofs have a soil depth of a foot or more, and require substantial structural elements to support the weight of the whole roof. They can sustain a wide range of plant species and typically require a fair amount of regular maintenance.
- Extensive roofs, much shallower, typically only 2 to 4 inches deep, and are planted with particularly hardy plants.
A green roof can reduce roof stormwater runoff, protect the roof membrane from sunlight (keeps the roofing material from degrading), keep the roof cooler, it’s a source of oxygen and provides a habitat for some birds.
Check the article at EcoGeek for more information.