A living green roof, is a layer of vegetation planted over a waterproofing system that is installed on top of a flat or slightly–sloped roof. Green roofs are also known as vegetative or eco–roofs. They fall into three main categories—extensive, intensive, and semi–intensive. Although there are no precise definitions of them, an extensive green roof has a shallow growing medium—usually less than six inches—with a modest roof load, limited plant diversity, minimal watering requirements, and is often not accessible. Intensive green roofs have more soil and a deeper growing medium—sometimes several feet—that can support a more diverse plant selection, including small trees. Thus, they have more substantial structural loads and need more frequent maintenance and watering. They are usually accessible. Semi–intensive green roofs include features of both types. The appropriate depth of any green roof depends on the roof structure, the plants chosen, annual rainfall, and storm water performance requirements.