2. Three types of green roofs

There are three coexisting processes for designing and building a green roof:

• The roof-garden or intensive vegetation requires a minimum of 30 cm of soil, which allows traditional vegetation to be planted;

• the vegetation layer, usually based on sedums (low-growing creeping perennials) or extensive vegetation that can make do with less than 10 cm of substrate;

• an intermediate or semi-intensive vegetation system with a substrate thickness of between 10 and 30 cm, allowing greater plant diversity (figure 1 ).