Greenroofs are vegetated roof covers, with growing media and plants taking the place of bare membrane, gravel ballast, shingles or tiles. The number of layers and the layer placement vary from system to system and greenroof type, but at the very least all greenroofs include a single to multi-ply waterproofing layer, drainage, growing media and the plants, covering the entire roof deck surface. There are two main types of greenroofs – extensive or intensive - although a greenroof is often designed with features of both and then are referred to as either semi-extensive or semi-intensive.