Green roofs are vegetated layers that sit on top of the conventional roof surfaces of a building. Usually a distinction is made between ?extensive? and ?intensive?. These terms refer to the degree of maintenance the roofs require.
Intensive green roofs are composed of relatively deep substrates and can therefore support a wide range of plant types: trees and shrubs as well as perennials, grasses and annuals. As a result they are generally heavy and require specific support from the building. Intensive green roofs (what most people think of as roof gardens) have in the past been rather traditional in their design, simply reproducing what tends to be found on the ground, with lawns, flower beds and water features. Because of their larger plant material and horticultural diversity, intensive green roofs can require substantial input of maintenance resources.
Extensive green roofs are composed of lightweight layers of free-draining material that support low-growing, tough drought-resistant vegetation. Generally the depth of growing medium is from a few centimetres up to a maximum of around 10cm. Extensive green roofs can be designed into new buildings or ?retrofitted? onto existing buildings.