Green roofs

Green roofs are roofs covered with a multi-layer system consisting of (from top down): growth medium, drainage layer and water-proofing membrane. Green roofs delay rainwater run-off, and the total run-off volume is less than that from conventional tiled roofs, for example. The degree of delay and volume reduction increases with the thickness of the growth medium. Green roofs insulate buildings against warming and can provide a habitat for certain insects and birds. Retained water evaporates.

Intensive green roofs, also known as roof gardens, are constructed with such a thick layer of soil that a wide range of plants – from grasses and herbaceous plants to bushes and small trees – can grow on the roofs (see the figure). The roofs are normally designed for use as recreational spaces and resemble gardens in terms of maintenance, watering and pruning. Intensive green roofs effectively retain rainwater. They are heavy and rarely installed on existing buildings.

Figure 1: Intensive green roof  (Photo: www.jetsongreen.com)

Figure 2: Sketch of the construction layers of green roofs