Green roof

A flat roof designed to attenuate rainfall by temporarily storing water on the roof and releasing it at a controlled rate through restricted outlets, reducing peak loads on drains and watercourses. Blue roofs form part of sustainable drainage (SuDS) strategies and place high demands on the reliability of the waterproofing and the structure.

A flat roof arrangement in which moisture-resistant insulation, typically extruded polystyrene, is laid above the waterproofing membrane and held down by ballast or paving. The membrane is protected from thermal cycling, ultraviolet exposure and mechanical damage, but the insulation must tolerate contact with water.

Loose material, typically rounded gravel or paving slabs, laid over a roof membrane or over inverted roof insulation to hold the system down against wind uplift and to protect it from ultraviolet light and mechanical damage. Ballasted systems avoid fasteners and adhesives but add significant dead load and make leak tracing more difficult.