Ballast

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.

The suction force exerted on a roof by wind flowing over and around a building, greatest at perimeters and corners. Roof systems resist uplift through mechanical fastening, adhesion or ballast, with design loads derived from wind actions codes and system resistance established by testing.

A roof deliberately covered with growing medium and vegetation over a waterproofing system, usually incorporating root barrier, drainage and filter layers. Extensive green roofs use shallow substrates with low-maintenance planting such as sedum, while intensive green roofs support deeper, landscaped schemes. They contribute to rainwater attenuation, biodiversity and reduced urban heat island effect.