Warm roof

A roof construction in which the insulation is placed at ceiling level below the structural deck, leaving the deck and the void above it at close to external temperature. The void normally requires ventilation to remove moisture vapour, and this arrangement is discouraged for flat roofs in current UK guidance because of condensation risk.

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.

A membrane or sheet installed on the warm side of insulation to restrict the passage of water vapour from inside the building into the roof or wall construction, reducing the risk of interstitial condensation. Materials range from polyethylene sheet to bitumen membranes and foil composites, and effectiveness depends on sealed laps, edges and penetrations.

Condensation that occurs within the layers of a roof, wall or floor construction rather than on its internal surface, when water vapour moving through the build-up reaches a plane at or below its dew point temperature. It can degrade insulation, corrode fixings and decay timber, and is controlled through vapour control layers, ventilation and correct layer sequencing.

Rigid insulation boards manufactured or cut to a wedge profile and laid to a designed scheme to create drainage falls on a flat roof, avoiding the need for screeds or firrings. Schemes combine tapered and flat boards to achieve both the required fall and the target U-value.