Flat roof

A roof construction in which the thermal insulation is placed above the structural deck, keeping the deck and supporting structure at close to internal temperature. A vapour control layer is normally installed on the warm side of the insulation to limit the risk of interstitial condensation.

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.

The designed slope built into a nominally flat roof so that rainwater drains towards gutters or outlets. Falls may be formed by the structure, screeds, firring pieces or tapered insulation, and design gradients are normally set steeper than the minimum finished fall to allow for construction tolerances and deflection.

A factory-manufactured polymeric waterproofing sheet, typically EPDM, TPO or PVC, installed in a single layer and joined at laps by welding, adhesive or tape. Single-ply systems may be mechanically fastened, fully adhered or ballasted, and are widely used on flat and low-pitched roofs.

Rainwater that remains standing on a flat roof after the rest of the surface has drained, typically caused by inadequate falls, structural deflection or blocked outlets. Persistent ponding adds load, accelerates deterioration of some coverings and can invalidate system guarantees.