Wind uplift

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.

The structural layer of a flat or low-pitched roof that supports the waterproofing build-up and imposed loads, commonly profiled steel, timber boarding, plywood, oriented strand board, or in-situ or precast concrete. The deck type influences fixing methods, falls, deflection and condensation behaviour.

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.

The overlap between adjacent sheets of roofing membrane or underlay where they are joined and sealed, by heat welding, solvent welding, adhesive, tape or bitumen bonding depending on the material. Lap integrity determines the watertightness of sheet systems, so minimum lap widths and the staggering of end laps are specified in system guidance.