Roofing glossary

Waterproofing sheet of plasticised polyvinyl chloride with a reinforcing scrim; seams are hot-air welded.

pvc roofing, single ply

Rigid closed-cell foam used for insulation boards; low thermal conductivity.

pir board, pir insulation

Layer below the insulation that keeps indoor moisture out of the insulation and prevents condensation.

vapour barrier, vapor retarder, vcl

Perimeter wall above the roof surface; a critical upstand and fixing detail.

parapet upstand

Water collection fitting at the roof low point, connected to the membrane and rainwater system.

roof drain, rainwater outlet

Trapezoidal steel sheet spanning between steel beams and carrying the roof build-up.

steel deck, trapezoidal deck

Fixing the membrane through the insulation into the deck with washers and screws, sized for wind uplift.

mechanically fastened, mechanically attached

Area where heat passes faster than through the rest of the build-up — e.g. fasteners, open joints.

cold bridge

Separation layer between PVC and polystyrene or bitumen that prevents plasticiser migration.

separation layer

Roof covering classification for external fire performance (EN 13501-5, CEN/TS 1187 test 2). Valid for a specific substrate.

external fire classification

Thermal transmittance, W/(m²·K): the lower it is, the less heat passes through the element.

thermal transmittance

Thermal conductivity of a material, W/(m·K). λD — manufacturer’s declared value.

thermal conductivity

Roof whose membrane is loose laid and held down by gravel or slabs.

ballast

Roof where insulation (XPS) is above the waterproofing.

inverted roof

Structural movement joint for thermal or settlement movement; on the roof — with a compensator.

movement joint

Pulling a welded seam apart to check weld quality.

seam test