Purlin

Steel or aluminium sheet cold roll-formed into trapezoidal, sinusoidal or other profiles to provide spanning stiffness, used as roof and wall cladding and as structural decking. Sheets are typically metallic-coated steel with a factory-applied organic coating, fixed through to purlins or rails, or used as the skins of built-up and composite panel systems.

A metal roofing system in which long sheets or trays of steel, aluminium, zinc or copper are joined by raised interlocking seams that stand proud of the drainage plane, with concealed clips that allow thermal movement. Suited to low pitches and long uninterrupted runs, standing seam roofs have no exposed through-fixings in the main roof area.

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.