Profiled metal sheet

A horizontal secondary structural member spanning between principal rafters, portal frames or walls, supporting the roof cladding or decking. In modern industrial buildings purlins are commonly cold roll-formed galvanised steel sections such as zed and cee profiles; in traditional construction they are timber.

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.

Corrosion that develops at the exposed cut edges of pre-coated steel roof sheets, typically at overlaps and eaves, where the steel core is not protected by the factory coating system. It appears as coating delamination and rust creeping back from the edge, and is treated by preparation and application of edge protection coatings before it spreads into the sheet.

Refurbishment of an existing roof by installing a new waterproofing or covering system over the retained build-up rather than stripping it, normally after a condition survey and condensation risk assessment. On profiled metal roofs the equivalent practice is called oversheeting, where new sheets are fixed over the old covering on spacer systems.