Overlay

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.

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.

Condensation that occurs within the layers of a roof, wall or floor construction rather than on its internal surface, when water vapour moving through the build-up reaches a plane at or below its dew point temperature. It can degrade insulation, corrode fixings and decay timber, and is controlled through vapour control layers, ventilation and correct layer sequencing.

A roof with a very low pitch, in UK practice generally taken as no more than 10 degrees to the horizontal. Flat roofs rely on a continuous waterproof covering rather than overlapping units, and must be laid to falls so that rainwater drains to outlets.