Metal roof sheeting

Metal roof sheeting is thin sheet steel or aluminium. On roofs it comes as trapezoidal or corrugated sheets, often galvanised and factory-painted, or as sandwich panels, two metal skins with insulationThermal insulationA layer of material that lets heat through only slowly, so a home keeps its warmth in winter and stays cool in summer. between them. Metal is also used for the flashings that close the joint between a roof and a wall or parapetParapetThe low wall around the edge of a flat roof or balcony. The joint between it and the floor is one of the most common places for leaks..

Plain metal sheeting doesn't insulate: in summer it gets scorching and passes the heat into the space below, and in winter it can "sweat" on the inside from condensationCondensationMoisture in the air turning to water on cold surfaces such as windows, outside corners and ceilings. It is the most common cause of mould indoors.. That's why it is often insulated, for example with spray polyurethane foamSpray polyurethane foamA foam sprayed on as a liquid that expands on the spot and sets into one seamless layer. It insulates against heat and cold and closes cracks and joints., and painted a light colour.

The weak points are the screws and their rubber washers, which age, the overlaps between sheets, the joints with walls, and spots where the paint has gone and rust starts. Checking and sealing them regularly keeps the roof dry.