Siding is the outer covering of a house meant to shed water and protect from the effects of weather.

Siding may be formed of horizontal boards, vertical boards, shingles, or sheet materials. In all four cases, avoiding wind and rain infiltration through the joints is a major challenge, met by overlapping, by covering or sealing the joint, or by creating an interlocking joint such as a tongue-and-groove or rabbet. Since building materials expand and contract with changing temperature and humidity, it is not practical to make rigid joints between the siding elements.

Siding may be made of wood, metal, plastic, or composite materials. It may be attached directly to the building structure (studs in the case of wood construction), or to an intermediate layer of horizontal planks called sheathing.