any material fastened to the structural members of a building to provide a base for plaster. Lath can be of wood, metal, gypsum, or insulated board. In older residential buildings, narrow wood strips were generally used.

One of the most common laths is gypsum lath. It is manufactured with an air-entrained gypsum core sandwiched between two layers of fibrous absorbent paper. Sheets with reflective foil backing provide insulation and act as a vapour barrier.

Metal lath, a mesh formed by expanding a perforated metal sheet, is made in a variety of forms (diamond-mesh, flat-ribbed, and wire lath). The sheets of metal are slit and drawn out to form numerous openings, creating an irregular surface for the keying of the plaster. It is often used in bathrooms and kitchens, where ceramic tile is applied over a plaster base.