Material used to surface the exterior of a building to protect against exposure to the elements, prevent heat loss, and visually unify the facade. The word siding implies wood units, or products imitative of wood, used on houses. There are many different types of siding, including clapboard, horizontal lap siding, vertical board siding, and shingles. Board and batten siding, sometimes found in Carpenter Gothic houses and very modest structures, differs from the common clapboard in that it consists of vertical wood boards with their butt joints covered by battens (narrow strips), imparting a seamed appearance. Both aluminum and polyvinyl-fluoride-coated siding (commonly called vinyl siding) were developed as maintenance-free alternatives to wood clapboard; they mimic its horizontal boards. Fiberboard, a pressed-wood-pulp product, is sometimes used, though its long-term durability is limited. In larger buildings, the exterior covering is called cladding, and may be of brick, glass in a metal framework, or stone, concrete, or metal panels.