A subfloor is the layer of decking between the floor joists and finished flooring such as carpet, tile or linoleum. If the subfloor is made of plywood or oriented strand board (OSB), flooring materials are often nailed into it. If the subfloor is concrete, flooring materials may have to be glued to it or allowed to "float" unattached above it.
Floor joists are akin to the studs in a wall, so they need to be covered with some form of subfloor to provide a level, smooth surface for inhabitants. Since a subfloor is designed to be more functional than decorative, many builders nail down large sheets of plywood or OSB directly to the wooden floor joists. A subfloor needs to be as level and as smooth as possible in order to give proper support to the finished flooring. Some builders may used concrete as subflooring, especially on ground-level floors and basements. A concrete subfloor needs to be just as smooth and level as a plywood or OSB subfloor. The finished flooring could suffer from improperly installed or warped subflooring.
A concrete subfloor may also contain the tubing for a radiant heating system, which are then covered with tiles or other heat-transferring flooring materials. A plywood subfloor is rarely modified, since nails may be driven into it as part of the finished flooring process. The material used for subflooring may have to change depending on the planned use of the space.
Rooms with high moisture content benefit from concrete subflooring, since the humidity and moisture can cause plywood subflooring to warp or decay. Other spaces may benefit from wooden subflooring, since it is much easier to install and more forgiving under foot. A loose plywood subfloor may creak under stress, however, so builders need to make sure there are no spaces between the floor joists and the subfloor during installation. Some contractors use a lighter weight concrete as a substitute for plywood subflooring on higher floors of residential buildings, but that method can limit the types of flooring materials that can be used.
A subfloor can also support a finished floor which literally floats unattached above it. A floating floor could be constructed from tongue-in-groove slats of real wood or a processed wood product such as Pergo. The finished floor is supported by the subfloor, but there are no actual adhesives, screws or nails holding them together.