Undergulvet

Undergulvet, or the subfloor, is the structural layer between the building's foundation or ground and the finished floor covering. It provides support for the load of people and furniture and serves as the base for the final floor material. It also contributes to the overall stiffness and, in some constructions, to thermal and acoustic performance.

In timber-framed buildings, the subfloor is usually a sheet material such as plywood or oriented strand board (OSB) fixed to the floor joists, forming a flat surface for the finished floor. In concrete buildings the structural slab itself can act as the subfloor; in such cases a separate underlayment or a floating subfloor may be installed to receive the finish floor. Subfloors can also be designed as ventilated cavity constructions or be insulated within the joist space to improve thermal efficiency.

Moisture control is important. A moisture or damp-proof membrane may be placed between the subfloor and concrete, and vapor barriers are used in some assemblies. In crawl spaces, ventilation helps limit moisture buildup; insulation between joists reduces heat loss and improves comfort.

For a good finish, the subfloor must be flat, clean, and dry. Any damaged or warped panels are replaced; subfloor joints are concealed; leveling compounds may be used to achieve a uniform surface before installing the final floor covering. Screws or nails are used to secure panels to resist squeaks and movement.

Common issues include moisture damage, mold, rot, squeaks, and movement due to improper fastening or framing. Regular inspection, moisture control, and timely repair help maintain the integrity of the subfloor and the finished floor above.