Subfloor

The subfloor is the structural layer that forms the base for finished flooring. It sits on top of the building’s floor joists or directly on a concrete slab and provides load-bearing support, stiffness, and a flat, continuous surface for the floor covering. Common subfloor materials include wood-based panels such as plywood or oriented strand board (OSB) in wood-framed buildings, and concrete slabs in many basement or slab-on-grade constructions. Older or specialized buildings may use plank subfloors or other materials.

Construction and installation considerations vary by material. Wood subfloors are attached to the tops of joists with nails or screws and are typically installed in panel form with joints staggered for stability. Edge spacing, panel thickness, and fastener patterns must meet local codes and structural requirements. Concrete slabs serve as both foundation and subfloor and may require a finish-approved surface or an additional underlayment, especially when moisture or radiant heating is present. Moisture control is crucial for wood subfloors, which should be acclimated to indoor conditions and, if needed, protected by a moisture barrier or vapor retarder to prevent damage to the finish floor.

Finished flooring and underlayment: The subfloor is usually covered by an underlayment or the finished floor product itself, depending on compatibility and manufacturer instructions. Subfloor flatness, moisture content, and fastener flushness influence the final appearance and performance of the floor.

Common issues include squeaks, cupping, warping, moisture damage, and nail pops. Repairs may involve replacing damaged panels, re-fastening, or resurfacing the surface.