Subfloors

Subfloors are the structural surface installed beneath the finished floor covering. They provide a flat, rigid base that transfers loads from the finished floor to the building’s framing and helps manage moisture movement within the floor assembly. The subfloor must be straight, free of major defects, and securely fastened to resist movement that could damage the finish floor.

In residential wood framing, subfloors are typically made of plywood or oriented strand board (OSB) panels laid over floor joists. Common thicknesses range from about 1/2 inch to 3/4 inch, depending on span and code. Panels are installed with staggered joints and fastened at regular intervals. In concrete constructions, the subfloor can be a concrete slab (slab-on-grade) that serves as the structural floor; when finishes are added, moisture barriers and sometimes a thin underlayment are used to provide a smooth surface.

Moisture control is a key consideration for subfloors. Wood subfloors rely on proper vapor barriers and ventilation to prevent warping or rot, while concrete slabs require moisture management to prevent finish-floor problems. Common issues include squeaks, cupping, buckling, or cracking, often caused by loose fasteners, movement of joists, or excessive moisture.

Maintenance and repair typically involve re-fastening loose panels, filling gaps, addressing sources of moisture, and ensuring a level, clean surface before installing the final floor.