Subfloor
The structural layer of plywood or OSB fastened directly to floor joists, forming the base for finished flooring.
The subfloor is the first structural layer of the floor assembly, attached on top of the floor joists. It is typically 3/4-inch tongue-and-groove plywood or OSB (oriented strand board). Its job is to provide a rigid, level platform that distributes loads across multiple joists and serves as the substrate for finished flooring materials like hardwood, tile, vinyl, or carpet.
A sound subfloor is critical for the performance of any finish floor. Squeaks are usually caused by the subfloor rubbing against a joist because the fasteners have worked loose or the glue bond failed. Tile installations are especially demanding—excessive subfloor flex will crack grout lines and even tile itself, so tile setters often add a layer of cement board or self-leveling compound.
When replacing flooring, contractors often discover soft spots, delaminated edges near exterior walls, or water damage around toilets and dishwashers. Subfloor repairs typically run $2–$8 per square foot depending on the extent of damage and whether the finish floor must be removed first.
Real-World Example
During the bathroom remodel, the tile setter found that the 1/2-inch subfloor had delaminated near the toilet—he replaced 20 square feet with 3/4-inch plywood before laying the new tile.