Undergulvets

Undergulvets, in construction, refers to the subfloor—the structural layer that lies between the finished floor and the floor framing. In most wood-frame buildings, the subfloor is formed by sheets of plywood or oriented strand board (OSB) laid over the floor joists and fastened with nails or screws. In concrete constructions, the subfloor is the concrete slab itself. The subfloor provides a flat, rigid surface for the finished floor, helps transfer loads from the finished floor to the joists, and contributes to overall floor stiffness. It can also host insulation, vapor barriers, and air sealing materials as required by the building envelope.

Installation and performance: Subfloors are typically installed with tongue-and-groove edges to improve alignment and stiffness, and are secured across the joist system. The thickness and fastener pattern depend on material, span, and local codes. The finished flooring is installed on top of the subfloor, and the subfloor must be dry, flat, and free of excessive movement. In many homes, the subfloor also acts as a base for underlayment or sound isolation products.

Common issues and maintenance: Moisture is a major concern for subfloors. Excess moisture can cause warping, cupping, mold, or rot, especially in basements or over improper vapor barriers. Squeaks indicate looseness and may require re-nailing or screwing the panels down, or replacing damaged sections. Regular inspection, proper moisture control, and timely repairs help maintain subfloor performance. Replacement is sometimes needed when panels are severely damaged or when structural degradation occurs.