Subfloor The layer of flooring that is fastened directly to the floor joists, and to which the finish flooring is applied. It is constructed of plywood sheathing or 2 x 6 planks, and may be a single or double layer.Related Articles: Glossary: Carpet PadGlossary: Parquet FlooringGlossary: Radiant FlooringGlossary: Staple-up SystemWarmzone Expands its Product Offerings