Definition
A subfloor is the structural layer of flooring that provides a solid base for finish flooring materials such as hardwood, tile, carpet, or laminate. Subflooring is installed over floor joists and beneath underlayment and finish flooring. Common materials include plywood (typically 3/4 inch for 16-inch joist spacing) and oriented strand board (OSB). Subfloor panels are rated by span rating-for example, 32/16 means 32 inches maximum roof span, 16 inches maximum floor span.
Formula
For 16-inch joist spacing, use 3/4 inch (23/32) tongue-and-groove plywood or OSB with a 24/16 or 32/16 span rating. For 24-inch spacing (common with I-joists), use 3/4 inch panels with 48/24 span rating or 7/8 inch panels. Stagger panel joints for strength and glue to joists to reduce squeaking.
Example
Subfloor Installation Example
For a 20x30 foot room with joists at 16 inches OC: Area = 600 sq ft. Using 4x8 sheets (32 sq ft each), you need 19 sheets. Add 10% for waste = 21 sheets total. Use 3/4 inch tongue-and-groove plywood with exterior glue (exposure 1) for moisture resistance. Install panels perpendicular to joists with construction adhesive and 8d nails or screws at 6 inches OC along edges and 12 inches OC in the field.