Subfloor Calculator
Building or replacing a subfloor? Enter the room dimensions and plywood sheet size to find out how many sheets you need plus joist spacing estimates.
Choosing the Right Subfloor Material
Plywood and OSB (oriented strand board) are the two main subfloor materials. Plywood is made from thin wood veneers glued in alternating grain directions, creating a strong, stable panel. OSB is made from wood strands mixed with resin and pressed into sheets. Both are rated by thickness and span ratings that tell you the maximum joist spacing they can handle.
For residential subfloors, 3/4-inch tongue-and-groove sheets are the gold standard. The tongue-and-groove edges lock together, preventing movement and eliminating squeaks. If you use square-edge sheets, you must install blocking between joists at every seam to support the edges.
Check the exposure rating. Exterior-rated or exposure-1 plywood resists moisture better than interior grades. This matters in bathrooms, basements, and humid climates where the subfloor might get wet during construction or from occasional leaks. Marine-grade or pressure-treated plywood is overkill for most subfloors but makes sense for wet areas or homes in flood zones.
Subfloor Installation Best Practices
Start by checking that your joists are level and properly spaced. Sistered joists, sagging beams, or out-of-level framing will cause problems no matter how good your subfloor is. Fix structural issues before laying sheets.
Snap chalk lines across the joists at 4-foot intervals to mark sheet edges. Lay the first row with the tongue facing the wall (cut it off for a tight fit). Stagger the seams between rows by at least 4 feet so weak points do not line up. Leave 1/8-inch gaps between sheets and 1/4-inch gaps at walls to allow for expansion.
Use subfloor adhesive and screws together. Run a bead of adhesive along each joist, lay the sheet, and immediately screw it down before the adhesive skins over. Screws hold better than nails and do not pop loose when the wood shrinks. Drive screws every 6 inches along edges and 12 inches in the field. Wipe up any adhesive squeeze-out before it dries.
Preventing Subfloor Squeaks and Failures
Squeaks happen when wood rubs against wood or fasteners. Eliminate movement by using adhesive, driving screws instead of nails, and making sure every sheet edge lands on solid backing. Tongue-and-groove edges that are glued together before installation form a monolithic floor that resists flexing.
Moisture is the other big enemy. Wet OSB swells and delaminates. Wet plywood warps and grows mold. Cover subfloors during construction if rain is expected. Do not install finished flooring until the subfloor is completely dry; trapped moisture causes cupping, mold, and adhesive failure.
Before covering the subfloor, walk every square foot and mark any spots that squeak or feel spongy. Drive extra screws into those areas or add blocking underneath. Fixing squeaks after the floor is covered requires expensive repairs and often damages the finished flooring.
Frequently Asked Questions
What thickness should I use for subfloor plywood?
3/4-inch tongue-and-groove plywood is standard for subfloors over joists spaced 16 inches on center. Use 5/8-inch for 12-inch joist spacing or 23/32-inch OSB as an economical alternative.
Should I use plywood or OSB for subfloor?
Both work. Plywood is lighter, stronger, and handles moisture better. OSB is cheaper and more dimensionally stable. For wet climates or bathrooms, use plywood or treated OSB rated for moist conditions.
How do I lay subfloor sheets?
Stagger the seams so they do not line up across rows. Leave 1/8-inch gaps between sheets for expansion. Run the long dimension perpendicular to the joists and attach with screws every 6-8 inches along joists and 12 inches in the field.
Do I need tongue-and-groove or can I use square edge?
Tongue-and-groove is better because it interlocks the edges between joists, preventing squeaks and adding stiffness. Square edge is acceptable if you add blocking between joists to support the edges.
Can I install subfloor over old flooring?
Only if the old flooring is solid, level, and structurally sound. Remove old carpet, vinyl, or damaged wood. Installing new subfloor over bad subfloor just hides problems until they get worse.