How many sheets of plywood?

An area count, not a cutting plan

A standard sheet is 4 by 8 feet, which is 32 square feet. Divide your area by 32, add an allowance, round up, and you have the number of sheets. For a 20 by 14 foot floor that is 280 ft², or ten sheets with 10% waste.

The honest caveat is that this is a floor, not a ceiling. Sheets only divide cleanly into a room when the room happens to be a multiple of 4 and 8 feet, and rooms rarely are. Every partial sheet at the edge leaves an offcut, and whether that offcut is usable depends on where the next cut falls.

Two rules push the real count above the arithmetic. Joints must land on framing — every sheet edge needs a joist, stud or rafter under it, which sometimes means cutting a sheet narrower than the leftover space. And joints should be staggered, so adjacent rows do not line up, which means starting alternate rows with a half sheet. Both cost material.

That is why 10% is the working figure rather than 5%. On a simple rectangle you may beat it; on anything with an angle, a stairwell or a chimney chase, plan on more.

Two worked examples

Subfloor: a 14 × 20 foot room is 280 ft². With 10% waste that is 10 sheets of 4 × 8 tongue-and-groove subfloor. Lay the long edge across the joists and stagger the end joints by half a sheet from row to row.

Roof sheathing: the 40 × 30 house in our roofing example with a 6/12 pitch has about 1,342 ft² of roof surface. Sheathing goes on the same surface area, so it needs 1,342 × 1.10 ÷ 32 = 47 sheets. Use the roof surface area from the roofing calculator, not the footprint, or you will be short by the pitch multiplier.

Which panel for which job

Every structural panel is stamped with a span rating, such as 24/16, that tells you the maximum spacing of framing it can span as roof or floor. Match the stamp to your framing.

Mistakes that cause squeaks and buckling

• Butting sheets tight. Panels swell with humidity. Leave about 1/8 inch between sheets at edges and ends unless the panel's own edge profile sets the gap.
• Running sheets parallel to the joists. The strength axis of a panel runs along its 8-foot length. Lay it across the framing.
• Skipping glue on a subfloor. Construction adhesive on the joists, plus screws or ring-shank nails, is what keeps a floor quiet for decades.
• Leaving panels out in the rain. OSB edges swell when soaked and do not shrink back fully. Store sheets flat and covered.

What it costs

Panel prices swing with the lumber market. As a rough guide, a 7/16 inch OSB sheet often costs $12 to $25 in the US and a 3/4 inch tongue-and-groove subfloor panel $35 to $70. For the 14 × 20 subfloor, 10 sheets come to roughly $350 to $700. Check current prices at your local yard.