Field Guide
How Many Sheets of Plywood or OSB Do I Need?
Sheathing and subfloors are bought by the sheet. Find the area of each floor, wall or roof plane, add waste for cuts, and divide by the sheet size. Roofs need one extra step: the pitch.
1. How to Calculate Sheets (The Math Explained)
Here's the exact process this calculator uses, with a 30 × 40 ft gable roof at 6/12 pitch as the example.
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1
Measure the footprint
Floors and walls are simple length × width (or length × height). For a roof, measure the flat footprint including the overhangs.
Footprint = 40 ft × 30 ft = 1,200 sq ft -
2
Apply the pitch factor (roofs only)
A sloped roof has more surface than its footprint. The pitch factor converts one to the other.
Pitch factor = √(6² + 12²) ÷ 12 = √180 ÷ 12 = 1.118 Roof area = 1,200 × 1.118 = 1,342 sq ft -
3
Add waste and divide by the sheet size
With 10% waste = 1,342 × 1.10 = 1,476 sq ft 4×8 sheets = 1,476 ÷ 32 = 46.1 → 47 sheets
2. Choosing Panels
| Panel | Typical use | Good to know |
|---|---|---|
| 7/16" OSB | Wall and roof sheathing | The most common sheathing; usually the lowest price per sheet. |
| 1/2" CDX plywood | Roof and wall sheathing | "CDX" means C and D face grades with exterior glue: made for sheathing, not for looks. |
| 5/8" CDX plywood | Roofs | Stiffer under heavier roofing, higher snow loads or wider rafter spacing. |
| 23/32" T&G subfloor | Floors | Sold as "3/4 in". Tongue-and-groove edges lock sheets together between joists. |
3. Installation Tips
Run the long edge across the framing and stagger the end joints from row to row.
Leave a 1/8 in gap at panel edges and ends so the sheets can swell without buckling.
Check the stamp. The span rating (such as 24/16 or "24 oc") must match your rafter, truss or joist spacing.
4. Frequently Asked Questions
How many sheets of plywood do I need for a roof?
Multiply the roof's footprint (including overhangs) by the pitch factor, add waste, and divide by 32 for 4×8 sheets. A 30 × 40 ft footprint at 6/12 is 1,200 × 1.118 = 1,342 sq ft of roof; with 10% waste that's 47 sheets.
How do I find the roof pitch factor?
The pitch factor is √(rise² + 144) ÷ 12, where rise is the inches the roof climbs per 12 in of run. A 4/12 roof is 1.054, 6/12 is 1.118, 8/12 is 1.202 and 12/12 is 1.414. Multiply the flat footprint by it to get the sloped area.
How many sheets of OSB do I need to sheathe a house?
Multiply the total length of the exterior walls by the wall height and divide by 32 for 4×8 sheets. 120 ft of 8 ft wall is 960 sq ft: 33 sheets with 10% waste. Don't subtract windows and doors; sheathing is usually run over them and cut out, and the offcuts are rarely reusable.
What thickness plywood or OSB do I need for a roof?
7/16 in OSB or 1/2 in plywood is common over rafters or trusses 24 in on center; 5/8 in is used for heavier roofing, high snow loads or wider spacing. The panel's span rating stamp (for example 24/16) tells you the maximum spacing it's rated for, and local code has the final say.
What thickness subfloor do I need?
23/32 in (sold as 3/4 in) tongue-and-groove plywood or OSB rated for 24 in on center is the usual choice for floor joists up to 24 in apart. Glue and screw it to the joists to prevent squeaks.
Is OSB or plywood better?
Both are code-approved structural panels. OSB is usually cheaper and very consistent from sheet to sheet. Plywood is lighter for its strength and recovers better if it gets wet during construction. For most sheathing and subfloors, either works when it carries the right span rating.
How much extra plywood should I buy?
About 10% for floors and simple walls, and 15% or more for roofs with hips, valleys or dormers, where angled cuts create more waste.