This guide walks through using a plywood / OSB calculator to scope a roof decking job. Topics include picking the right sheet thickness for your rafter spacing (15/32" plywood or 7/16" OSB for 24" o.c.), calculating sheet count from total roof area (each sheet covers 32 sq ft), choosing a waste factor (10% simple, 15%+ complex), and accounting for cuts at hips, valleys, and the starter row. Includes a span-rating lookup table from the APA Engineered Wood Association.
Roof sheathing is sold in 4x8 sheets covering 32 square feet each, and the sheet count follows the sloped roof area rather than the footprint. This guide works through the measurement, the sheet count, the thickness decision and the fixing pattern.
Measure the building on the ground to the outside of the eaves and rakes, multiply length by width for each rectangle, and add them. Then multiply by the pitch factor. The pitch factor is what turns a flat outline into the surface you actually have to sheathe.
| Pitch | Angle | Slope % | Multiplier | 2,000 sq ft footprint becomes |
|---|---|---|---|---|
| 1/12 | 4.8° | 8.3% | 1.0035 | 2,007 sq ft |
| 2/12 | 9.5° | 16.7% | 1.0138 | 2,028 sq ft |
| 3/12 | 14.0° | 25.0% | 1.0308 | 2,062 sq ft |
| 4/12 | 18.4° | 33.3% | 1.0541 | 2,108 sq ft |
| 5/12 | 22.6° | 41.7% | 1.0833 | 2,167 sq ft |
| 6/12 | 26.6° | 50.0% | 1.1180 | 2,236 sq ft |
| 7/12 | 30.3° | 58.3% | 1.1577 | 2,315 sq ft |
| 8/12 | 33.7° | 66.7% | 1.2019 | 2,404 sq ft |
| 9/12 | 36.9° | 75.0% | 1.2500 | 2,500 sq ft |
| 10/12 | 39.8° | 83.3% | 1.3017 | 2,603 sq ft |
| 12/12 | 45.0° | 100.0% | 1.4142 | 2,828 sq ft |
| 14/12 | 49.4° | 116.7% | 1.5366 | 3,073 sq ft |
| 16/12 | 53.1° | 133.3% | 1.6667 | 3,333 sq ft |
| 18/12 | 56.3° | 150.0% | 1.8028 | 3,606 sq ft |
| 20/12 | 59.0° | 166.7% | 1.9437 | 3,887 sq ft |
| 24/12 | 63.4° | 200.0% | 2.2361 | 4,472 sq ft |
| Footprint | Sloped area | Sheets at 10% waste | Bundles of 50 |
|---|---|---|---|
| 1,000 sq ft | 1,118 sq ft | 39 | 0.8 |
| 1,500 sq ft | 1,677 sq ft | 58 | 1.2 |
| 2,000 sq ft | 2,236 sq ft | 77 | 1.5 |
| 2,500 sq ft | 2,795 sq ft | 97 | 1.9 |
| 3,000 sq ft | 3,354 sq ft | 116 | 2.3 |
Ten percent waste suits a simple gable. A hip roof or a roof with several valleys runs 15%, because every diagonal cut leaves an offcut too small to use.
If the existing deck is sound you may need no sheets at all — the new roof goes straight over it. What you cannot know until the shingles are off is how much of it is rotten, which is why contractors quote a per-sheet rate for replacement decking as an extra rather than including a guess in the headline price.
Budget for some. On a roof that has leaked, on an older house, or anywhere the shingles have been on past their life, replacing several sheets around the eaves and any valley is normal rather than exceptional.
These come up constantly when ordering, and the answer is different for every material — some figures are fixed by definition and some are printed on the wrapper and vary by brand.
How many sheets of plywood are in a bundle? · How many sheets of OSB are in a bundle? · How many linear feet does a bundle of ridge cap cover? · Every coverage figure in one table
A 2,000 square foot footprint at 6/12 pitch is about 2,236 square feet of roof. At 32 square feet per 4x8 sheet with 10% waste that is about 77 sheets, or a bundle and a half.
Half inch on rafters at 16-inch centres, five eighths at 24-inch centres. Heavy tile or significant snow load calls for five eighths regardless of spacing. The span rating stamped on the sheet is the number that governs.
Fifty is the standard bundle at most lumber yards, though it varies by supplier and thickness. Whole bundles are usually cheaper per sheet than part bundles.
Usually not. If the existing sheathing is sound the new roof goes straight over it. Contractors normally quote a per-sheet rate for whatever bad decking turns up after tear-off, because none of it is visible beforehand.