Calculate the number of plywood or OSB sheathing boards needed to deck a roof, allowing for pitch and waste.
Roof sheathing (called “roof decking” in the US or “roof boarding” in the UK) is the sheet layer fixed over the rafters. Enter the plan length and width the roof covers (its footprint, including both slopes of a gable roof) and the roof pitch in degrees — the tool converts the flat footprint into the true sloping area and returns the number of sheets.
| Pitch | Area multiplier |
|---|---|
| 15° | × 1.04 |
| 30° | × 1.15 |
| 45° | × 1.41 |
A roof with a 10 m × 8 m footprint at 30° pitch has a true area of 80 × 1.15 ≈ 92 m². With 8 × 4 ft (≈ 2.98 m²) sheets that is 92 ÷ 2.98 ≈ 31 sheets, plus a waste allowance.
Tip: lay sheets with the long edge across the rafters and stagger the joints; leave a small expansion gap so the boards do not buckle.
For most domestic roofs, 11–18 mm (7/16–23/32 in) OSB3 or plywood is used, depending on the rafter spacing.
Use a roof pitch calculator from your rise and run, or measure the angle directly. A common house roof is around 30–45 degrees.
Yes. Complex roofs generate more offcuts, so allow 15% waste rather than 10% if your roof has hips, valleys or dormers.
Lay the boards with their long edge across the rafters and stagger the joints between rows. Leave a small expansion gap and support every edge on a rafter or noggin.
Divide the sloped roof area by the sheet area (about 2.98 m² for an 8×4 sheet) and add waste. The calculator does this allowing for pitch.
