Roof Sheathing Calculator
Last reviewed How we calculate
Roof sheathing, also called decking, is the layer of plywood or OSB that the underlayment and roof covering are fastened to. This turns a roof area into a sheet count, with a waste allowance for the cuts at hips, valleys and edges.
Sheets of plywood or OSB for a roof deck
How many sheets of plywood or OSB a roof deck needs, from the roof area, the sheet size and a waste allowance.
This calculator needs JavaScript to work. The measurements below are the ones you would need to do the sum by hand.
Sheets to order Roof area plus waste, divided by the area of one sheet.
- Roof area
- Area including waste
- Area of one sheet
- Roofing squares
A sheet count only. Panel thickness and grade are set by the rafter spacing and your local code. Stripping an old roof often turns up rotten decking nobody planned for, so keep a few spare sheets on site rather than ordering to the exact number.
How the calculation works
Take the sloped roof area, not the footprint of the house, add the waste allowance, and divide by the area of one sheet. A standard 4 x 8 ft sheet covers 32 sq ft, so the arithmetic is quick. Rounding up to whole sheets and adding waste is where hand estimates tend to come up short.
A worked example
A roof of 1,250 sq ft with 10 per cent waste needs sheathing for 1,375 sq ft. That is 42.97 sheets of 4 x 8, so 43 to order. If you do not know the sloped area, the roof area calculator works it out from the footprint and pitch.
Choosing a waste allowance
Sheets are laid with their long edge across the rafters and their ends staggered, so every row starts or finishes with a cut piece. On a plain gable most offcuts can be used further along the row, and 10 per cent is usually enough. Hips and valleys cut sheets at an angle and leave triangles that cannot be used anywhere else, which is why complicated roofs need 15 per cent or more.
Thickness and grade
The calculator counts sheets; it does not choose them. The thickness a roof deck needs depends on the rafter or truss spacing and on the loads in your area, and building codes and panel manufacturers publish span ratings for exactly that. On a re-roof, expect some of the existing deck to be damaged once the old covering is off. A few spare sheets are cheaper than stopping work to fetch more.
Frequently asked questions
Is plywood or OSB better for roof sheathing?
Both are accepted as roof sheathing when their span rating suits the rafter spacing. OSB usually costs less; plywood is often said to cope better with getting wet during construction.
What thickness of sheathing do I need?
It depends on the rafter or truss spacing and on the loads in your area. Each panel is stamped with a span rating, and that rating has to match your framing. Your building code or the panel maker can confirm the minimum.
Why are gaps left between sheathing panels?
Panels expand as they take up moisture. Panel manufacturers call for a small gap, typically about an eighth of an inch, at edges and ends so the deck does not buckle.