How do you calculate roof decking?
Short answer. Find the roof's true sloped area, divide by 32 square feet per 4x8 sheet, and add about 10 percent for waste on a simple roof and more on a cut-up one. To get sloped area, multiply each plane's flat footprint, including overhangs, by the pitch factor, such as 1.118 for a 6 in 12 roof.
Step by step
- Measure each roof plane's footprint, length times horizontal depth, including the overhang past the walls.
- Multiply by the pitch factor to convert flat area to sloped area.
- Add the planes together for total roof area.
- Divide by 32 to get the number of 4 by 8 foot sheets.
- Add waste: about 10 percent for a simple gable, 15 percent or more for hips, valleys and dormers where many angled cuts are needed.
Common pitch factors
- 4 in 12: 1.054
- 6 in 12: 1.118
- 8 in 12: 1.202
- 10 in 12: 1.302
- 12 in 12: 1.414
Worked example
A gable roof with a 60 by 32 foot footprint, overhangs included, at 6 in 12 pitch has 1,920 square feet of footprint. Times 1.118 is about 2,147 square feet of deck. Divided by 32 is 67 sheets, and 10 percent waste brings it to 74.
On a reroof, you rarely replace it all
During a replacement, most decks need only a few sheets where leaks or rot have damaged the wood. That is why contracts usually price decking per sheet, with a stated number included. A full redeck is typically reserved for old 1x board decks with wide gaps, widespread delamination, or a deck too thin for the framing spacing.
Choose the material with OSB vs plywood roof decking, and see how roofers measure a roof for area basics.