How do you use a roof pitch multiplier table?
Short answer. Find your pitch in the table, then multiply the roof's flat footprint area, including overhangs, by the multiplier beside it. A 2,000 square foot footprint at 7/12 has a multiplier of 1.158, so the sloped roof surface is about 2,316 square feet, or 23.2 squares before waste.
The common multipliers
- 4/12: 1.054
- 5/12: 1.083
- 6/12: 1.118
- 7/12: 1.158
- 8/12: 1.202
- 9/12: 1.250
- 10/12: 1.302
- 12/12: 1.414
Worked example, step by step
- Measure the footprint at the drip line, not the walls. A 50 by 36 foot house with 1 foot overhangs is 52 by 38, or 1,976 square feet.
- Apply the multiplier for the pitch: at 7/12, 1,976 × 1.158 = 2,288 square feet.
- Divide by 100 to get squares: 22.9.
- Add waste separately, usually 10 to 15 percent depending on how cut up the roof is.
The factors most tables leave out
Hips and valleys run diagonally, so they are longer than a common rafter. When both sides share the same pitch, multiply the horizontal run of a common rafter by the hip and valley factor: 1.453 at 4/12, 1.500 at 6/12, 1.563 at 8/12, 1.641 at 10/12 and 1.732 at 12/12. Rakes, the sloped gable edges, use the regular multiplier. These lengths drive ridge cap, valley metal and leak barrier quantities.
Mixed pitch roofs
If the main roof is 8/12 and the porch is 3/12, split the footprint by plane and apply each multiplier separately. Using one average multiplier for the whole house is the most common way homeowners misjudge a quote. See how roofers measure a roof and roof replacement cost in Georgia.