What are the roof pitch factors for each pitch?
Short answer. Roof pitch factors range from 1.003 at 1/12 to 1.414 at 12/12. The common ones are 1.054 for 4/12, 1.118 for 6/12, 1.202 for 8/12 and 1.302 for 10/12; multiply plan area by the factor to get true roof area.
Area factor and rafter length per foot of run
The second number is the common rafter length in inches for each foot of horizontal run, the same value printed on a framing square.
- 1/12: 1.003, 12.04 in
- 2/12: 1.014, 12.17 in
- 3/12: 1.031, 12.37 in
- 4/12: 1.054, 12.65 in
- 5/12: 1.083, 13.00 in
- 6/12: 1.118, 13.42 in
- 7/12: 1.158, 13.89 in
- 8/12: 1.202, 14.42 in
- 9/12: 1.250, 15.00 in
- 10/12: 1.302, 15.62 in
- 11/12: 1.357, 16.28 in
- 12/12: 1.414, 16.97 in
Hip and valley length per foot of common run
Hips and valleys run diagonally, covering 16.97 inches of plan for every 12 inches of common run, so their length per foot of run is √(rise² + 288):
- 4/12: 17.44 in
- 6/12: 18.00 in
- 8/12: 18.76 in
- 10/12: 19.70 in
- 12/12: 20.78 in
How to use the chart
For area, multiply each section's plan area by its factor. For rafters, multiply the run in feet by the inches per foot. For hips and valleys, multiply the common run in feet by the hip value, then divide by 12 for feet. See how roofers measure a roof and roof anatomy explained.