How do you figure roof pitch from the gable height or rafter length?
Method 1: gable height and half span
- Measure the building's width across the gable end, wall to wall. Half of that is the run.
- Measure or estimate the height from the top of the wall to the ridge. That is the rise.
- Pitch = rise divided by run, times 12.
Example: a 30 foot wide house has a run of 15 feet. If the ridge is 7.5 feet above the wall top, 7.5 divided by 15 is 0.5, times 12 is 6. That is a 6/12 roof.
Method 2: rafter length
If you can measure a rafter in the attic from the wall plate to the ridge, use the right triangle relationship: rise equals the square root of (rafter length squared minus run squared).
Example: a rafter 16.8 feet long over a 15 foot run. 16.8 squared is 282.2, 15 squared is 225, the difference is 57.2, and its square root is about 7.6 feet. Then 7.6 divided by 15, times 12, is about 6.1, so roughly 6/12. Measure along the same line the run was measured, not including the overhang.
Method 3: siding courses
On a gable with lap siding, count courses from the wall top to the ridge and multiply by the exposure of each course to estimate rise from the ground.
Sanity check
Compare your result with how the roof looks: 4/12 to 6/12 looks gentle; 8/12 and above looks steep.
See how roofers measure a roof to turn pitch into roof area.