How do you work out roof pitch from the outside?
Short answer. To work out roof pitch without getting on the roof, find the run, which is half the width of the gable end, and the rise, which is the height from the top of the wall to the peak, then divide rise by run and multiply by 12. A gable 30 feet wide that rises 7.5 feet to the peak has a 6/12 pitch.
Step 1: Find the run
Measure the width of the house at the gable end, wall to wall, along the ground. On a symmetrical gable, the run is half of that number. If the roof is offset or one side is longer, measure from the wall to the point directly under the peak instead.
Step 2: Estimate the rise
The rise is the height of the triangle, from the top of the wall where the roof begins to the peak. Ways to get it without climbing:
- Count siding courses: lap siding usually has a consistent exposure, often around 4 to 8 inches depending on the product. Measure one course you can reach, count the courses in the gable triangle, and multiply.
- Count brick courses: standard modular brick runs three courses to about 8 inches. Count courses in a brick gable and convert.
- Use a photo: take a straight-on photo of the gable and measure the triangle's height and half-width in the image. Because both are at the same scale, the ratio holds.
Step 3: Do the math
Pitch = rise divided by run, times 12. Examples:
- Rise 6 feet, run 12 feet: 6 divided by 12, times 12 = 6/12.
- Rise 10 feet, run 15 feet: 10 divided by 15, times 12 = 8/12.
- Rise 4 feet, run 14 feet: about 3.4/12, so likely a 3.5/12 or a low-slope 3/12 section.
Check the result
Photo and siding methods are estimates. Round to the nearest common pitch, and verify with a level in the attic if accuracy matters. See how roofers measure a roof for how professionals do it.