Key Takeaways
- Pitch is rise over a 12 inch run, written as “x in 12”
- You can measure it directly with a level and tape along a rafter
- Pitch converts to an angle in degrees and a percent grade
- The rafter length factor tells you how much material a slope actually needs
Roof pitch describes how steep a roof is, and it is measured the same way regardless of the roof’s size: vertical rise for a fixed horizontal run, conventionally normalized to 12 inches. A roof that climbs 6 inches for every 12 inches it runs horizontally is a “6 in 12” pitch.
Measuring pitch yourself
You do not need to climb onto the roof to measure pitch, and generally should not for safety reasons. From inside an attic, or along an exposed rafter or gable end, hold a level perfectly horizontal against the underside of the rafter. Mark a point 12 inches along the level from where it touches the roof, then measure straight down from that mark to the roof surface. That vertical distance, in inches, is your rise per 12 inches of run, which is your pitch.
Converting pitch to angle and percent
The same slope can be expressed three ways: as a pitch (x in 12), as an angle in degrees, or as a percent grade. A 6 in 12 pitch works out to about 26.6 degrees and a 50 percent grade. These are all describing the exact same steepness, just in the vocabulary a particular trade or spec sheet uses.
Why the rafter length factor matters
A roof’s actual surface, the part that has to be covered in decking and roofing material, is longer than its flat horizontal footprint once you account for the slope. The rafter length factor, calculated as the square root of pitch squared plus 144, divided by 12, captures exactly how much longer. A steeper roof needs proportionally more material to cover the same footprint, which is the same multiplier the roof square footage and shingle calculators apply to a building’s plan dimensions.
Use the roof pitch calculator to convert your own rise and run measurement into pitch, angle and rafter factor. If you already know a slope in degrees or percent and want to convert it to pitch notation instead of measuring rise and run, the roof slope calculator converts directly between all three. To turn a footprint and pitch into actual roof area, see the roof square footage calculator.