How to measure roof pitch
Four ways to get the pitch of a roof you can stand on or under: with a level and a tape in the attic, with a speed square on a rafter, from the ground against the gable, and by reading the rise over a 12 in run. Then convert it to degrees and percent.
Updated
Skip the arithmetic
Measured the rise over 12 already? The roof pitch calculator converts it to degrees, percent, and ratio, in any direction.
The short version
Roof pitch is the rise in inches for every 12 in of horizontal run. To measure it, hold a level dead level against the roof or a rafter, mark 12 in along the level, and measure straight down from that mark to the roof surface. That drop in inches is the rise: a 6 in drop over a 12 in level run is a 6/12 pitch. Convert it to an angle by taking the arctangent of rise over run, and to a percent grade by dividing rise by run.
Four ways to measure it
Pick the one that matches where you can safely reach the roof. All four land on the same rise-over-12 number.
- 01
In the attic, against a rafter
The safest method and the one that needs no ladder work. Hold a 12 in level horizontally against the underside of a rafter or the roof sheathing. Level the bubble, then measure straight down from the 12 in end of the level to the rafter. That vertical distance is the rise per 12 in of run. Read it right off the tape.
- 02
With a speed square on a rafter tail
A speed square has a pivot and a degree scale. Rest the square's lipped edge on the rafter or fascia, let it hang from the pivot until it is plumb, and read the degree mark where the roof line crosses the scale. That is the roof angle; the pitch scale on some squares reads the rise per 12 directly.
- 03
From the ground, against the gable
When you can see the gable end, hold a level horizontal with one end touching the rake board and center the bubble. Measure 12 in out along the level, then measure the vertical gap from that 12 in mark to the rake. That gap is the rise per 12 in of run. A phone inclinometer app held flat against the rake gives the angle instead, which you convert below.
- 04
On the roof surface, with a level and tape
If you are already up top and tied off, lay a level flat on the shingles, raise the downhill end until the bubble centers, mark 12 in from the low end, and measure the gap from that mark down to the roof. Same rise-over-12 as every other method.
Worked example: a 6 in drop over 12 in
Say the level reads level, you measure 12 in along it, and the drop to the roof is 6 in. That is a 6/12 pitch. Here is the same pitch in the three forms an inspector, an estimator, and a framer each want.
| Rise over run | 6/12 |
| Angle | 26.57° |
| Percent grade | 50% |
Pitch, angle, and percent for common roofs
Every row is the same pitch read three ways. The angle is the arctangent of the rise over 12; the percent grade is the rise over 12 as a percentage. These are computed by the same code the CutOnce app runs, so the four representations can never disagree.
| Pitch | Angle | Percent grade |
|---|---|---|
| 2/12 | 9.46° | 16.7% |
| 3/12 | 14.04° | 25% |
| 4/12 | 18.43° | 33.3% |
| 6/12 | 26.57° | 50% |
| 8/12 | 33.69° | 66.7% |
| 9/12 | 36.87° | 75% |
| 10/12 | 39.81° | 83.3% |
| 12/12 | 45° | 100% |
What the number decides
Pitch is the gate on your roof covering. Anything below 2/12 is treated as low-slope and generally needs a membrane, not shingles; most asphalt shingle manufacturers set 2/12 as the floor and call for a double underlayment from 2/12 up to 4/12 (IRC R905.2.2 and the product's own instructions, which is what the inspector checks). Measure the pitch before you order the covering, not after.
Common questions
- How do you measure roof pitch without going on the roof?
- From inside the attic. Hold a 12 in level horizontally against a rafter, center the bubble, and measure straight down from the 12 in mark to the rafter. That drop in inches is the rise per foot of run. It is the safest method and needs no ladder.
- What is roof pitch measured in?
- In inches of rise per 12 in of horizontal run, written as a ratio like 6/12. You can express the same slope as an angle in degrees or as a percent grade, but framers and shingle instructions use the rise-over-12 ratio.
- How do I turn a measured angle into a pitch?
- Take the tangent of the angle and multiply by 12. A 26.57 degree roof is tan(26.57) x 12, which is about 6, so it is a 6/12 pitch. The roof pitch calculator does the conversion in either direction.
- Does a phone app measure roof pitch accurately?
- A phone inclinometer gives you the angle, which converts to a pitch, but only as accurately as the phone sits flat against a straight part of the roof. Held against a rake board or a rafter it is close; laid on wavy shingles it drifts. Confirm with a level and tape when the number matters.
Measure once, cut from a plan
The app takes the pitch you measured and carries it through to rafter line length, the birdsmouth, ridge height, and a printable cut plan.
See also