How to Measure Roof Pitch: Two Practical Methods

Measure horizontal run and vertical rise

Run is the horizontal projection, not the sloping roof length. Measure it along a level held in the air, not with a tape resting on the shingles. Rise is the vertical height change across that run. The two measurements form a right angle.

• A: the contact point where the level’s reference end touches the roof or rafter.
• B: the 12-inch mark on the same horizontal edge of the level. A to B is the run.
• C: the roof or rafter point directly above or below B. B to C is the vertical rise.

Both diagrams below use these points. A to C follows the slope and must not be entered as run. This guide uses the familiar rise-per-12 notation; GAF’s field guide distinguishes that roof-slope notation from the architectural rise-to-span definition of pitch.

Choose a safe measuring position

Start with an accessible attic if it has a secure floor or work platform. Ceiling drywall and insulation are not walking surfaces. If the rafter cannot be reached without balancing, disturbing wiring, or moving through unsupported space, use existing drawings or ask a roofer to measure it.

The exterior method is for someone already equipped and trained to work safely on that roof. Do not climb onto a roof just to obtain this number. A shallow slope still has fall hazards; wet roofing and loose material add risk. OSHA’s residential fall protection guidance describes the protections roofing work requires.

Have a straight 24-inch bubble level, tape measure or rigid ruler, pencil, and a way to record the result. Choose one end as zero, then mark 12 inches from it. The zero end becomes A; the 12-inch mark becomes B.

Method 1: Level and tape on the roof

Work in the direction water drains, directly toward the eave rather than diagonally across the roof. The level sits above the roofing. This follows the geometry in CertainTeed’s roof-slope measurement diagram.

1. Set the zero end at point A

Touch the lower corner of the level’s zero end to a sound spot on the roof. This is A. Extend the level toward the eave so the 12-inch mark, B, is downhill from A. Avoid curled shingles and raised seams.

2. Make the 12-inch reference truly horizontal

Keep the contact point in place and lift the downhill end until the bubble is centered. On a 6:12 plane, the lower edge at your 12-inch mark will be approximately 6 inches above the roof.

3. Measure the vertical gap at the mark

At B, measure straight down from the level’s lower edge to the roof at C. A 6-inch gap over the 12-inch run gives 6:12. Keep the tape vertical, at a right angle to the level, not perpendicular to the roof.

4. Repeat before recording the pitch

Check another sound spot on the same plane. If readings are 6 inches and 6¼ inches, check level position and shingle thickness before choosing a value. Record the actual readings rather than automatically rounding to a whole-number pitch.

Method 2: Measure from inside the attic

Use a straight common rafter whose underside is parallel to the roof deck. Avoid hip or valley rafters, internal truss webs, and ceiling members. The Home Depot measurement guide also describes taking a level-and-tape reading inside the attic.

1. Identify a 12-inch horizontal measuring space

From your secure position, choose a rafter section with enough room for the level’s 12-inch reference. Use an uninterrupted edge, away from a notch or joint. For this example, the rafter rises 5 inches per foot.

2. Hold the level below the rafter

Touch the upper corner of the level’s zero end to the rafter’s underside at A. Extend the 12-inch mark, B, horizontally toward the ridge. Center the bubble. The level stays below the rafter rather than resting along its slope.

3. Measure up to the same underside

At B, measure vertically up from the level’s top edge to the rafter’s underside at C. A 5-inch gap over 12 horizontal inches gives 5:12. Measuring from the bottom of a level that is 2 inches tall would incorrectly add 2 inches to the rise.

4. Verify another section

Repeat on a straight section of the same rafter or a neighboring common rafter on that plane. Two 5-inch readings support 5:12. If one reads 6 inches, investigate alignment or a changed roof plane before using it.

Common roof pitch measurement mistakes

Using the sloping length as run

On a 6:12 roof, the 12-inch horizontal run and 6-inch rise make a sloping length of about 13.42 inches. Entering that diagonal as run gives 6 ÷ 13.42 × 12 ≈ 5.37:12, understating the actual pitch. Keep the tape off the diagonal: measure A–B horizontally and B–C vertically.

Other errors to check before using the result

• Confusing span with run. For a symmetrical gable with a centered ridge and 24-foot span, one side’s wall-to-ridge horizontal run is 12 feet. A porch addition may have a different run and pitch.
• Using the whole level without converting. An 8-inch rise across 24 horizontal inches is 4:12, not 8:12. Calculate 8 ÷ 24 × 12 = 4.
• Measuring across the slope. A diagonal line toward a valley can read shallower than the main plane. Align with a common rafter’s direction and keep the 12-inch run horizontal.
• Rounding away a meaningful difference. A 3¾-inch rise over 12 inches is 3.75:12. Keep that value when checking a product’s minimum slope rather than calling it 4:12.

Label measurements by plane: “main roof, 6:12” and “rear porch, 3:12,” for example. Keep the measured rise and run alongside the converted pitch. That lets you recheck the math and avoids applying the main roof’s angle to every section when estimating area or materials.