How to Measure Roof Pitch
The short answer
To measure roof pitch, hold a level horizontally against the roof, mark 12 inches (30 cm) along it, and measure straight down from that mark to the roof surface. The inches of drop are your pitch: a 6-inch drop over a level 12 is a 6:12 pitch, which is a 26.6° angle.
That’s the whole trick - roof pitch is just the rise per 12 inches of level run. You can take that reading three ways, and two of them don’t require setting foot on the roof:
- In the attic, against the underside of a rafter (safest).
- On the roof surface, if it’s walkable and dry.
- From the gable end, using the total rise and total run.
Method 1 - in the attic (no roof walk)
The most accurate reading, because you’re measuring the bare rafter instead of shingles:
- In the attic, hold a level horizontal with one end touching the underside of a rafter.
- Make a mark on the level at 12 inches from that contact point.
- Measure straight up from the 12-inch mark to the underside of the same rafter.
That vertical distance is your pitch. 7 inches up means a 7:12 roof.
Method 2 - on the roof surface
Same geometry, on top of the shingles:
- Pick a flat spot mid-slope - not on a ridge cap, a curled shingle, or a course edge.
- Hold the level horizontal with one end on the roof, mark 12 inches, and measure straight down from the mark to the surface.
Only do this on a dry, low roof you can walk safely; the attic and gable methods give the same number from the ground.
Method 3 - from the gable end
No level needed - use the roof’s overall triangle instead:
- Measure the total rise: from the top of the wall (top plate) straight up to the peak.
- Measure the total run: the horizontal distance from the wall to a point directly under the ridge (half the building width on a symmetrical gable).
- Divide rise by run and multiply by 12.
If the peak is 60 in above the top plate and the ridge sits 120 in in from the wall: 60 ÷ 120 × 12 = 6:12.
Measuring the angle instead (phone or speed square)
A phone inclinometer app or a speed square gives you degrees, not x:12. Convert with pitch = tan(angle) × 12. A 30° reading is tan(30°) × 12 = 6.9, so call it a 7:12. (Metric-country roofs are usually quoted in degrees or percent directly - the table below crosses all three.)
Pitch to angle conversion table
Angle = arctan(rise ÷ 12), slope factor = √(rise² + 12²) ÷ 12, grade = rise ÷ 12 × 100 - the same formulas the calculator runs. The slope factor is the number you multiply the horizontal run by to get rafter length, and the flat plan area by to get true roof area.
| Pitch | Angle | Slope factor | Grade |
|---|---|---|---|
| 1:12 | 4.8° | 1.003 | 8.3% |
| 2:12 | 9.5° | 1.014 | 16.7% |
| 3:12 | 14.0° | 1.031 | 25.0% |
| 4:12 | 18.4° | 1.054 | 33.3% |
| 5:12 | 22.6° | 1.083 | 41.7% |
| 6:12 | 26.6° | 1.118 | 50.0% |
| 7:12 | 30.3° | 1.158 | 58.3% |
| 8:12 | 33.7° | 1.202 | 66.7% |
| 9:12 | 36.9° | 1.250 | 75.0% |
| 10:12 | 39.8° | 1.302 | 83.3% |
| 12:12 | 45.0° | 1.414 | 100.0% |
The pitch also tells you what can go on the roof (US IRC R905.2.2): under 2:12 is too flat for shingles, 2:12 to under 4:12 needs a double underlayment, and 4:12 or steeper is a standard shingle install.
Worked example
Say the gable-end method gave you a 60 in rise over a 120 in run, and the roof run (wall to ridge) is 10 ft:
- Pitch: 60 ÷ 120 × 12 = 6:12
- Angle: arctan(60 ÷ 120) = 26.6°
- Slope factor: √(60² + 120²) ÷ 120 = 1.118
- Rafter length: 1.118 × 10 ft = 11.18 ft - before adding the overhang and the ridge and birdsmouth cuts
👉 Skip the arithmetic: the Roof Pitch Calculator turns your rise and run into the x:12 pitch, the angle, the slope factor, the percent grade and the rafter length as you type - and flags whether the pitch suits asphalt shingles.
Watch-outs
- Keep the run level and the rise plumb. A tape laid along the sloping surface reads the pitch too shallow, and every rafter and shingle number downstream inherits the error.
- Measure one surface, consistently. Don’t mix a rafter-underside rise with a shingle-top run; either surface works alone because the planes are parallel.
- Check both slopes if the roof looks asymmetrical. Saltbox and shed-dormer roofs have different pitches per side - measure each.
- Ladder and roof safety first. If the roof is steep, wet, or high, take the reading from the attic or the gable end - the number is identical.
Related
- Roof Pitch Calculator - pitch, angle, slope factor and rafter length from rise and run.
- Roofing tools - the full roofing toolset.
- What is the minimum roof pitch for shingles - what your pitch number means for material choice.
- How to calculate shadow length - the same right-triangle math, pointed at the sun.