What roof pitch means
In the US and Canada, roof pitch is written as the number of inches the roof rises for every 12 inches it runs horizontally. A 6/12 roof ("six-twelve") climbs 6 inches for every foot of horizontal distance. Builders use the same idea for rafters, stairs and ramps, and the pitch is what shingle manufacturers and building codes refer to when they set minimum slopes.
Pitch = rise ÷ run × 12 (inches per foot)
Angle = arctan(rise ÷ run) · Percent slope = rise ÷ run × 100
Pitch factor = √(1 + (pitch ÷ 12)²) · Rafter length = run × pitch factor
You can measure the rise and run in any unit, as long as both are in the same one: inches, centimetres or a 24-inch level all work, because pitch is a ratio.
Common roof pitches
| Pitch | Angle | Percent slope | Pitch factor |
|---|---|---|---|
| 2/12 | 9.46° | 16.7% | 1.014 |
| 3/12 | 14.04° | 25% | 1.031 |
| 4/12 | 18.43° | 33.3% | 1.054 |
| 5/12 | 22.62° | 41.7% | 1.083 |
| 6/12 | 26.57° | 50% | 1.118 |
| 7/12 | 30.26° | 58.3% | 1.158 |
| 8/12 | 33.69° | 66.7% | 1.202 |
| 9/12 | 36.87° | 75% | 1.250 |
| 10/12 | 39.81° | 83.3% | 1.302 |
| 12/12 | 45° | 100% | 1.414 |
The pitch factor (also called the slope multiplier) converts a flat, horizontal measurement into the length along the roof surface. Multiply the footprint area by it to get the roof area, or a rafter's horizontal run by it to get the rafter length.
Worked example 1: a 6/12 gable roof
- Rise 6, run 12: pitch = 6 ÷ 12 × 12 = 6/12.
- Angle = arctan(0.5) = 26.57°; percent slope = 50%.
- Pitch factor = √(1 + 0.5²) = 1.118.
- A rafter that runs 14 ft from the wall to the ridge is 14 × 1.118 = 15.65 ft long along the slope, before adding the overhang and allowing for the ridge board.
Worked example 2: measuring with a level
You hold a 24-inch level flat against the underside of a rafter and measure 9 inches straight down from its end to the roof. Rise 9, run 24: pitch = 9 ÷ 24 × 12 = 4.5/12, an angle of 20.56°. Because pitch is a ratio, you did not need a 12-inch run.
Worked example 3: from an angle
An app or angle finder reads 30°. Pitch = tan(30°) × 12 = 6.93/12, which is close to a 7/12 roof.
How to measure roof pitch safely
- From the attic: hold a level horizontally against a rafter, mark 12 inches along it, and measure vertically from that mark up to the underside of the rafter. That number is the pitch.
- From a gable end: measure the vertical height from the top of the wall to the ridge (rise) and the horizontal distance from the wall to the centre of the ridge (run).
- From a ladder at the eave: place a level on the shingles with one end touching the roof and measure the gap at 12 inches. Stay off steep or wet roofs.
- From an angle: a phone inclinometer or digital angle finder on a rafter gives degrees; choose the angle option above.
Why pitch matters for roofing materials
Steeper roofs shed water faster, so building codes and manufacturers set minimum slopes. Under the International Residential Code, asphalt shingles may only be used on slopes of 2/12 or steeper, and slopes from 2/12 up to 4/12 need a double layer of underlayment. Very low slopes need membrane roofing instead. Pitch also affects safety and price: crews usually need extra equipment once a roof is too steep to walk on, and the larger surface of a steep roof needs more material, which the roofing calculator works out from the footprint and pitch.
- Low slope (below 3/12): sheds water slowly; check the manufacturer's minimum slope and underlayment rules.
- Conventional (4/12 to 9/12): the most common range on houses; walkable at the lower end.
- Steep (above 9/12): more surface per square foot of footprint and harder to work on.
Common mistakes
- Mixing units, such as a rise in inches and a run in feet. Use the same unit for both.
- Measuring along the slope instead of horizontally. The run is always the level, horizontal distance.
- Using the full building width as the run on a gable roof. Each rafter runs only half the span, from the wall to the ridge.
- Treating degrees and pitch as the same number. A 6/12 roof is 26.57°, not 6° or 60°.
Frequently asked questions
What is a 4/12 roof pitch in degrees?
A 4/12 roof rises 4 inches per foot, which is arctan(4 ÷ 12) = 18.43°, or a 33.3% slope.
How do I convert degrees to roof pitch?
Take the tangent of the angle and multiply by 12. For 30°, tan(30°) × 12 = 6.93, so the pitch is about 7/12.
What is the most common roof pitch?
Most houses have pitches between 4/12 and 9/12. Lower pitches are common on porches and additions, steeper ones on older or snowy-climate houses.
What is the minimum pitch for asphalt shingles?
The International Residential Code allows asphalt shingles on slopes of 2/12 or steeper, with double underlayment required from 2/12 up to 4/12. Always check the shingle manufacturer's instructions too.
What is a pitch factor?
It is the ratio between the length along the roof and the horizontal distance: √(1 + (pitch ÷ 12)²). For 6/12 it is 1.118, so every 10 ft of horizontal run is 11.18 ft along the roof.
Sources
- International Code Council, International Residential Code (IRC), Section R905.2.2 Slope and R905.2.7 Underlayment application for asphalt shingles.
- Pitch, angle and slope relations: standard right-triangle trigonometry (tan θ = rise ÷ run).
Last reviewed 31 October 2026. Results are estimates; see our disclaimer.