How to measure roof pitch
Pitch is the number of inches a roof rises for every 12 inches it runs horizontally. You only need a level and a tape measure. Pick whichever spot is safest to reach.
- In the attic: hold a level horizontally against the underside of a rafter, mark 12 inches along the level, and measure straight up from that mark to the rafter. That vertical distance is the pitch.
- On a gable end: measure the horizontal run and the vertical rise of the trim from a ladder, then enter both. Any pair of rise and run works; they do not have to be 12 inches.
- From an angle: a phone level app laid on a rafter or a roof photo gives an angle in degrees. Switch the calculator to angle mode and enter it.
How the numbers are calculated
Pitch = rise ÷ run × 12
Angle = arctan(rise ÷ run)
Slope percent = rise ÷ run × 100
Pitch factor = √(1 + (pitch ÷ 12)²)
Rafter run = span ÷ 2 − ridge thickness ÷ 2
Rafter length = (rafter run + overhang) × pitch factor
Worked example
A house is 24 feet wide with a 6/12 roof, a 1 1/2 inch ridge board and a 12 inch overhang. The rafter run is 144 − 0.75 = 143.25 inches. The pitch factor for 6/12 is 1.118, so the rafter is 160.2 inches (13' 4 3/16") to the plate line. The overhang tail adds 13 7/16", for a total line length of 14' 5 9/16". That fits in a 16 foot board.
Common roof pitches
| Pitch | Angle | Slope | Pitch factor |
|---|---|---|---|
| 2/12 | 9.46° | 16.7% | 1.0138 |
| 3/12 | 14.04° | 25% | 1.0308 |
| 4/12 | 18.43° | 33.3% | 1.0541 |
| 5/12 | 22.62° | 41.7% | 1.0833 |
| 6/12 | 26.57° | 50% | 1.118 |
| 7/12 | 30.26° | 58.3% | 1.1577 |
| 8/12 | 33.69° | 66.7% | 1.2019 |
| 9/12 | 36.87° | 75% | 1.25 |
| 10/12 | 39.81° | 83.3% | 1.3017 |
| 12/12 | 45° | 100% | 1.4142 |
What the pitch means for your project
- Below 2/12: treated as low slope. Asphalt shingles are not allowed under the International Residential Code, so these roofs use membranes such as EPDM, TPO or modified bitumen.
- 2/12 to 4/12: shingles are allowed, but with a double layer of underlayment.
- Over 4/12: OSHA classes it as a steep roof. Working on it calls for fall protection, and roof brackets help on anything you cannot walk comfortably.
- Estimating materials: the steeper the roof, the more surface it has over the same footprint. A 12/12 roof has about 41 percent more area than its footprint.
Frequently asked questions
What does a 6/12 roof pitch mean?
The roof rises 6 inches for every 12 inches it runs horizontally. That is an angle of 26.57 degrees and a slope of 50 percent.
How do I convert roof pitch to degrees?
Divide the rise by the run and take the arctangent. For a 4/12 roof, arctan(4 ÷ 12) is 18.43 degrees. An 8/12 roof is 33.69 degrees, and a 12/12 roof is exactly 45 degrees.
What is the pitch factor used for?
The pitch factor (also called the slope multiplier) converts a horizontal measurement into the length along the roof. Multiply the plan area of a roof by it to get the real roof area, or multiply the run by it to get the rafter length. The shingle calculator uses it to turn a house footprint into roofing squares.
What is the lowest pitch for asphalt shingles?
The International Residential Code allows asphalt shingles on slopes of 2/12 or steeper. From 2/12 up to 4/12 it requires a double layer of underlayment. Below 2/12 you need a low-slope system such as a membrane.
Which pitches count as a steep roof?
OSHA defines a steep roof as one with a slope greater than 4 in 12, and a low-slope roof as 4 in 12 or less. Many homeowners find anything above about 6/12 hard to walk on, so use roof brackets and fall protection.
Does the calculator include the birdsmouth and plumb cuts?
No. It gives the line length of the rafter: from the ridge (after deducting half the ridge board) to the outside of the wall, plus the overhang tail. Lay out the plumb cut at the ridge, the seat cut at the wall and the tail cut from those points.