Roof Pitch Calculator
Roof pitch is rise per 12 inches of run: divide the rise by the run and multiply by 12. A roof that rises 6 in over 12 in is 6/12, or 26.6°, with a pitch multiplier of 1.118.
Roof pitch
What roof pitch means
Pitch (or slope) is written as rise over a 12-inch run: a 4/12 roof climbs 4 inches for each foot it extends horizontally. IKO's estimating guide defines it as the ratio of rise in inches to horizontal run in feet, where the run is half the span of the building. Degrees and percent grade describe the same slope another way.
pitch (x/12) = rise ÷ run × 12 degrees = arctan(pitch ÷ 12) multiplier = √(1 + (pitch ÷ 12)²) hip/valley factor = √(2 + (pitch ÷ 12)²)
Roof pitch chart: degrees and multipliers
Degrees and multipliers below are calculated. The IKO column is the factor printed in IKO's Table 1 so you can compare (IKO rounds 7/12 and 11/12 down by 0.001).
| Pitch | Degrees | Multiplier | IKO area factor | IKO hip/valley |
|---|---|---|---|---|
| 1/12 | 4.76° | 1.003 | – | – |
| 2/12 | 9.46° | 1.014 | – | – |
| 3/12 | 14.04° | 1.031 | – | – |
| 4/12 | 18.43° | 1.054 | 1.054 | 1.452 |
| 5/12 | 22.62° | 1.083 | 1.083 | 1.474 |
| 6/12 | 26.57° | 1.118 | 1.118 | 1.500 |
| 7/12 | 30.26° | 1.158 | 1.157 | 1.524 |
| 8/12 | 33.69° | 1.202 | 1.202 | 1.564 |
| 9/12 | 36.87° | 1.25 | 1.250 | 1.600 |
| 10/12 | 39.81° | 1.302 | 1.302 | 1.642 |
| 11/12 | 42.51° | 1.357 | 1.356 | 1.684 |
| 12/12 | 45° | 1.414 | 1.414 | 1.732 |
Using the multiplier
Measure the house footprint including the overhangs, multiply length by width, then multiply by the pitch multiplier. A 40 × 30 ft footprint at 6/12 is 1,200 × 1.118 = 1,342 sq ft of roof. The same factor turns a rafter's horizontal run into its length along the slope: a 15 ft run at 6/12 is 16.77 ft, before the overhang and ridge cuts.
Low-slope limits for shingles
- Below 2/12: asphalt shingles are not allowed (IRC R905.2.2; GAF instructions).
- 2/12 to under 4/12: two layers of underlayment are required.
- 4/12 and up: one layer of underlayment under GAF's instructions.
Frequently asked questions
What is a 6/12 roof pitch in degrees?
A 6/12 roof rises 6 inches for every 12 inches of horizontal run. That is arctan(6 ÷ 12) = 26.57 degrees, with a pitch multiplier of 1.118.
How do I find my roof pitch?
Hold a level horizontally against the roof or a rafter, mark 12 inches along the level, and measure straight up from that mark to the roof. That measurement is the rise per 12 inches, so 5 inches means a 5/12 pitch. IKO defines slope the same way: rise in inches per foot of horizontal run.
What is the pitch multiplier?
It is how much longer the sloped roof is than its flat footprint: √(1 + (rise ÷ 12)²). Multiply the horizontal area by it to get the roof area. IKO's estimating guide lists 1.054 for 4/12, 1.118 for 6/12 and 1.414 for 12/12.
What is the minimum roof pitch for shingles?
2/12. The IRC (R905.2.2, as summarized by the City of Hopkins, MN) and GAF's installation instructions allow asphalt shingles only on slopes of 2:12 or more, and from 2:12 up to 4:12 they require two layers of underlayment.
What is a 12/12 roof pitch?
A 12/12 roof rises as much as it runs: 45 degrees, with a multiplier of 1.414. Every square foot of footprint becomes about 1.41 sq ft of roof.
Sources
- Estimating How Much Roofing Is Required, IKO. Used for: slope definition (rise per foot of run), area/rake factors (Table 1) and hip/valley factors (Table 2).
- Asphalt Shingles (IRC R905.2), City of Hopkins, MN. Used for: IRC R905.2 minimum 2:12 slope for asphalt shingles and double underlayment from 2:12 to 4:12.
- Installation Instructions: Timberline HDZ, UHDZ and related shingles (©2023), GAF (manufacturer PDF, hosted by Home Depot). Used for: GAF minimum slope 2:12 and two underlayment layers below 4:12.
Estimates only. Check local codes and your product's label. Found a number that does not match its source? Send a correction.