How roof pitch works
Roof pitch is the rise in inches for every 12 inches of horizontal run, written as a ratio like 6/12. The same slope can be written as an angle (6/12 is about 26.6°) or a percentage (50%). Enter whichever you know and the calculator gives the others, then works out rafters, roof area and shingles for the building.
Roof pitch chart
| Pitch | Angle | Pitch factor | Typical use |
|---|---|---|---|
| 2/12 | 9.5° | 1.014 | Lowest pitch for asphalt shingles |
| 3/12 | 14.0° | 1.031 | Sheds, porches, low-slope additions |
| 4/12 | 18.4° | 1.054 | Ranch houses |
| 6/12 | 26.6° | 1.118 | Common for houses |
| 8/12 | 33.7° | 1.202 | Steeper houses, hip roofs |
| 10/12 | 39.8° | 1.302 | Steep gables |
| 12/12 | 45° | 1.414 | A-frames, steep gables |
How the numbers are worked out
For a gable or hip roof, the common rafter covers half the building width, less half the ridge board. Its length is that run plus the eave overhang, times the pitch factor (√(1 + (pitch ÷ 12)²)). Roof area is the footprint including overhangs times the pitch factor, which holds for any roof whose planes share one pitch. A square is 100 sq ft, and shingles come 3 bundles per square.
Questions people ask
How do I find my roof pitch?
Hold a level horizontally against a rafter or the roof surface, measure 12 in along the level, then measure straight up to the roof. That number is the pitch: 6 in up means a 6/12 pitch. You can also enter any measured rise and run, or the angle, and the calculator converts it.
What is a roofing square?
A roofing square is 100 sq ft of roof surface. Most asphalt shingles come 3 bundles to a square, so a 25-square roof needs about 75 bundles before waste.
What is the pitch factor?
The pitch factor is how much longer a sloped roof is than its flat footprint. Multiply the footprint area, including overhangs, by the pitch factor to get the roof surface area. A 6/12 roof has a factor of about 1.118.
How much waste should I add for shingles?
About 10% for a simple gable roof and 15% or more for hip roofs and roofs with valleys and dormers, because of the extra cutting.
What is the minimum pitch for asphalt shingles?
The International Residential Code allows asphalt shingles on slopes of 2/12 or steeper. Between 2/12 and 4/12 they need two layers of underlayment.