Roof pitch tells you how steep a roof is. In the United States it is written as inches of vertical rise for every 12 inches of horizontal run, so a 6:12 roof climbs 6 inches for each foot it travels toward the ridge. That one number decides how much surface the roof has, which coverings are allowed on it and how the work has to be staged.
Pitch, angle and slope are three names for one measurement
The same steepness can be written three ways. Pitch is the rise per 12 of run. Slope percentage is rise divided by run, times 100. The angle is the arctangent of rise divided by run. A 6:12 pitch is a 50% slope and an angle of 26.57°. It is not 6 degrees, and it is not half of 45 degrees either: the angle grows more slowly than the pitch as roofs get steeper.
Run is always measured level, not along the roof surface. If you measure along the rafter you get the hypotenuse of the triangle and your pitch comes out too low.
| Pitch | Angle | Slope | Area multiplier | Roof area over 1,500 sq ft of footprint |
|---|---|---|---|---|
| ¼:12 | 1.19° | 2.1% | 1.0002 | 1,500 sq ft |
| ½:12 | 2.39° | 4.2% | 1.0009 | 1,501 sq ft |
| 1:12 | 4.76° | 8.3% | 1.0035 | 1,505 sq ft |
| 2:12 | 9.46° | 16.7% | 1.0138 | 1,521 sq ft |
| 3:12 | 14.04° | 25.0% | 1.0308 | 1,546 sq ft |
| 4:12 | 18.43° | 33.3% | 1.0541 | 1,581 sq ft |
| 5:12 | 22.62° | 41.7% | 1.0833 | 1,625 sq ft |
| 6:12 | 26.57° | 50.0% | 1.1180 | 1,677 sq ft |
| 7:12 | 30.26° | 58.3% | 1.1577 | 1,737 sq ft |
| 8:12 | 33.69° | 66.7% | 1.2019 | 1,803 sq ft |
| 9:12 | 36.87° | 75.0% | 1.2500 | 1,875 sq ft |
| 10:12 | 39.81° | 83.3% | 1.3017 | 1,953 sq ft |
| 12:12 | 45.00° | 100.0% | 1.4142 | 2,121 sq ft |
| 14:12 | 49.40° | 116.7% | 1.5366 | 2,305 sq ft |
| 16:12 | 53.13° | 133.3% | 1.6667 | 2,500 sq ft |
| 18:12 | 56.31° | 150.0% | 1.8028 | 2,704 sq ft |
What the area multiplier is for
A plan or a tape measure on the ground gives you the horizontal footprint of a roof. The sloped surface is larger, and the multiplier in the table is the conversion. Multiply the footprint, including overhangs, by the multiplier for the pitch and you have the surface that needs covering.
The effect is small on shallow roofs and large on steep ones. Over 1,500 sq ft of footprint a 4:12 roof has 1,581 sq ft of surface, a 9:12 roof has 1,875 sq ft and a 12:12 roof has 2,121 sq ft. Going from 4:12 to 12:12 adds 34% more material on the same house. This is why an estimate based on floor area alone is unreliable.
How to find the pitch of an existing roof
From the attic
Hold a level against the underside of a rafter with one end touching it. Mark 12 inches along the level, then measure straight up from that mark to the underside of the rafter. That vertical distance in inches is the pitch. Seven inches means 7:12. The method works from inside, on a dry surface, without stepping on the roof.
From the gable end
On a gable wall you can measure the rise and run of the roof triangle directly. Measure the level distance from the outside of the wall to the point below the ridge, and the height from the top of the wall to the ridge. Divide rise by run and multiply by 12. With 7 ft 6 in of rise over 15 ft of run, 7.5 ÷ 15 × 12 = 6, a 6:12 roof. The roof pitch calculator does this conversion for any pair of measurements in the same unit.
From drawings
Construction drawings show pitch as a small right triangle on the roof line in the elevations, with 12 on the level leg and the rise on the vertical leg. If you have the drawings, use that figure and confirm it with one of the measurements above, because roofs are sometimes built differently from the plan.
Pitch limits for common roof coverings
Building codes tie roof coverings to a minimum slope because shallow roofs shed water slowly. The model residential code text, as adopted in the California Residential Code, sets these minimums:
- Asphalt shingles: only on slopes of 2:12 or greater. From 2:12 up to 4:12 a double layer of underlayment is required; from 4:12 a single layer.
- Metal panels with lapped, nonsoldered seams and no lap sealant: 3:12.
- The same panels with applied lap sealant: ½:12.
- Standing-seam metal roof systems: ¼:12.
These are code minimums. A manufacturer can require more for a specific product, and the installation instructions for the product are part of what an inspector checks. Our calculators flag sections below 2:12 because shingle quantities for such a roof are arithmetic only.
Mistakes that change the answer
- Mixing units. Rise and run must be in the same unit before you divide. Feet over inches gives a pitch twelve times too small.
- Measuring along the slope. The 12 is level run. Measured along the rafter, a true 6:12 reads as about 5.4:12.
- Assuming one pitch for the whole house. Porches, dormers and additions often have their own pitch. Enter them as separate sections in the roof area calculator, each with its own value.
- Applying the multiplier twice. If a supplier or a measurement report already gives sloped area, do not multiply by the pitch factor again.
Sources
- 2025 California Residential Code, Section R905, Requirements for Roof Coverings. Minimum slopes for asphalt shingles (R905.2.2), underlayment layers (R905.1.1) and metal roof panels (R905.10.2). The California code is based on the International Residential Code; other states adopt their own editions and amendments.
Building codes are adopted and amended locally, and product specifications change. Check the code edition your jurisdiction enforces and the current data sheet of the product you buy.