Roof Pitch Calculator
Roof Parameters
Calculation Results
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How to Calculate Roof Pitch
- Measure rise and run, or the angle. Run is the level distance; rise is how far the roof climbs over it. Measuring 12 inches level and reading the rise gives the pitch directly (rise 6 in. over 12 in. = 6:12). If you have the angle (from a phone app or a plan), choose "Angle in degrees".
- Pitch (x:12) = rise ÷ run × 12, or tan(angle) × 12.
- Angle = arctan(rise ÷ run). A 6:12 roof is 26.57°.
- Percent slope = rise ÷ run × 100. A 6:12 roof is a 50% slope.
- Area factor = √(1 + (rise ÷ run)²): roof area = footprint × area factor. A 6:12 roof is 1.1180.
- Slope class: under 2:12 flat; 2:12 to under 4:12 low slope; 4:12 to under 9:12 conventional; 9:12 to 12:12 steep; over 12:12 very steep.
Rise and run can be in inches or centimeters; the pitch is the same either way.
Roof Pitch Examples
Example 1: Standard house roof
Rise 6 in., run 12 in.
- 6:12, 26.57°, 50%
- Area factor 1.1180 · Conventional slope
Example 2: 4:12 roof
Rise 4 in., run 12 in. (result shown in degrees)
- 18.43°, 4:12, 33.33%
- Area factor 1.0541 · Conventional slope (4:12 is where conventional starts)
Example 3: Steep roof
Rise 12 in., run 12 in.
- 12:12, 45°, 100%
- Area factor 1.4142 · Steep slope
Example 4: Metric
Rise 25 cm, run 100 cm
- 3:12, 14.04°, 25%
- Area factor 1.0308 · Low slope
Example 5: From the angle
Angle 30°
- 6.93:12, 57.74%
- Area factor 1.1547 · Conventional slope
Roof Pitch Chart (Degrees and Area Factor)
| Pitch | Angle | Area factor (roof area ÷ footprint) |
|---|---|---|
| 1:12 | 4.76° | 1.0035 |
| 2:12 | 9.46° | 1.0138 |
| 3:12 | 14.04° | 1.0308 |
| 4:12 | 18.43° | 1.0541 |
| 5:12 | 22.62° | 1.0833 |
| 6:12 | 26.57° | 1.1180 |
| 7:12 | 30.26° | 1.1577 |
| 8:12 | 33.69° | 1.2019 |
| 9:12 | 36.87° | 1.2500 |
| 10:12 | 39.81° | 1.3017 |
| 11:12 | 42.51° | 1.3566 |
| 12:12 | 45.00° | 1.4142 |
Multiply the footprint by the area factor to get the roof area (see the roofing calculator).
Minimum slopes in the International Residential Code (2024 IRC, Section R905):
- Asphalt shingles: 2:12 minimum; between 2:12 and 4:12 they need double underlayment (R905.2.2).
- Clay and concrete tile: 2-1/2:12 minimum; double underlayment from 2-1/2:12 to 4:12 (R905.3.2).
- Metal panels: 3:12 for lapped, non-soldered seams without lap sealant, 1/2:12 with lap sealant, 1/4:12 for standing seam (R905.10.2).
- Slate: 4:12 (R905.6.2). Wood shingles and shakes: 3:12 (R905.7.2, R905.8.2).
- Flat (low-slope) roofs still need about 1/4:12 to drain (the minimum for built-up and membrane roofs).
Local codes can differ; check your jurisdiction's adopted edition.