Calculate 4/12 roof pitch rafter length, rise, slope angle, and roofing area from span, run, overhang, or roof length and width in feet or meters.
4/12 Roof Pitch Formulas
A 4/12 pitch means the roof rises 4 inches vertically for every 12 inches of horizontal run. The calculator uses these formulas depending on the tab:
Rise = Run × (4 / 12) Rafter Length = Run × √(4² + 12²) / 12 = Run × 1.0541 Roof Angle = arctan(4 / 12) = 18.43° Roof Area = Length × Width × 1.0541
- Run — horizontal distance from outside wall to roof centerline (half the span on a symmetric gable).
- Rise — vertical distance from wall plate to ridge.
- Rafter Length — sloped length from wall plate to ridge, before any overhang.
- Length × Width — building footprint covered by the roof.
- 1.0541 — the slope multiplier for 4/12, equal to √160 / 12.
The area formula assumes a simple gable with no dormers, hips, or valleys. Add overhang to the run before multiplying if you want rafter length to the fascia. For materials, multiply the sloped area by 1 plus your waste factor.
Reference Values for 4/12 Pitch
Common conversions and run-to-rise values you can use without re-running the calculator:
| Property | Value |
|---|---|
| Rise per 12 in run | 4 in |
| Roof angle | 18.43° |
| Percent grade | 33.3% |
| Slope multiplier | 1.0541 |
| Radians | 0.3217 rad |
| Walkability | Walkable, low-slope feel |
| Run (ft) | Rise (ft) | Rafter (ft) |
|---|---|---|
| 8 | 2.67 | 8.43 |
| 10 | 3.33 | 10.54 |
| 12 | 4.00 | 12.65 |
| 14 | 4.67 | 14.76 |
| 16 | 5.33 | 16.87 |
| 20 | 6.67 | 21.08 |
Worked Example and FAQ
Example: A gable roof spans 24 ft with a 1 ft overhang. Run to wall is 12 ft, so the ridge rise is 12 × 4/12 = 4 ft. Rafter length to the wall is 12 × 1.0541 = 12.65 ft. Adding the overhang, total rafter is 13 × 1.0541 = 13.70 ft. For a 40 ft long building, sloped roof area is 24 × 40 × 1.0541 = 1,011.9 sq ft, or about 10.12 squares.
Is 4/12 considered low slope? No. Code typically defines low slope as under 2/12. A 4/12 pitch is the lower end of standard slope and accepts most asphalt shingles without special underlayment.
Can you walk on a 4/12 roof? Yes, with normal caution and proper footwear. It feels noticeably sloped but does not require staging or roof jacks for most people.
Why use 1.0541 instead of measuring the rafter? The multiplier comes from the Pythagorean theorem applied to the 4-12 triangle. Multiplying any horizontal run by 1.0541 gives the sloped distance over that run for a 4/12 pitch.
