Pitch, degrees and slope are three views of one ratio
Pitch is written as rise-per-12-inches-of-run, so 4:12 means the roof climbs 4 inches over a foot. That single ratio also gives the angle (arctangent of rise over run) and the percent slope (rise over run, times 100). The common mistake is reading "4:12" as 4 degrees — it is actually 18.4.
Common pitches, in every unit
| Pitch | Angle | Slope % | Typical use |
|---|---|---|---|
| 1:12 | 4.8° | 8.3% | Roll roofing, low-slope |
| 2:12 | 9.5° | 16.7% | Shingle minimum (needs ice & water shield) |
| 4:12 | 18.4° | 33.3% | Standard asphalt shingle |
| 6:12 | 26.6° | 50% | Common steeper pitch |
| 8:12 | 33.7° | 66.7% | Steep — slate, wood shake |
| 10:12 | 39.8° | 83.3% | Very steep |
| 12:12 | 45° | 100% | A perfect 45° |
A 12:12 pitch is exactly 45 degrees and 100 percent slope — the point where rise equals run.
Two things the pitch decides
- What material you can use. Asphalt shingles need at least 2:12 (with a full underlayment) or 4:12 as standard; standing-seam metal can go as low as 1/4:12. Go below a material's minimum and water works back under the surface.
- How long the rafters run. A rafter is always longer than the run: length = √(rise² + run²). A 6:12 roof over a 20 ft run needs a rafter of √(10² + 20²) ≈ 22.4 ft, not 20.
Common questions
What does a 4/12 roof pitch mean?
The roof rises 4 inches for every 12 inches it runs horizontally. In degrees that is about 18.4, and as a slope it is 33 percent. 4:12 is the most common pitch on US homes.
How do I convert roof pitch to degrees?
Take the arctangent of rise divided by run. For 6:12 that is arctan(6/12) = arctan(0.5) = 26.6 degrees. Do not read the numbers directly: 4:12 is not 4 degrees.
What is the difference between pitch and slope?
They are the same steepness expressed differently. Pitch is the ratio (4:12); slope is the percentage, rise divided by run times 100 (a 4:12 pitch is a 33 percent slope).
