How to measure roof pitch
Roof pitch is the rise over the run — how many inches a roof climbs for every 12 inches it runs horizontally. To measure it, hold a level straight out from the roof, mark 12 inches along it, and measure straight down from that mark to the roof. That drop is the rise:
Pitch = rise : 12 — so a 6-inch rise over a 12-inch run is a 6:12 pitch.
You can measure safely from inside the attic against a rafter, or from a ladder at the eave — there's no need to climb onto a steep roof just to find the pitch.
Pitch in degrees and percent
The same slope can be written three ways, and this calculator shows all of them:
- Ratio (X:12) — the roofer's standard, like 6:12.
- Angle — degrees from horizontal: angle = arctan(rise ÷ run).
- Slope / grade — a percentage: rise ÷ run × 100.
Common roof pitches
| Pitch | Angle | Slope | Slope factor |
|---|---|---|---|
| 3:12 | 14.0° | 25% | ×1.031 |
| 4:12 | 18.4° | 33% | ×1.054 |
| 5:12 | 22.6° | 42% | ×1.083 |
| 6:12 | 26.6° | 50% | ×1.118 |
| 8:12 | 33.7° | 67% | ×1.202 |
| 10:12 | 39.8° | 83% | ×1.302 |
| 12:12 | 45.0° | 100% | ×1.414 |
Slope factor and roof area
The slope factor is what turns a flat footprint into real roof area: a steeper roof has more surface over the same ground. Multiply your roof's footprint (length × width) by the factor to get the sloped area — for a 6:12 roof that's footprint × 1.118. To go from there to roofing squares, bundles of shingles, and a cost, use the roofing calculator.
Rafter length
A rafter is the hypotenuse of the rise-and-run triangle, so rafter length = √(rise² + run²). Enter your actual rise and horizontal run (not the 12-inch shorthand) and the result is the true rafter length, before any overhang or ridge-board adjustments.
Good to know
- Low slope ≠ flat. Anything under about 3:12 is "low slope" and usually needs a membrane or rolled roofing rather than standard shingles.
- Walkable vs. steep. Roofs up to about 6:12 are generally walkable; steeper than that needs roof jacks and fall protection.
- Measure safely. Take the rise from the attic or a ladder — never climb a steep or wet roof just to measure, and hire a pro for steep or complex roofs.
Why roof pitch matters
Pitch is more than a number on a plan — it drives real decisions. Drainage and materials: steeper roofs shed water and snow quickly and take almost any covering, while low slopes pond water and need membranes or rolled roofing instead of standard shingles. Cost: a steeper roof has more surface area than its footprint (that's the slope factor) and is slower and riskier to work on, so both materials and labor climb with pitch. Usable space: a steep pitch creates attic or vaulted-ceiling room; a shallow one doesn't. And walkability sets how the work gets done — anything past about 6:12 needs roof jacks and fall protection.
Finding pitch without climbing
You don't need to get on the roof. From inside the attic, hold a level horizontally against the underside of a rafter, measure 12 inches along it, and measure straight down to the rafter — that drop is your rise over 12. A smartphone angle/level app laid on a rafter (or on the roof from a ladder at the eave) reads the angle directly, which you can match to the table above. A magnetic pitch gauge does the same in seconds.