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Roof Pitch & Rafter Length Calculator

Find a roof's pitch (X:12), slope, angle, and rafter length from rise and run, with an IRC/OSHA-informed slope classification.

Pitch
6 : 12
Slope
50%
Roof angle
26.57°
Rafter length (for this run)
33.54 cm
Roof area multiplier
×1.118
Classification
Conventional slope -- the standard range for asphalt shingles

Where this falls

6 falls in the "Conventional" range.

How it works

Roofing pitch is conventionally written as "X:12" — X inches of rise for every 12 inches of horizontal run — so this calculator scales your entered rise:run ratio to that same 12-unit basis (pitch = rise ÷ run × 12) regardless of the actual units you measured in. From the same right triangle, slope% = rise ÷ run × 100, the roof angle is the arctangent of rise ÷ run, and the rafter length spanning that run is the Pythagorean hypotenuse, √(rise² + run²).

The rafter-length-to-run ratio (the "roof area multiplier") tells you how much more surface area the sloped roof actually has compared to its flat footprint — for example, a 6:12 roof's multiplier works out to √(1 + 0.5²) ≈ 1.118, meaning a 1,000 sq ft footprint needs about 1,118 sq ft of actual roofing material. This is pure Pythagorean geometry, not a looked-up table, and it reproduces the ~1.054 and ~1.118 multipliers commonly published for 4:12 and 6:12 roofs as an exact cross-check.

The slope is also classified against roofing-industry bands verified via web search — OSHA officially treats anything under 4:12 as "low-slope" for fall-protection purposes, while roofing references generally put a practical minimum around 2:12 for standard shingles and treat 9:12+ as steep enough to need specialty fastening. Since rise:run is a pure ratio, every result here is identical whichever unit system rise and run are both measured in together, the same unit-invariant-ratio reasoning this site's Ramp Slope calculator already established — no metric/imperial conversion happens internally.

FAQ

Is 4:12 always the line between "low-slope" and a regular roof?

It's OSHA's official cutoff for when additional roofer fall-protection rules kick in, but roofing-material manufacturers and local building codes don't all draw the line in the same place — some treat anything at or below 2:12 or 3:12 as needing special low-slope membrane roofing instead of standard shingles. Check your specific roofing material's manufacturer instructions and local code rather than treating one number as universal.

Why does the roof area multiplier matter?

It converts a building's flat footprint area into the actual sloped surface area that needs to be covered — since a steeper roof has more surface area than its footprint suggests, multiplying footprint area by this factor gives a much better estimate of the shingles, underlayment, or sheet material you'll actually need than the footprint number alone.

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