What is a roof pitch?

Roof pitch describes a roof’s steepness. InterNACHI’s technical definition divides rise by the full span; everyday rise-per-12 notation describes slope instead. For a home roofing project, clarify which measurement a proposal uses: NRCA explains that slope helps asphalt shingles drain water and recommends against shingles below 18 degrees.

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By InstaBids editorial team with AI research agents. Updated 31 October 2026

What this means for your home project

Ask whether the proposal’s pitch figure uses rise over span or rise per 12 inches of run. Source: InterNACHI, read 2026-10-31.
Ask the roofer to explain how the proposed asphalt-shingle system will drain water at your roof’s slope. Source: National Roofing Contractors Association, read 2026-10-31.
If asphalt shingles are proposed below 18 degrees, ask how the proposal addresses NRCA’s recommendation against that slope. Source: National Roofing Contractors Association, read 2026-10-31.

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What does a 4/12 roof pitch mean?

In rise-per-foot notation, 4/12 describes vertical rise against horizontal distance, as InterNACHI explains below. “A roof that rises 4 inches for every 1 foot or 12 inches of run is said to have a “4 in 12” slope.” Source: InterNACHI, read 2026-10-31.

What does roof pitch mean in technical terminology?

InterNACHI uses pitch for rise divided by the full span, rather than the run. “Pitch is the incline of the roof expressed as a fraction derived by dividing the rise by the span, where the roof span is the distance between the outside of one wall's top plate to another.” Source: InterNACHI, read 2026-10-31.

Why isn’t a 6/12 pitch 22.5 degrees?

Using InterNACHI’s right-triangle geometry, our calculation is arctan(6/12) = 26.57 degrees. Angles do not scale linearly with rise. “If the rise is 6 inches for every 12 inches of run, then the roof slope is “6 in 12.”” Source: InterNACHI, read 2026-10-31.

What roof pitch is considered steep?

For federal construction fall-protection terminology, OSHA draws the boundary at 4:12. This classification describes roof steepness; it does not establish that a roof is safe to walk on. “Low-slope roof means a roof having a slope less than or equal to 4 in 12 (vertical to horizontal).” Source: Occupational Safety and Health Administration, read 2026-10-31. “Steep roof means a roof having a slope greater than 4 in 12 (vertical to horizontal).” Source: Occupational Safety and Health Administration, read 2026-10-31.

Why does roof pitch matter?

Pitch matters when discussing drainage and the covering for your roof. NRCA explains the drainage principle for asphalt shingles below; its separate recommendation is to avoid shingles below 18 degrees. Ask how the proposed roof system suits your slope. “Asphalt shingle roofs rely on gravity and roof slope to effectively drain water.” Source: National Roofing Contractors Association, read 2026-10-31.

Sources

Measuring Roof Slope and Pitch — InterNACHI; read 2026-10-31.
29 CFR 1926.500(b): Roof definitions — Occupational Safety and Health Administration; read 2026-10-31.
Roofing guidelines: Resources and homeowner questions — National Roofing Contractors Association; read 2026-10-31.