Roof Pitch Calculator

Roof pitch is the slope of a roof expressed as rise over run. Standard notation: X/12 where X is how many inches the roof rises for every 12 inches of horizontal run. A 6/12 pitch rises 6 inches for every foot of run. It is equivalent to 26.6 degrees and is one of the most common residential pitches.

Minimum for asphalt shingles: 2/12 (with underlayment). Flat roofs: 1/4:12 to 1/2:12 (uses membrane, not shingles). Common residential: 4/12 to 9/12. Steep: 9/12 to 12/12. Extra steep or mansard: above 12/12. Low-pitch roofs need more waterproofing attention; steep roofs shed water better but cost more to build.

Rafter length = horizontal run × rafter factor. Rafter factor = sqrt(rise² + run²) / run. For 6/12 pitch: factor = sqrt(36+144)/12 = sqrt(180)/12 = 1.118. If roof run is 20 feet: rafter length = 20 × 1.118 = 22.36 feet. Add overhang/eave length and ridge board deduction for actual cut length.

Higher pitch = more surface area = more materials. Multiply the flat footprint area by the rafter factor to get actual roof area. For a 6/12 pitch, factor = 1.118: a 2,000 sq ft footprint needs 2,000 × 1.118 = 2,236 sq ft of roofing. Steeper roofs also require more care and labor, increasing cost.

Steep pitch (8/12+): heavy snow loads (steep sheds snow faster). High rainfall areas: 4/12+ for good runoff. Hot dry climates: lower pitch acceptable since rainfall is rare. Hurricane/wind: lower pitch with proper fasteners — steep roofs catch more wind. Architectural preference and HOA rules also affect pitch choice.