Roof Pitch Calculator
Roof pitch determines rafter cuts, shingle selection, and structural loads. Enter the rise and run to get the pitch ratio, angle in degrees, and rafter length.
Understanding Roof Pitch Ratios
Roof pitch is always expressed as rise over run. The run is standardized at 12 inches, so a 4/12 pitch rises 4 inches per foot of horizontal distance. Steeper pitches have larger rise numbers.
Flat roofs are actually low-slope, typically 1/12 to 2/12, to allow drainage. Anything below 2/12 requires special waterproofing and cannot use standard asphalt shingles.
Steep roofs start around 9/12 and go up from there. Mansard and A-frame designs can hit 18/12 or more. Steep pitches shed snow and rain aggressively but cost more to build and roof because of the extra material and labor involved.
How Pitch Affects Roofing Materials
Asphalt shingles work on pitches from 2/12 up. Below that, water sits too long and seeps under the shingles. Manufacturers void warranties on installations below the minimum slope.
Metal roofing tolerates lower pitches, down to 1/12 with proper sealing. Tile and slate need at least 4/12 to prevent water from backing up under the laps. Roll roofing and membrane systems handle flat or near-flat applications.
Pitch also drives ventilation needs. Steeper roofs create taller attic spaces with better natural airflow. Low-slope roofs trap heat and require more aggressive mechanical ventilation to prevent moisture buildup and premature aging.
Calculating Rafter Length from Pitch
Rafter length is the hypotenuse of a right triangle formed by the rise and run. If your building is 40 feet wide, each rafter spans from the ridge to the wall, covering half that distance horizontally, or 20 feet.
The rise is determined by the pitch. A 6/12 pitch on a 20-foot run rises 10 feet (6 inches per foot). Use the Pythagorean theorem: rafter length = √(20² + 10²) = √500 ≈ 22.36 feet.
Add extra length for overhangs and tail cuts. Rafter calculators provide the bare minimum from ridge to wall. Most designs include 12 to 24-inch overhangs for eave protection and aesthetics.
Frequently Asked Questions
What is roof pitch?
Roof pitch is the steepness of a roof, expressed as a ratio of rise over run. A 6/12 pitch means the roof rises 6 inches for every 12 inches of horizontal distance.
What is a normal roof pitch?
Residential roofs typically range from 4/12 to 9/12. A 6/12 pitch is common because it sheds water well while remaining walkable for roofers.
How do I measure roof pitch?
Use a level and tape measure. Hold the level horizontally from the roof surface, measure 12 inches along it, then measure the vertical distance from the roof to the level. That vertical distance is your rise.
Does pitch affect shingle choice?
Yes. Low-slope roofs under 3/12 need special underlayment and may require roll roofing instead of shingles. Steep roofs over 12/12 may need extra fasteners to prevent shingles from sliding.
Can I convert pitch to degrees?
Yes. Use the arctangent function: angle = arctan(rise / run). A 6/12 pitch equals approximately 26.6 degrees. This calculator shows both ratio and degrees.