Roof Pitch Calculator
Pitch, angle, slope factor and rafter length from rise and run.
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Step by step
The formula
angle = arctan(rise ÷ run); rafter = √(rise² + run²)
Roof pitch describes steepness as rise over run, traditionally expressed per 12 units of run. It determines which materials are suitable, how much surface area the roof actually has, and how long the rafters need to be — all from one right triangle.
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Example calculations
Common questions this calculator answers — select one to load its values.
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Frequently asked questions
What is the slope factor for?+
A sloped roof has more surface than the ground it covers. Multiply the flat footprint area by the slope factor to get the actual area you need to buy materials for.
What pitch do I need for shingles?+
Asphalt shingles are typically rated for 4:12 and above. Between 2:12 and 4:12 they usually need a doubled underlayment, and below 2:12 you need a fully sealed membrane system instead.
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