Calculate pitch angle, actual roof area, rafter length and ridge length for gable, hip and skillion roofs.
To convert a pitch ratio (rise:run) to degrees, use arctan(rise/run). For example, a 1:3 pitch gives arctan(1/3) = 18.4°. A 1:2 pitch gives arctan(1/2) = 26.6°. This calculator does the conversion automatically.
The pitch factor is the ratio of actual rafter length to horizontal run. It equals √(1 + (rise/run)²). Multiply your plan (footprint) area by the pitch factor to get the actual roof area needed for materials.
Calculate the plan footprint (length × width), then multiply by the pitch factor for the actual slope area. For a gable roof, double the slope area of one side. Add a waste allowance of 10–15% for cuts and overlaps.