How this calculator works
The calculator divides rise by run to create the roof pitch ratio and uses arctangent to estimate the roof angle. The pitch factor is the sloped rafter length divided by horizontal run. Roof area is the flat projected area multiplied by pitch factor, then increased by the selected waste allowance.
Example: a 6 in rise over a 12 in run is a 6:12 pitch. The angle is about 26.57 degrees and the pitch factor is about 1.118. A 40 ft by 30 ft section becomes about 1,475.8 square feet with 10% waste.