How this calculator works
For a simple symmetrical gable or shed roof, pitch increases the sloped surface area above the flat building footprint. The result is then converted to roofing squares, where one square equals 100 sq ft.
Formula
Roof area \u2248 footprint \u00d7 \u221a(1 + (rise \u00f7 12)\u00b2)Worked example
A 30 \u00d7 40 ft footprint is 1,200 sq ft. A 6:12 pitch multiplies that footprint by about 1.118 before waste.