Calculate the true sloped surface area of a roof from its footprint and pitch, in square feet, square metres and roofing squares.
A pitched roof is larger than the flat area it covers because it slopes. The steeper the pitch, the bigger the true surface area. This calculator multiplies the plan area by a slope factor to give the real surface you need to cover.
Because the roof slopes. A flat plan area only tells you the footprint — the actual sloped surface is larger by the slope factor, which grows with the pitch.
If your pitch is given as rise-in-12 (e.g. 6:12), the angle in degrees is arctan(rise ÷ run). A 6:12 pitch is about 26.6° and a 12:12 pitch is 45°.
A square is the standard unit for roofing materials and equals 100 square feet (about 9.29 m²). Shingles and felt are often sold and quoted by the square.
Yes — for a simple hip or gable roof the plan area multiplied by the slope factor gives the total surface area. Very complex roofs should be broken into separate sections.
Measure the building footprint at ground level, then multiply by the pitch (slope) factor for your roof angle. The calculator turns your plan size and pitch into the true sloped area.
