Roof Area Calculator

Calculate the true sloped surface area of a roof from its footprint and pitch, in square feet, square metres and roofing squares.

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How to Use This Calculator

How to Use the Roof Area Calculator

  1. Choose metric or imperial units.
  2. Enter the length and width of the building footprint the roof covers (measured on the ground / plan).
  3. Enter the roof pitch in degrees. Add an eaves overhang per side if you want it included.

Why Footprint Isn't the Same as Roof Area

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.

How the Numbers Are Worked Out

  • Plan area = (length + 2 × overhang) × (width + 2 × overhang).
  • Slope factor = 1 ÷ cos(pitch angle).
  • Roof area = plan area × slope factor.
  • Roofing squares = roof area in square feet ÷ 100 (a “square” is the standard roofing unit).

Tips

  • This works for simple gable, hip and mono-pitch roofs measured on plan. Complex roofs should be split into sections and added together.
  • To convert a pitch given as rise-in-12 to degrees, the angle is arctan(rise ÷ 12) — e.g. 6:12 ≈ 26.6°.
  • Once you have the roof area, use our roofing or shingle calculators to work out materials.

Common Questions

Why is my roof area bigger than the floor area?

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.

How do I convert roof pitch to degrees?

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°.

What is a roofing “square”?

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.

Does this work for a hip roof?

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.

Why is the roof area bigger than the building footprint?

Because a pitched roof slopes, its surface is larger than the flat area it covers. The calculator multiplies the footprint by a pitch factor to give the true area.

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