How To Calculate The Area Of A Roof

Calculating the area of a roof requires a combination of simple mathematical principles and an understanding of the roof’s shape and design. Here’s a general guide in English:

Flat Roofs

For a flat roof, the calculation is straightforward:

  1. Measure the total length and width of the entire roof in feet.
  2. Multiply the length by the width to get the area in square feet.


If your roof is 30 feet long and 20 feet wide, the area is 30 ft x 20 ft = 600 square feet.

Pitched Roofs

For a pitched (sloped) roof, you need to account for the slope:

  1. Measure the length and width of the building’s footprint in feet.
  2. Calculate the area as if it were flat.
  3. Determine the roof’s pitch (how much it rises for each foot of width). This is typically given in terms of rise over run (e.g., 4 in 12, meaning 4 inches of rise for every 12 inches of run).
  4. Find the slope factor using a pitch factor chart or calculate the square root of (pitch² + 12²) / 12.
  5. Multiply the footprint area by the slope factor to get the roof’s area.


If the footprint is 30 feet by 20 feet, and the pitch is 4 in 12:

  • Footprint area = 30 ft x 20 ft = 600 square feet.
  • The slope factor for a 4 in 12 pitch is √(4² + 12²) / 12 = 1.06.
  • Roof area = 600 square feet x 1.06 = 636 square feet.

Gabled Roofs

For a gabled roof, you would:

  1. Calculate the area of each rectangular section (like a flat roof).
  2. For the triangular sections, measure the base and height and use the formula for the area of a triangle (Area = 0.5 x base x height).
  3. Add the areas of all sections together to get the total roof area.

With two rectangular sections, each measuring 30 ft by 10 ft, and two triangular sections with a base of 10 ft and a height of 5 ft:

  • Rectangular area = 2 x (30 ft x 10 ft) = 600 square feet.
  • Triangular area = 2 x (0.5 x 10 ft x 5 ft) = 50 square feet.
  • Total roof area = 600 square feet + 50 square feet = 650 square feet.

Special Roof Shapes

If the roof is more complex (like hexagonal), you must divide it into regular shapes, calculate each area, and then sum them up. Always consider extra features like dormers or valleys that can alter the simple rectangular or triangular calculations.

For accurate roofing material estimates or construction work, it’s advisable to get precise measurements and calculations, often done by professionals using special tools and accounting for local building codes and material requirements.

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