How to Calculate the Roof Area Using the Building Square Footage & the Pitch of the Roof

by Bill Butler
    You could calculate the roof area of any one of these houses without ever setting foot on a ladder.


    So you're thinking of reroofing the house yourself and want an estimate for how much roofing material you'll need, but you don't want to go climbing around with a tape measure and scare your spouse half to death. You're in luck, because math can rescue you from pulling out that ladder and tape measure.

    Straight to the answer


    Divide your roof pitch by 12. Roof pitch is given as the number of inches in height change over the distance of one foot. Dividing by 12 gives the ratio of inches in rise per distance. For example, a standard roof pitch is likely to be about 4. So for a pitch angle of 4, divide by 12 and get 1/3.


    Square the result of Step 1. So keeping with our roof pitch of 4 this gives us 1/9.


    Add 1, then take the square root. Adding 1 gives us 1 + 1/9 = 10/9, then taking the square root gives us about 1.0541.


    Multiply by the square footage of a single floor of your house. So continuing our example, if you've got a two story house with a total square footage of 2,700 square feet, with each floor having 1,350 square feet, multiply 1,350 by 1.0541 to get 1,423 square feet. So you'd need 1,423 square feet of roofing.

    Things You Will Need

    • Square footage of your house
    • Pitch of your roof
    • Calculator
    • Paper and Pen


    • This calculation will give a solid estimate for a single slope roof and a simple gable roof with a single pitch. For more complex roof lines with multiple pitch angles, it's a little more complicated but the same steps can be followed; just break down the roof into separate sections with a different square footage for each different pitch angle.


    • Use only the square footage directly covered by the roof. On a multistory house, use only the square footage of a single floor.
    • Minor differences in calculations occur based on chimneys and other variations.

    About the Author

    Dr. Butler received his PhD in General Physics in 2004. His dissertation topic included gravitational cosmological background radiation detection, digital signal analysis, feedback control, thermal noise, statistical mechanics, and LASER optics. Additional research topics include charge, parity, time inversion investigation, spectroscopy, optical state transitions, and non-linear effects in optical cavities. After his PhD Dr. Butler has worked for 8+ years in industry gaining experience in areas of silicon manufacturing, CPU architecture, design, validation and debug, process management, project management, and computer programming. Time permitting Dr. Butler also enjoys volunteering as a science fair judge for the local high school and junior high school age competitions.

    Photo Credits

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