How to calculate how much roofing felt is needed?

Written by andrew hazleton
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How to calculate how much roofing felt is needed?
Felt is commonly applied underneath new roof shingles. (roof turss image by Greg Pickens from

Roofing felt, commonly called tar paper, is an important component of any roofing system. It provides the roof with additional waterproofing and protects the wood sheathing of a roof until the installation of shingles is complete. Building codes and shingle manufacturers typically require the use of felt. The lifetime and flammability guarantees of roofing systems require the use of roofing felt prior to shingle installation. Roofing felt is rated based on the weight of a 100 -square-foot section, with 15 and 30-pound felt being the most common.

Skill level:

Things you need

  • Tape measure
  • Calculator

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  1. 1

    Enter the attic of the home, or any part where the roof framing is visible.

    How to calculate how much roofing felt is needed?
    Determine the square footage of the roof. (tape measure image by Joann Cooper from
  2. 2

    Measure and mark a 12-foot distance along the floor in the same direction as the pitch of the roof.

  3. 3

    Measure the height of the roof above the floor at each end of the 12-foot distance.

  4. 4

    Subtract the lower height measurement from the larger height measurement.

  5. 5

    The difference calculated is the roof pitch. Typically roof pitch is expressed as a change in elevation per 12-foot distance. If your difference calculated in step four was three feet, your roof pitch is 3/12.

  1. 1

    Measure the length and width of the structure at ground level in feet. Multiply the measurements to determine the total area covered by the roof. If you have an irregularly shaped building, break the structure into several rectangles and measure each separately.

  2. 2

    Multiply the total area determined in step one by a correction factor to account for the roof pitch. This correction factor will take into account the greater area of the roof due to the pitch.

    If your roof pitch is 3/12 or 4/12, multiply by 1.25. For roof pitches of 5/12 to 7/12, use a factor of 1.3. Use 1.4 for up to a 9/12 pitch. For higher pitches, a factor of 1.5 to 1.7 can be used, although with less accuracy.

  3. 3

    Multiply the result in step two by 1.10 to account for waste. If the roof has complicated hips and valleys, add another 10 to 25 per cent to account for the added area.

  4. 4

    Divide the result from step 3 by 100. Round up to convert square feet to squares. Roofing materials are always specified in squares. One square is equal to 100 square feet. Most roofing felt is sold in rolls that are one square each.

Tips and warnings

  • This procedure will provide only a rough estimate, although it is usually close enough. Most building material suppliers will allow you to return extra material with no charge.
  • Always use caution when working on a ladder.
  • Never walk on a roof with a greater than 4/12 pitch without using proper safety harnesses. Consider professional roofers for high-pitched roofs.
  • Use extreme caution when working on roofs. Be alert for electrical wires.

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