Before deciding on a roofing company for your home or business, it’s a good idea to calculate how much roofing material will be required so you can double-check the estimates of the contractors. It’s best and easiest to have someone else do this for you aside from the roofing contractors you’re considering—such as an insurance adjuster, for example—but if that’s not possible, follow these steps to calculate it on your own.

Measure Like a Roofer

  1. Measure the outside dimensions of the building, making sure to include overhangs. If the architecture is complicated, you may need to measure one section or wing at a time.
  2. Determine the slope of the roof:
    • Lay a board (either a 1×2 or 2×4 that’s at least 16” long will work) on the roof perpendicular to an eave of the house, so that one end points toward the peak of the roof and the other end points toward the edge or overhang.
    • Hold a tape measure so that one end touches the board at least 12” down from the end pointing toward the roof peak. Hold the other end straight up from where it’s touching the board.
    • Place a level on the end of the board pointing toward the peak and hold it at a level position so it intersects the tape measure 12” down the board. Note how many inches above the roof the level is when it intersects the tape measure. If there are 3 inches between the board and the edge of the level, that would be a 3 in 12 slope. If this is the case, you would multiply the total square feet of the building by 1.03.
    • Using the following chart, determine the slope of your roof, then multiply the total square feet of the structure by the amount indicated to determine the amount of roofing material you’ll need. Remember to add about 10% to your estimate since that is typically the amount of material wasted when a new roof is installed.
    • Slope Multiply by
      2 in 12 1.02
      3 in 12 1.03
      4 in 12 1.06
      5 in 12 1.08
      6 in 12 1.12
      7 in 12 1.16
      8 in 12 1.20
      9 in 12 1.25
      10 in 12 1.30
      11 in 12 1.36
      12 in 12 1.41

Read more about measuring roof size

Image: Hundred House roof lines / Groton School / CC BY 2.0