How to Calculate a Flat Roof Pitch

Written by james marshall
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The pitch of a roof is a measure of its steepness. Carpenters typically measure roof pitch as the number of inches the roof rises for each foot of its run. The rise of the roof is its increase in height, and its run is the roof's vertical distance. Steep roofs tend to be more common in colder climates, where the increased pitch prevents snow from accumulating on the roof. Flat roofs are easier to construct than steep roofs.

Skill level:
Moderately Challenging

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Things you need

  • Carpenter's level
  • Framing square

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Instructions

  1. 1

    Hold the carpenter's level in the vertical, or plumb, position. Position the level so that the bubble is centred within its container, indicating the level is vertical.

  2. 2

    Position the framing square so that its tongue points up and is flush against the carpenter's level. The body of the framing square will be pointed toward the roof.

  3. 3

    Move the tongue of the framing square down along the carpenter's level until the body of the framing square touches the roof. Ensure the tongue of the framing square remains flush against the carpenter's level.

  4. 4

    Measure the distance from the corner of the framing square to the roof, as indicated by the carpenter's level. This distance is the rise of the roof. For this example, assume this distance is 4 inches.

  5. 5

    Measure the length of the framing square's body, which indicates the run of the roof. For this example, assume this distance is 16 inches.

  6. 6

    Divide the rise of the roof by its run to obtain the absolute pitch of the roof. This result is 4 divided by 16, or 0.25.

  7. 7

    Multiply the result from step 6 to obtain the rise of the roof for 12 inches of run. This result is 0.25 times 12 inches, or 3 inches. The roof in this example therefore has a pitch of 3/12.

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