How to Calculate Roof Rafter Length

Written by james marshall
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A rafter is a beam that runs from the peak of the roof to the edge of the roof. The length of a roof rafter is an important factor in determining the cost of materials for a roof repair. One method of calculating the rafter's length uses the slope of the rafter and the width of the house. This approach allows you to determine the length of the rafter from ground level.

Skill level:
Moderately Challenging

Things you need

  • Angle finder
  • Tape measure

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  1. 1

    Measure the span of the roof with a tape measure. This is the distance from the eave on one side of the house to the eave on the opposite side of the house. Assume the span of the roof is 34 feet in this example.

  2. 2

    Divide the roof span by two to get the run of the roof. This is the horizontal distance between the two ends of the rafter. The span of the roof in this example is 34 feet, so the run of the roof is 34 / 2 = 17 feet.

  3. 3

    Place an angle finder under the rafter according to the manufacturer's instructions. The measurement from the angle finder is the angle between the rafter and the horizontal plane, also known as the roof's slope. Assume for this example that the slope of the roof is 29 degrees.

  4. 4

    Divide the run of the roof by the cosine of the roof's slope to get the length of the rafter. The run of the roof in this example is 17 feet and the slope of the roof is 29 degrees. The length of the rafter in this example is 17 / cosine (29 degrees) = 17 / 0.875 = 19.4 feet.

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