How to Figure Roof Rafters

A variety of methods exist for calculating a roof rafter's length without measuring the rafter directly. One of the simplest uses the Pythagorean theorem to calculate the roof rafter length from the rise and run of the roof. This theorem states that the square of the length of a right triangle's hypotenuse is equal to the sum of the squares of the length of the other two sides--if your roof's peak forms a right angle, this calculation will work to figure your rafter length.

Instructions

- 1
  Measure the distance from the edge of the outer wall on the end of the house to the opposite outer wall on the same end of the house. This measurement is the span of the roof. Assume the span of the roof is 32 feet in this example.
- 2
  Divide the span of the roof by two to obtain the run of the roof. The span of the roof is 32 feet in this example, so the run of the roof is 32 / 2 = 16 feet.
- 3
  Measure the vertical distance from the peak of the roof to the top of the wall to obtain the rise of the roof. Assume for this example that the rise of the roof is 22 feet.
- 4
  Add the square of the rise of the roof to the square of the run of the roof. The rise of the roof is 22 feet and the run of the roof is 16 feet, so the sum of their squares is (22 x 22) + (16 x 16) = 740.
- 5
  Calculate the square root of the sum from Step 4 to obtain the length of the roof rafter. The square root of 740 is 27.2, so the length of the roof rafter in this example is 27.2 feet.

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