How to Calculate the Dimensions of Roof Rafters

A roof rafter is a beam of wood that runs down the roof from the roof's peak to the top of the outer wall. Carpenters typically need to know the dimensions of a roof rafter to determine the materials required for a repair. The width and thickness of a rafter is generally easy to obtain by direct measurement. Carpenters must typically calculate the length of a rafter from indirect measurements to avoid the necessity of climbing onto the roof.

Angle finder
Tape measure
Scientific calculator

Instructions

- 1
  Locate the end of a rafter under an eave. Measure the thickness and height of the rafter with a tape measure. A common width for rafters is 6 inches and a common thickness is 2 inches.
- 2
  Measure the slope of the roof with an angle finder according to the manufacturer's instructions. This generally requires you to place the angle finder against the underside of one of the rafters and record the angular measurement from the angle finder. Assume for this example that the roof slopes at a 29 degree angle.
- 3
  Measure the distance between the outer walls on one end of the house with a tape measure. This distance is also called the span of the roof. Assume the roof span is 27 feet in this example.
- 4
  Divide the roof span by 2 to get the run of the roof. The roof span in this example is 27 feet, so the run of the roof is 27 / 2 = 13.5 feet.
- 5
  Divide the run of the roof by the cosine of the roof angle to get the length of the rafter. The roof angle is 29 degrees and the run of the roof is 13.5 feet, so the length of the rafter is 13.5 / cos (29) = 15.4 feet.

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