How to Measure the Angles of a Roof for Rafters

Rafters are structural beams within a sloping roof. They span the gap from the hip or ridge board in the middle of the roof to the tops of the walls. The weight of the roofing materials push the rafters outward, so they are firmly attached to ceiling joists that prevent the rafter ends from moving. The angle between the joist and the rafter is known as the roof angle. If the angle is wrong, the roof won't fit.

Architect's or builder's plans
Calculator

Instructions

- 1
  Ascertain the pitch for the roof. This describes the relationship between the rise and the run of the roof or how far up it goes for every 12 inches along the ground. The pitch is expressed either as a ratio or as a short sentence. For example, a roof that rises 6 inches in every 12 inches may be described as a "6-in-12" roof or as a "6-inch rise over every foot."
- 2
  Substitute the rise and run values into this equation: Tangent of the angle = run / rise. For example, with a rise of 6 inches and a run of 12 inches, the equation becomes the following: tangent of the angle = 6/12, or tangent of the angle = 0.5
- 3
  Ascertain the arc-tangent of the value obtained in Step 2. The result is the angle of the roof. Use the arc-tangent function on a calculator or look up the value in a table of arc-tangent values. For example, the arc-tangent of 0.5 is 27, so the angle of the roof is 27 degrees.

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