Enter the span of the building and the pitch angle into the calculator to determine the rafter span. This calculator can also evaluate any of the variables given the others are known.

Rafter Span Formula

The following formula is used to calculate the rafter span. 

RS = (2 * S * cos(A)) / cos(2 * A)

Variables:

  • RS is the rafter span (ft or m) S is the span of the building (ft or m) A is the pitch angle (degrees)

To calculate the rafter span, multiply the span of the building by 2 and the cosine of the pitch angle. Divide this result by the cosine of twice the pitch angle.

What is a Rafter Span?

A rafter span is a distance that a rafter can safely support the weight of a roof without requiring additional support. This measurement is determined by factors such as the type and size of the rafter material, the weight of the roof materials, and the local building codes. The rafter span is crucial in the design and construction of a roof, as it helps to ensure the structural integrity and safety of the building.

How to Calculate Rafter Span?

The following steps outline how to calculate the Rafter Span using the given formula:

  1. First, determine the span of the building (S) in feet or meters.
  2. Next, determine the pitch angle (A) in degrees.
  3. Next, calculate the cosine of the pitch angle (cos(A)).
  4. Next, calculate the cosine of twice the pitch angle (cos(2 * A)).
  5. Finally, use the formula RS = (2 * S * cos(A)) / cos(2 * A) to calculate the rafter span (RS) in feet or meters.

Example Problem:

Use the following variables as an example problem to test your knowledge:

span of the building (S) = 20 ft

pitch angle (A) = 30 degrees