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

4/12 Pitch Roof Formula

The following formula is used to calculate the length of a rafter for a 4/12 pitch roof.

R = S / cos(θ)


  • R is the length of the rafter (ft or m) S is the span of the roof (ft or m) θ is the angle of the roof pitch (degrees)

To calculate the length of the rafter, divide the span of the roof by the cosine of the angle of the roof pitch. For a 4/12 pitch roof, the angle is typically 18.43 degrees.

What is a 4/12 Pitch Roof?

A 4/12 pitch roof refers to a roof slope that rises by 4 inches for every 12 inches across. This is a common pitch for residential homes, as it provides a balance between efficient water runoff and architectural aesthetics. The pitch of a roof can impact its overall style, cost, and suitability for certain climates or weather conditions.