# How to figure the length of rafter on a porch or shed.

Submitted by Ray Thornburg on Mon, 09/19/2011 - 14:27 This is the easy way to figure the length of a shed or porch roof rafter! How to figure the length of a rafter on a porch or shed roof.

You must know three variables to solve this equation.

Run in feet = f             to changes inches to feet divide by 12, ex 9/12=.75’

Rise main roof = d                  expressed as x units in 12 for ex. 8/12 roof use 8

Rise shed roof = e                   expressed as x units in 12, for ex. 4/12 roof use 4

(180˚ - (tan⁻¹(d÷12))) = g ;   f · sin g ÷ (sin(180 – ( g + (tan⁻¹(e ÷ 12))) = length of rafter in feet

To changes decimal measurements of feet to feet and inches subtract the integer portion and multiply it by 12 to get inches.  For example 10.975’; .975 x 12 =11.7 so the answer will be 10’ 11.7”; now multiply .7 x 16 to get 11/16 of an inch.

In the equation above tan⁻¹ is the inverse function of tangent.

If you have a programmable calculator the above formula can be used to quickly derive the length of rafter on with just a few key strokes. Just add the tail, birds mouth and make the top cut and you have it. Make the top cut according to the instruction on this page.

If you're looking for how to determine the pitch of a shed roof rafter or how to calculate the wall height difference on a shed roof see the "how to determine the pitch of a shed roof rafter" page.

For even more roof framing secrets and formulas buy my book. The Roof Framers Field Manual has it all!