Rafters are the diagonal members that support the roof of a building. Designing rafters is more complicated than floor and wall framing, but there are several helpful techniques that make the process fairly simple, even for inexperienced carpenters. The first and most important step in designing a rafter is calculating the rafter length.
Choose the pitch of your roof. This is typically expressed as x:12, where x indicates the inches of rise for every 12 inches of run. If your roof rises 6 inches over a horizontal distance of 12 inches, you have a 6:12 roof.
Measure your building from the outermost part of the wall to the centre of the building. If your rafter will be installed after your sheathing or siding, make sure to include these materials in your measurement. This is your roof's overall "run."
Calculate the pitch of your roof as a decimal. If your roof is 6:12, 6 divided by 12 = 0.5.
Multiply this decimal by your overall run to obtain the overall rise. For example, if your run is 12 feet and your roof is 6:12, your overall rise is 6 feet.
Calculate the length of the rafter using the Pythagorean Theorem, which is the hypotenuse of a right triangle: square root of (run squared + rise squared). In this example, 12 squared + 6 squared = 180, square root of 180 = 13.4 feet. The length of this rafter is 13.4 feet.
Calculate the length of your rafter overhang using the same method. After choosing the horizontal overhang distance ("overhang run"), multiply by roof pitch to obtain "overhang rise," then use the Pythagorean Theorem again.
If your roof uses a ridge beam, adjust your calculation by subtracting half of the width of the ridge beam from the overall roof run. To change your numbers from decimal feet to feet plus inches, multiply the decimal portion by 12.