Laying out a common rafter begins with preparing a detailed roofing plan and obtaining the pitch of the roof. Get the roof span, and make a triangle called a birds-mouth. Calculate the rafters for hip roofs falling on four sides, and use the difference to calculate the lengths of the jack rafters before laying out all common rafters.
- Get the pitch of the roof
The pitch of the roof is necessary when laying out a common rafter. The pitch must be expressed in units of rise per foot of run. For example, in gable roofs, a rafter of 6-by-12 pitch rises 6 in every foot of the run. After knowing the pitch and total run, you can lay out the rafter.
- Get the span of the roof
Get the measurement of the span from the roofing plan, and figure the length needed for the common rafter with a table on the blade. Using the pitch, determine the distance which a rafter run will cover.
- Calculate and install common rafters
Get the measurement for hip roof beams and valley rafters. Use the difference in length of valley and hip rafters, and find the length of hip rafters from the peak to the corners of the wall. Consider the rafters for unusual roofs that have many slopes per side, and use the same formula to lay them.