How to Make Rafter Angles

Obtain two basic dimensions to determine rafter angles: the run or distance each rafter must support from a peak to a wall, and the pitch or slope of the roof in terms of inches per foot, as in 5/12 for a rise of 5 inches every foot. Those figures and a framing square are all you need to calculate rafter angles.

Figure the basic top or plumb cut of a common rafter by putting the point or heel of a square at the bottom of a rafter board on its wide face with the slender side or tongue of the square to the right. Set the pitch mark on the tongue (5 inches for a 5-pitch roof) and the 12-inch mark on the wide blade at the top of the board. This creates an angle at the end of the board for the plumb cut.

Check the "length of common rafters per foot of run" table on the square's blade to determine the placement of a birdsmouth, a triangle that will fit on the top of the wall. Look under the pitch mark for the difference; for a 5-pitch roof it is 13, so a rafter must be 13 inches long for every foot of run. That's easy on a 12-foot run -- 13 times 12 or 156 inches. Mark that spot, measure up 1 inch into the bottom of the rafter board, then measure 3 1/2 inches on the bottom of the board back toward the plumb cut. That makes a triangle to fit the wall top.

Calculate the final angle by figuring an overhang or eave and adding that length to the total rafter length. Mark that spot and make an end or tail cut the reverse of the plumb cut angle. Put the square heel at the top of the rafter and repeat the plumb cut calculation.

Make one final common rafter angle calculation if the rafter or truss will have a ridge board at the top to join the rafters side by side and end to end. Measure down 3/4 inch from the first plumb cut line and mark an identical angle to create the space for a ridge board.

Calculate hip and jack rafters also by using the "difference in length" tables on the square. These will vary for jack rafters -- short ones of varying length that connect hip rafters to walls -- according to the rafter spacing.

Look at the "side cut" line on the framing square table under the appropriate pitch mark for the hip roof. That will give numbers for both hip and jack rafters. Place the blade of the square at the 12-inch mark and the tongue at the given number (such as 11 1/16 or 11 1/2 for jack or hip rafters on a 5-pitch roof) aligned on the edge of the rafter board. That will mark the proper angle.