How to Cut a Hip Roof Rafter

Mark the common rafters first. Put the point of a square on the bottom at one end of a rafter board. Align the pitch mark in inches on the slender tongue and the 12-inch mark on the wide blade at the top of the board to form an angle on the end of the board called a top or plumb cut. For a 6/12 roof, which rises 6 inches per foot of rafter run, use the 6-inch mark on the tongue.

Use the table on the square's blade to figure the length of rafter needed for a triangle notch called a birdsmouth to fit over the wall. Look under the pitch mark; for a 6-pitch that's 13.42 or roughly 13 3/4 inches for each foot of run. Multiply that by the length of run, half the width of the roof. On a 12-foot run, that's 161 inches.

Measure 161 inches from the plumb cut and form a triangle notch 1-inch deep by 3 1/2 inches long in the bottom of the board. Add an overhang and cut the rafter square at that tail end. Make two more common rafters than you need to frame the gable portion of the roof; those extra commons go in the center of the hip ends, from the ridge board to the outer wall.

Make four hip rafters with similar calculations, using the second line on the rafter table, length of hip rafters. Make the top cuts on the hip rafters at a 45-degree angle, to slope from the ridge board peak to the corners. For a 6-pitch, the difference in length for a hip rafter is 18, or 18 inches of rafter for every foot of run. Multiply that by the run to figure and cut a birdsmouth on the hip rafters. Add the same overhang as on the common rafter to get the total length.

Figure jack rafters similarly, using one of the two jack rafter length differentials, depending on whether they are spaced 24 inches apart or 16. Jack rafter lengths will decrease from the first rafter at the top of the hip rafter to the last jack where the hip rafter meets the wall. Use the length differential table, but vary the multiplication by adjusting the run, since each jack rafter will cover a different length.