How to Cut Rafters For a Roof Pitch

Cut rafters to pitch a roof using a framing square, a pencil, a tape measure and a circular saw. Mark the top or plumb cut first. Put the point of the square at the bottom edge of a 2-by-4-inch rafter board, with the 4-inch side face up. Use the inch marks on the thin tongue as a guide. Align the pitch mark, like the 5-inch spot for a rafter with a 5-inch rise per foot, and the 12-inch mark on the wide blade at the top of the board.

Draw a line with a pencil along the tongue to mark that plumb cut angle. Calculate the length of rafter needed with the "length of common rafter" table on the square's blade. Look under the inch mark for the pitch--again, 5-inch for a rise of 5 inches per foot. Multiply that differential--13 inches per foot for a 5-pitch roof--times the run of the rafter, the distance between the wall and the peak, such as 12 feet on a 24-foot wide roof.

Measure that distance--156 inches in the example--down the bottom of the board from the bottom of the plumb cut line. Use the square to draw a vertical line from that point 1 inch up into the bottom of the board. Measure 3 1/2 inches back toward the plumb cut from the bottom of that line and draw a line to connect that point and the top of the vertical line to form a triangle, called a birdsmouth.

Add any extra length for an overhang or eave--typically 18 inches--and draw another angle. Make this like the plumb cut, but with the point of the square at the top of the board so the angle will be the reverse of the plumb cut. Move back to the plumb cut, measure in 3/4 of an inch, and draw an identical angled line; this allows space for a 2-inch ridge board between rafters at the peak.

Test that pattern rafter by putting it on the roof. Set the triangular birdsmouth firmly on the wall cap boards with the vertical line against the edge of the cap and the horizontal line firmly on top of the cap. Make sure the plumb cut is at the center of the roof. Use that pattern to cut all needed rafters.