How to Measure & Cut an 8/12 Rafter

Lay a rafter board on a flat surface with the wide side face up. Use one at least a third longer than the rafter run, a 16-footer, for instance, on an 8/12 roof with a 12-foot run (half the width of a 24-foot building). Check lumber tables to see what dimension lumber to use, probably a 2-by-6-inch board for rafters spaced every 16 inches under a moderate load. A heavy load or wider rafter spacing may require larger lumber.

Put the point or heel of a framing square at the bottom of one end of the board. Position the pitch, 8 inches, on the shorter tongue and the 12-inch mark on the wider blade at the top of the board to form an angle along the tongue at the end of the board. That is the end or plumb cut, where two rafters will meet at a ridge board or peak.

Look up "length of common rafters per foot of run" on the table on the square's blade. Under 8 that will show 14.42: The rafter must be 14 3/4 inches long for every foot of run. Multiply that times the run. For a 12-foot run, for instance, that is 173.04 inches; ignore the .04.

Measure 173 inches along the bottom of the rafter board and mark a point. Measure 1 inch into the bottom of the board and mark another point. Measure 3 1/2 inches back toward the plumb cut along the bottom of the board and mark a point. Connect the upper mark and the mark into the board with a triangle to create a birdsmouth notch to fit on top of the wall cap board.

Add an overhang or eave and mark that spot for an end or tail cut. An 18-inch overhang will make a rafter 197 inches long or 15.9 feet. Use the framing square, with the heel at the top of the board, to make a tail cut angle, like the plumb cut but reversed. Take another 3/4-inch off the plumb cut angle to make space for a 2-inch ridge board. Use a circular saw to cut the plumb cut, birdsmouth and tail cut.