- 1). Get the pitch, span and run for the roof rafters; all should be marked on a house plan. The pitch is the angle of slope from wall the peak, in inches per foot of rise; most house roofs are 5/12 or 6/12, sloping up 5 or 6 inches every foot. The span is the width of the roof from wall to wall. The run is half the span, the space each individual rafter must cover between peak and wall.
- 2). Make a pattern rafter first, using a board long enough to cover the run plus a couple of feet. For a 12-foot run (on a 24-foot roof), use a 16-foot 2-by-4-inch board, laid with the 4-inch side up. Set the square at one end, with the tongue to the right and the heel at the bottom of the board. Place the 5-inch mark of the tongue and the 12-inch mark of the blade at the top of the board, to create an angle on the tongue side. That is the top or plumb cut. Mark that line.
- 3). Use the table on the inside of the blade marked "length of common rafters per foot of run." Look under the pitch mark (5), to get the length each rafter must be for a foot of run. For a 5, that is 13, or 13 inches for every foot of run. On a 12-foot run, that's easy ---- 13 feet.
- 4). Measure that distance (156 inches) with a tape measure down the bottom of the board from the plumb cut and mark the point. Measure another 3 1/2 inches and mark it. Measure 1 inch up into the board at that mark, then draw a diagonal line from the top of the 1-inch mark to the 156-inch mark. That will form a "birdsmouth" triangle, a notch that will fit over the 2-by-4-inch wall cap board (which is actually 3 1/2 inches wide).
- 5). Add any desired overhang, typically 18 inches, from the bottom of the birdsmouth and use the square to mark an angle the same as the top cut, but reversed. Put the heel at the top of the board instead of the bottom and mark that angle. Move back to the plumb cut, measure down 3/4-inch and draw an identical angle line; this creates space for a 2-inch (actually 1 1/2-inch) ridge board to fit between the rafter tops.
- 6). Cut the plumb cut, birdsmouth and end cut with a circular saw. Put the pattern rafter on the roof, with the bottom of the birdsmouth flush with the outside of the wall cap. Use a short 2-by-4 at the top to test the rafter fit. Mark all other rafters with the pattern, and cut them the same. Divide the length of the house by 24, for 24-inch rafter spacing, to get the number of rafters needed; multiply by two, since there will be one rafter on each side of the roof.