Laying out Loft Rafters for Cabins

Base your cabin rafter layout on the width of the cabin and the height needed inside. A 6/12 roof that rises 6 inches per foot from wall to peak will have a 6-foot clearance in the center of a 20-foot wide cabin, but has less as the rafters slope to the wall. Use a 10/12 or 12/12 pitch for most cabin lofts, to allow for more space on the edges.

Lay out rafters once a pitch is chosen using a framing square and pencil. Put a rafter board, typically 2-by-4-inch lumber at least 18 feet long, on a flat surface with the 4-inch edge up. Set a framing square at one end, with the point or heel at the bottom of the board. Align the 10-inch mark on the thin tongue of the square and the 12-inch mark on the wide blade at the top of the board.

Mark the angle along the tongue on the board with a pencil for a top or plumb cut to form a peak. Look up the "length of common rafters" table on the square's blade and multiply that times the rafter run, or the distance between the peak and wall. On a 10/12 pitch with a 10-foot run, this will be 169.7 inches. Use a tape measure to mark that distance from the top cut on the bottom of the board.

Draw a triangle notch, called a birdsmouth, at that point. Measure 1 inch up from the bottom of the board and 3 1/2 inches back up to make the triangle. Add an overhang, typically at least 12 inches, and mark another angle, similar to the top cut but with the heel of the square at the top of the board. Go back to the plumb cut and take off another 3/4 inch if you plan to use a center ridge board.

Cut all these angles with a circular saw on a test rafter. Set it in place on the roof, with the birdsmouth flush on top of the call cap and the plumb cut in the center of the roof. Use that as a pattern to lay out and cut all loft rafters.