DIY Rafter Construction
- 1). Figure the angles, lengths and other dimensions for rafters using a roofer's square, also called a carpenter's square or framer's square. Start with a roof pitch, the angle at which the roof slopes from peak to eaves. That varies with house design, locale and preference, but typically will be 4/12, 5/12 or 6/12; that is 4, 5 or 6 inches of rise to every 12 inches of run, or distance the rafter must cover.
- 2). Measure with a tape measure the span, the width of the building, and the rafter run, half that distance on a gable roof, which slopes on two sides. Make a pattern rafter using a 2-by-4-inch board. Put the point of the square, the heel, at the bottom of the board, with the short side (tongue) and the wide side (blade) along the board. Put the tongue at the pitch mark -- for a 5/12 roof, that would be the 5-inch mark. Put the blade at the 12-inch mark. The angle at the end of the board is the top or plumb cut for the rafter. Cut that with a circular saw.
- 3). Calculate the length of the rafter using the table on the blade of the square. Under 5, it will show 13, meaning the rafter must be 13 inches long for every foot of rafter run. For a 12-foot run (on a 24-foot wide building), the rafter must be 156 inches long to reach the wall. Measure that on the pattern rafter, mark 1 inch up from the bottom of the board, then measure down 4 inches from that point and mark the triangle. That will be a notch, called a birdsmouth, which fits on top of the wall cap board. Cut that notch with the circular saw.
- 4). Add any length of overhang or eave, usually at least 12 inches, to get the final rafter length. For this example that would be 172 inches -- 156, plus 4 of the birdsmouth, plus 12 of the overhang. Cut the pattern to those dimensions with the circular saw, angling the bottom cut the reverse of the top cut. Cut 1 inch off the top cut to allow for a ridge board, then test fit that rafter to make sure the dimensions are correct.
- 5). Cut all other rafters to that pattern. Figure the number of rafters needed by dividing the length of the building in inches by 24, since rafters are spaced 24 inches apart, and multiply by 2 for rafters on each side. For a 30-foot building that is 30 rafters: 360 inches divided by 24 is 15, times 2 is 30.
- 6). Set the first pair of rafters at one end of the roof. Nail sides to the wall cap through the birdsmouth and let the top cuts lay against each other. Brace these rafters plumb, using 2-by-4-inch boards nailed to the rafters and to stakes in the ground. Move to the other end of the roof and set another pair of rafters the same way. Slide a ridge board, usually a 2-by-6-inch board, up under the rafter plumb cuts at either end and nail it in place with a hammer and framing nails.
- 7). Install all other rafters in pairs, 24 inches apart, keeping rafters plumb and the ridge board level. Splice the ridge board, if necessary to cover the length, by cutting matching boards and nailing on either side of the joint. Place the joint between rafters, so each end is fully supported by a rafter pair. Add a gable stud, a 2-by-4, nailed vertically between the bottom of the ridge board and the end wall cap at each end. For additional support, nail 2-by-4 cross-tie boards horizontally between the rafters below the ridge board.
