How to Increase the Slope of a Roof

Inspect the cap boards on the walls where the old rafters were fastened. Look for any splits, breaks or other irregularities. Replace cap boards if necessary to get a solid base for the new rafters; cap boards are nailed on top of wall top plates and can be changed by pulling or cutting the nails and fastening new caps with 16d framing nails and a hammer.

Make rafters for the new slope or pitch, figured in inches of rise per foot from the wall to the peak at the center. Change a flat or low-slope roof to a 5/12, rising 5 inches per foot, or increase that slope to a steeper one, like 8/12, sloping 8 inches per foot. Base the pitch decision on the need to shed rain and snow efficiently or to provide more attic space under the rafters. Using a 5/12, for instance, will give you 5 feet of space under a peak on a typical house.

Mark one rafter board for a pattern with a framing square and pencil. Lay a 2-by-4-inch board with the 4-inch face up. Put the point of a framing square at the bottom of one end of the board. Place the 12-inch mark on the square's wide blade at the top of the board and align the inch-mark on the thin tongue for the desired pitch, 5-inch for a 5/12 roof, at the top. Mark the angle on the end of the board with a pencil for a top angle.

Calculate the length of rafter needed with differentials from a table on the square's blade. Look under the inch-mark for the pitch, 5 for a 5/12 roof, and multiply that figure, 13 in this example, by the rafter run, or space each rafter must support from peak to wall. Measure that length with a tape measure from the bottom of the top angle, it would be 156 inches for a 5/12 roof with a 12-foot run for a 24-foot wide roof.

Make a triangle at that length by drawing a vertical line an inch up into the rafter board and connecting the top of that line to a point 3 1/2 inches up the rafter board; this is called a birdsmouth and will fit exactly on top of the wall cap board. Add any desired overhang, mark that spot and use the framing square to figure another angle, using the top angle process but with the square point at the top of the board. Take another 3/4-inch off the top cut to make space for a ridge board.

Cut all needed rafters to that pattern with a circular saw. Get the number needed by dividing the length of the roof by 24, for rafters spaced 24 inches apart, and doubling that number, for a rafter on each side; a 30-foot roof, for instance, will need 15 pairs of rafters or 30 boards in all. Use the same method of cut rafters for any roof pitch; just match the inch mark to the desired pitch.

Place one pair of rafters at the back end of the roof. Nail them to the wall caps with three 16d nails in each rafter side, two nails on one side of the board, one on the other. Use a level to get them plumb and brace them with boards nailed to the rafters and to stakes in the ground. Let the rafter tops rest against each other.

Erect another pair of rafters at the other end of the roof. Mark a 2-by-6-inch board for a ridge board. Draw a line across one end of the board 1 1/2 inches from the end to locate the end rafters. Measure 23 1/4 inches in from the end and draw another line, on both sides of the board, to mark the outside edge of the second rafters. Draw lines at 24 inch intervals, based on that second line, to the other end of the roof.

Lift the ridge board to the roof and slide it between the tops of the end rafters from the bottom. Level it with a 4-foot level and nail both end sets of rafters to it with two framing nails through each rafter top. Install other rafters down the roof at the marked locations, working in pairs so both sides are supported equally. Keep rafters plumb and the ridge board level. Cut two 2-by-4s to fit between the bottom of the ridge board and the wall cap on each end and toenail them in place with framing nails diagonally through the post into the ridge and cap.

Add hurricane clips to each rafter where it connects to the wall cap. Nail these metal brackets in place with 8d galvanized nails, two nails into the side on the wall cap and two into the side that goes up the rafter.

Nail 4-by-8-foot panels of oriented strand board on top of the rafters. Set the panels horizontal, with the edges flush with the outside ends of the rafters and the rough face of the oriented strand board up. Cut panels as needed with a circular saw to fit. Overlap panels at the peak, so the edge of one panel goes across the edge of the panel on the other side.

Roll on roofing paper, overlapping layers by 6 inches from the top. Overlap the peak with roofing paper folded onto both sides. Fasten paper with a construction stapler. Install metal drip edge flashing on the bottom sides or eaves and the sloped or rake edges. Put flashing under roofing paper on eaves, over it on rakes.

Use a utility knife to cut tabs off enough shingles to cover the bottom of the roof. Nail these upside down, through the cut edge, with the other edge just slightly over the roof decking. Cut one tab width off a shingle to start a second layer, so seams don't line up. Shingle both sides of the roof to the peak. Cover the peak with special cap shingles or fold the extra tabs over the peak and nail on both sides. Overlap shingles to cover the nails and seal the last nails with roofing cement.