Raising the Roof Line on a Garage

Decide how much height you need under the peak of the new roof, and then choose a pitch for the roof. A 5/12 pitch roof, which slopes up at 5 inches per foot from wall to peak on a 24-foot-wide garage, will provide about 5 feet, and a 7/12 pitch roof about 7 feet, depending on how rafters are braced. Use the steeper peak to gain more width between rafters as well.

Strip off the old garage roof, using a shingle stripper, pry bar and hammer. Peel off shingles first, throw them to the ground, and put them in a construction dumpster. Remove roofing paper next, then pull up plywood or other decking with a pry bar. Strip the roof down to the cap boards on the exterior walls and the roof joists between walls.

Lay big tarpaulins over the joists to seal the garage during construction. Lay plywood or oriented strand board sheets over the joists to provide a stable working surface. Check the wall cap boards for any splits or other damage. Pull out any remaining nails, and replace any defective cap boards by prying the nails loose with a pry bar, cutting new 2-by-4-inch caps with a circular saw and nailing them on with 16d framing nails and a hammer.

Make new rafters with 2-by-4-inch boards. Mark a pattern first. Lay a board on a level surface with the 4-inch face up. Put the point of a framing square at the bottom on one end. Lay the 12-inch mark on the wide blade at the top of the board. Move the square until the inch mark of the pitch (7 inches in this example) is also at the top. Mark the angle at the end of the board with a pencil for a top or plumb cut.

Check the table on the square's blade for length of common rafters per foot of run. Look under the pitch mark -- 7 in this example, which will show 13.89. That means a rafter must be 13.89 inches for every foot from the peak to the bearing point on the wall. Multiply that times the rafter run, or the space it must support; 12 feet in this example, resulting in 166.68 inches.

Round that number off to the nearest fraction, 166 3/4 for this example, and mark that distance with a tape measure from the bottom of the plumb cut. Draw a vertical line 1 inch into the rafter at that point. Measure 3 1/2 inches back toward the plumb cut from the bottom of that line, and connect that point to the top of the line to form a triangle to fit over the wall cap.

Add a foot for an overhang or eave, and draw another angled line. Put the point of the square at the top of the board, and align the sides like the plumb cut. Take 3/4 inches off the plumb cut, and draw another line at the same angle to allow for a ridge board between rafters at the peak.

Cut all those angles with a circular saw. Use that rafter as a pattern to cut and mark all the rest of the rafters. Divide the roof length by 24 to get the number of rafters needed; multiply that by two for a rafter on each side of the peak. Mark the wall caps for rafters at 24-inch intervals. Mark the inside of the first rafter 1 1/2 inches from the wall end. Mark the outside for the second rafter 23 1/4 inches from the end. Measure other 24-inch spaces based on that line.

Tack bracing boards to stakes in the ground at the back of the roof, and lay the tops on the end wall. Lift the first rafters into place. Set them upright, and get them plumb using a level. Nail the ends to the wall caps with three 16d framing nails and a hammer. Put two nails on one side, one on the other, on each rafter. Nail the braces to the rafters to hold them plumb. Let the tops rest together.

Install another set of rafters at the other end of the garage. Put a 2-by-6-inch ridge board between the end rafters. Slide this in place from the bottom between rafter tops. Use a 4-foot level to get it level, and nail it in place on both ends with 16d nails through the rafters, two nails in each rafter. Add other rafter sets down the roof. Check each pair to make sure they are level and the ridge board to make sure it stays level

Make king post braces for each set of end rafters. Cut a 2-by-4-inch board to fit between the bottom of the ridge board and the wall cap. Toenail it to ridge and cap with 16d framing nails driven diagonally. Add internal bracing to fit spacing needs inside the garage attic. Nail 1-by-4-inch purlins diagonally between rafters, from the top of one rafter to the bottom of a rafter three spaces away, for instance. Cut 2-by-4 posts to set vertically between rafters and roof joists about halfway between peak and outer wall for different bracing.

Finish the roof with new decking, roofing paper and shingles or other roofing material, such as metal panels.