How to Change the Garage Roof Shape

Measure the garage with a tape measure to determine the length of the roof and the width or span. Divide the span by 2 to figure the rafter run, the length from peak to wall. Decide how much inside space is needed under the peak and use that and the run to calculate a pitch or slope of the roof sides; if you want 5 feet under the peak and the run is 12 feet, on a 24-foot garage, the roof must rise 5 inches per foot and will be a 5/12 roof.

Remove the old roof, using a pry bar and hammer. Take off shingles, roofing paper, wooden decking down to the old rafters and discard it in a rented trash bin. Remove the old rafters and inspect the wall tops to make sure they are in good condition. Replace any wall cap boards that are split or seriously damaged.

Cut new rafters, starting with a pattern. Lay a 2-by-4-inch board flat with the 4-inch face up. Put a framing square at one end with the point at the bottom of the board, the 12-inch mark on the wide blade to the left at the top of the board and the inch mark on the thin tongue for the pitch -- 5-inch for a 5/12 roof -- at the top of the board. Mark the angle along the outside of the tongue with a pencil for a top or plumb cut for the peak.

Figure the rafter length with a table on the square blade. Look under the pitch mark -- 5 inches in this example; it will show 13, meaning a rafter must be 13 inches long for every foot of run or 156 inches in this example. Measure that distance on the bottom of the rafter board and use the square to draw a vertical line 1 inch up into the board. Measure 3 1/2 inches toward the plumb cut and draw a line to connect with the top of the vertical line, creating a triangle to fit over the wall top.

Add an overhang, typically a foot on a garage, and draw another angle. Figure this like the plumb cut but with the point of the square at the top of the board. Use a circular saw to cut all those angles. Test fit the rafter by putting the triangle, a birdsmouth, firmly on top of the wall and making sure the plumb cut is in the center. Cut another 3/4 inch off the plumb cut end to make space for a ridge board. Cut all rafters to that pattern.

Mark a 2-by-6-inch ridge board for rafters. Measure 1 1/2 inches from one end of the board and draw a line across it to mark the first rafter. Measure 23 1/4 inches from the end and mark the edge for the second rafter. Measure and mark down the length of the board in 24-inch increments based on that second line. Mark both sides of the ridge board.

Erect the first pair of rafters at the back end of the roof. Nail them to the wall top at the birdsmouth with a hammer and three 16d framing nails, two on one side of the rafter, one on the other. Let the rafter tops rest against each other and brace both rafters with a board nailed to the rafter and to a stake in the ground. Use a level to set them plumb before bracing.

Set another pair of rafters at the other end of the roof. Slip the ridge board between the plumb cuts of rafters on both ends -- get help for this, it is not a one-person job. Nail the rafters to the ridge board after making certain it is level. Put two nails in each rafter top.

Add other rafters in pairs at the marked spots on the ridge board. Make a "king" post for each set of end rafters. Cut a 2-by-4 to fit between the bottom of the ridge board and the top of the wall. Toenail it to the ridge board and wall top with 16d nails driven diagonally.

Deck the roof with 4-by-8-foot sheets of oriented strand board. Run it perpendicular to the rafters, with the rough side up, and fasten it to the rafters with 8d galvanized nails. Make it flush with the ends of the rafters. Overlap panels at the peak, so the edge of one panel covers the edge of the one on the other side of the roof. Cut panels as needed to fit with a circular saw.

Fasten roofing paper over the OSB with a construction stapler. Overlap it by 6 inches in layers up the roof. Run it over the peak and staple it on both sides. Install metal drip edge flashing on the eaves and rake or sloped ends of the gable. Put flashing under the roofing paper on the eaves and over it on the rakes. Nail it with galvanized shingle nails.

Cut the tabs off enough shingles to cover the bottom of one roof side with a utility knife. Nail those with shingle nails through the cut edge, with the other edge flush to the edge of the decking or slightly over it. Cut the width off one tab off the first shingle on the next row, so the seams will not line up. Install shingles on both sides to the peak.

Cover the peak with special cap shingles or by bending the cut-off tabs over the peak. Nail peak shingles on both sides so one cap covers the nails of the preceding one and seal the last nails with roofing cement. Run cap shingles the direction of any prevailing wind.