How to Replace a Flat Roof on a Garage With a Pitched Roof

Remove all the old roofing, using a pry bar. Strip the roof down to the four walls and the joists between them that supported the flat roof decking. Rent a dumpster to hold the removed material, the outer covering of asphalt and gravel, and any weatherproofing membrane under it. Leave plywood or oriented strand board decking in place if it is in good shape.

Check the walls to make sure the tops are level; flat roofs are almost never truly flat and sometimes shims are added on wall caps to provide a slant. Take off any sloping shims on wall caps or on roof decking. Use a long board and a 4-foot level to check the roof for level, in all directions. Check the joists between the walls to make sure they are sturdy, in good condition and level.

Measure the garage with a tape measure to find the span or width of the roof and the run of each rafter, half the span and the distance each rafter must support between a peak and a wall. Decide on a pitch or slope to provide the drainage needed and the interior space desired; most garages will have a 5/12 to 7/12 pitch, meaning the roof slopes at 5 to 7 inches per foot from wall to peak.

Mark a pattern rafter to the desired pitch. Set a 2-by-4-inch board flat with the 4-inch (actually 3½ inches) face up. Place the point of a framing square at the bottom of one end. Align the 12-inch mark on the wide square blade and the inch mark for the pitch, such as 5 inches for a 5/12 roof, at the top of that board. Mark the angle that forms at the end of the board for a top or plumb cut to form the peak.

Figure the length of rafter with a table on the square blade. Look under the pitch inch-mark for a differential between the horizontal distance and the length needed for the sloped rafter; that will be 13 for a 5/12 or 13 inches per foot of run. Multiply that differential times the run, -- 12 feet for a 24-foot wide garage, for instance, which comes to 156 inches.

Make a bird's mouth to fit over the wall cap; this is a triangular cut, 1 inch by 3½ inches, that exactly conforms to the 2-by-4 boards on the wall cap and positions the rafter onto the wall. Measure the differential down the rafter, 156 inches in our example, and draw a vertical line 1 inch up into the bottom of the rafter. Measure 3½ inches back from the bottom of that line and connect that point to the top of the vertical line to make the triangle. Add any desired overhang, usually about a foot, and mark another angle, the reverse of the plumb cut by putting the square point at the top of the rafter.

Take another 3/4 inch off the plumb cut to allow for a ridge board, and use a circular saw to cut the angles, then use that rafter as a pattern to cut all others. Get the number needed by dividing the length of the garage by 24 and doubling that number; in other words, a 24-foot long garage will need 12 pairs of rafters, or 24 in all to put one rafter on each side.

Set the first pair of rafters at the back of the garage. Nail the bird's mouths to the wall caps with three 16d framing nails and a hammer, two nails on one side of the rafter, one on the other. Plumb the rafters with a level and brace them with boards nailed to the rafters and to stakes in the ground. Let the plumb cuts rest against each other.

Install another pair of rafters the same way at the other end of the garage. Slide a 2-by-6-inch ridge board between the rafter tops on both ends, lifting it from the bottom, making sure it is level, and nail both pairs of rafters to it. Set other rafters the same way down the roof, 24 inches apart, working in pairs, making sure that each set of rafters is plumb and that the ridge board stays level.

Cover the garage roof with OSB decking, fastening it to the rafters with 8d galvanized nails. Lay 4-by-8-foot OSB panels horizontally -- perpendicular to the rafters. Fasten roofing paper over the OSB with a construction stapler. Add shingles to match the house or put on some type of metal roofing.