How to Change the Pitch of the Existing Roof

Remove the existing roof one layer at a time. Start with the shingles and work through the underlayment and wood decking using a shingle stripper and pry bar. Strip it down to the existing rafters. Use the pry bar and a hammer to pull the existing rafters or trusses loose from a center ridge board if there is one, and then from the wall caps on each side.

Get a large tarpaulin to cover the roof while reconstruction is underway, which will take several days. Clean the wall caps of any nails or other objects. Replace any 2-by-4-inch caps that are split or damaged so they will hold new nails securely. Mark the caps for prefabricated trusses, which is the cheapest and easiest replacement framing.

Measure with a tape measure 1 1/2-inches in from the back end of the wall. Use a speed square and pencil to draw a line across the wall cap. Measure 23 1/2 inches from the end and draw another line, marking the outside edge of a second truss. Draw lines at 24-inch intervals from that line throughout the length of the roof. Mark the other wall cap the same way. The last space at the other end may be slightly less than 24 inches.

Lay plywood or oriented strand board panels across roof joists or interior wall caps to provide a platform on which to work. Lift the first truss upside down and have workers on the roof erect in on the marked spot. Plumb it with a level and brace it with 2-by-4-inch boards nailed with a hammer to the truss tops and to stakes in the ground outside. Fasten both truss ends to the wall caps with 16d framing nails and a hammer with two nails on one truss side, one on the other.

Add a second truss at the other end of the roof if trusses have slots at the top for a ridge board. Install that ridge between the two end trusses, setting it level with a carpenter's level. Install a second truss at the marked line if there is no ridge board and add a third truss at the next spot.

Brace trusses starting with the third one according to a pattern recommended by the truss supplier. This can vary with the type of truss and length of the roof. Install braces atop the horizontal bottom chords diagonally from the outside edge of the end truss to the center of the third and so on. Put diagonal braces from the top of the end truss chords to the bottom of the third truss as another bracing pattern.

Complete the truss installation and check all trusses for plumb. Nail short pieces of 1-by-2-inch lumber vertically on the ends of each truss rafter chord to set roof decking. Lay 4-by-8-foot panels of oriented strand board horizontally along the roof from end to end. Use the 1-by-2s to align them with the truss ends and hold them while they are fastened with 8d galvanized nails driven into the truss chords.

Start a second row of OSB decking, or board made of compressed wood, with a half panel so the panel seams do not line up. Cover the roof with full panels to the peak and trim overhangs with a circular saw. Overlap panels at the peak, so the end of one panel is covered by the panel from the other side. Remove the 1-by-2s when all decking is installed.

Lay roofing paper over the decking. Staple it with a construction stapler. Overlap layers by at least 6 inches from the top and run one layer across the peak, stapled on both sides. Trim paper as needed with a utility knife or big shears. Nail metal edge flashing on all roof sides with galvanized shingle nails. Place it under the paper on eaves, over the paper on sloped gable ends. Put a strip of bent flashing over the peak.

Install shingles starting at the bottom of one side with galvanized shingle nails. Nail on a starter strip if the shingle supplier furnishes one, or cut the tabs off a row of three-tab shingles and nail the cut sides up on the roof. Align the factory sides with the edge of the roof decking. Use four nails per shingle about 1 inch in from the top of the shingle.

Cut the width of one tab off a shingle to start a second row so the seams do not align. Add rows of shingles to the peak from both sides. Cover the peak with special cap shingles if supplied, or with 12-inch pieces of shingle. Use the cut-off tabs if possible. Bend cap shingles over the peak and fasten on both sides with one nail in each end side. Overlap so the next shingle covers the nails and then seal the nails on the last cap with roofing cement.