If you're building a gable roof extension or dormer into the existing roof of your home, you need to join the old and new parts of the roof together to form a seamless and weather-tight whole. But making an aesthetically pleasing and weather-tight join aren't your only priorities. The way the gable ties into the roof must also transfer its weight adequately to the existing roof rafters. Make sure the rafters can support the weight of the new gable roof before you begin construction.
- Skill level:
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Things you need
- Valley boards
- Waterproof shingle underlayment, 3-foot
- Thin, flat bar
- Roofing cement
- Tape measure
- Framing square
Snap chalk lines from the ridge point of the gable roof to the point where the new "walls" of the gable intersect the old roof. This is where you will install the valley boards -- where the gable meets the roof.
Measure the valley lines. Cut two boards to that length with a 45-degree angle on each end. Nail in place along the valley lines.
Assemble and frame the gable addition to the roof. When you've assembled the gable, snap a chalk line 2 inches up from the centre of the valley on the existing roof shingles. Cut through the shingles and the underlayment beneath them.
Unseal the shingle tabs with a thin, flat bar or a joint-taping knife. Remove the nails from the overlying course. Remove to a distance of two shingles on either side of the gable valleys until you reach the bottom of the gable. Save and number the shingles to replace later.
Lift the underlayment of the roof away, and lay down a new 3-foot waterproof underlayment in the valley.
Remove two shingles on each side of the new valleys until you reach the bottom. Save those old shingles, and number them for replacing later. Lift the underlayment out of the way, and then lay a 3-foo-wide roll of waterproof shingle underlayment in the valley.
Shingle the roof addition, letting the new shingles fold through the valley centre and onto the old roof deck by at least 1 foot. Re-lay the shingles you removed from the existing roof. Bed the end of each shingle in a 2-inch-wide ribbon of roofing cement placed 4 inches up from the valley centre.
Shingle the gable roof. Fold the new shingles through the valley centre and onto the existing roof to a distance of 1 foot. Re-lay the shingles you removed from the existing roof. Apply a 2-inch-wide ribbon of roofing cement to the end of each shingle, placed 4 inches up from the centre of the valley.
Tips and warnings
- Have an engineer examine your existing roof to determine if it can support the gable before beginning construction.
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