Tying a gable roof into an existing roof is a long, expensive project, usually undertaken only when there is a major addition or renovation to a house. It is not a job for beginners. You may need a structural engineer to help with design of trusses and rafters. You will need building help -- and a large construction dumpster for debris. You also will need a way to weatherproof the existing structure while roofing is being removed and replaced.
Tie a Gable Roof to an Old Roof
Determine where your new gable roof will need to tie into the existing roof. Tying an addition onto the gable roofed end of a house will require one sort of framing; tying a roof perpendicular to the existing roof will be another matter. Take careful measurements to determine how many trusses you will need. To tie into the side of an existing roof, put a new roof truss on the walls of the addition, brace it upright, then snap chalklines on the shingles to mark the centerline of the new roof valleys, where old and new roofs will connect.
Tie into the end of an existing gable roof by removing existing facing boards, wall siding and any other obstructions back to the first truss of the old roof. Then mount new trusses on the new addition walls -- butt the first new truss against the old one. Then install new ridge board, tying it to the old roof by cutting back part of the original ridge board so there is overlap. Once new trusses are secured, sheath the roof with oriented strand board (OSB), lay down roofing paper, then nail on shingles. Install metal drip edge at the eaves. Then add siding and trim to the new gable roof end.
Start tying in a new gable roof perpendicular to the old one by removing old shingles and nailing 2-inch-by-6-inch valley boards along the chalk lines you marked. Miter these at the top to fit a ridge board. Then cut "jack rafters" - short 2-inch-by-6-inch rafters that will connect the valley board and new ridge board up to the point of the first new truss. Raise your new trusses into place, brace them temporarily upright, then nail them to the new wall and add a ridge board from the outside truss back to the old roof to complete the framing for the new roof.
Finish the new roof by nailing on OSB sheathing, covering it with roofing paper and nailing in metal flashing along the valleys and metal drip edge at the eaves. Then nail shingles on the new roof and exposed areas of the old roof. Fold new shingles over the new valley onto the old roof, overlapping by at least a foot.
Complete the new roofing job by finishing the new gable end with siding to match the existing house and fascia and trim boards at eaves and edges.