How do you frame a new roof to existing roof?

From a licensed Builder:


Because the framing for the addition will overlay the existing truss-framed roof, you should have an engineer determine whether the trusses can support the weight of the new roof. If you get the engineer's OK for the project, your top priority is keeping your house weather-tight during construction. First, set the new rafters or trusses on the addition walls up to the point of the eave of the main house, and snap chalklines on the shingles to mark the center of the new valley.

You can strip the shingles out of the area that the new roof will cover, but to be safe, I often leave the old shingles in place and frame over them.
You'll need to install a valley board or nailer over the old roof shingles to create a solid nailing surface for installing jack rafters (see drawing). After nailing down these two boards, dab roofing cement on the nails and along the outside edge. The roofing cement will resist possible leaks overnight.

The easiest way to tie in the new shingles with the old is with a cut valley (see Four Ways to Shingle a Valley). After the addition roof is framed and sheathed, snap a line on the existing roof shingles 2 in. up from the valley center, and cut through the shingles and underlayment. Starting two courses higher than where the new ridge meets the old roof, unseal the shingle tabs (a thin flat bar or a joint-taping knife works well), and pop the nails of the overlying course.
Remove two shingles to 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-ft.-wide roll of waterproof shingle underlayment in the valley.

Shingle the addition roof, letting the new shingles fold through the valley center and onto the old roof deck by at least 1 ft. Re-lay the shingles you removed from the existing roof. Bed the end of each shingle in a 2-in.-wide ribbon of roofing cement placed 4 in. up from the valley center.