Old Roof New Roof Joined Rafters

Nail oriented strand board osb decking over all the new rafters.

Old roof new roof joined rafters.

A new level 2 6 ridgeboard and rafters then are assembled over the old roof and are attached to the existing structure with nails and lag screws driven into the underlying wall top plates. Put siding on the new roof end replacing any removed from the old roof area. 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. They form an upside down v with the point at the top of the addition roof.

Then set one new truss in place against the old roof and use it to snap chalk lines to mark the location on the old roof for diagonal 2 inch by 6 inch valley boards which will hold jack rafters to tie the roofs together. Cover the roof with roofing paper overlapping seams generously then nail on shingles. Remove shingles and other roofing material where the new roof will join down to the deck board of the old roof. Cut two rafters with a circular saw to the pitch of the addition roof and nail them across the rafters on the old roof.

A properly installed layover roof can look and function as well.