The sheathing must be stripped from the old roof for a couple feet along the bottom to get the new ceiling joists in.
Framing a new roof over an old roof.
To provide a framework for the new trusses nail 2 by 6 inch strips of wood around the perimeter of the roof.
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.
Your second row of shingles should be cut to 10 inches wide.
Butt these against the bottom of the old roof s third row and nail into place.
Installing the new roof.
Closed cell polyurethane foam insulation is sprayed around the new roof frame sealing edges and providing support for the rafters and the ridgeboard.