You can frame a new roof over the old but it will need to be designed for the span and pitch.
Framing a new roof over an old roof.
Use 1 5 or 1 75 inch roofing nails.
To provide a framework for the new trusses nail 2 by 6 inch strips of wood around the perimeter of the roof.
The sheathing must be stripped from the old roof for a couple feet along the bottom to get the new ceiling joists in.
Your second row of shingles should be cut to 10 inches wide.
Installing the new 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.