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.
Or if you want to raise the roof a bit more install fascia around the roof perimeter attached to the new soffit and nail the new roof deck to crown molding above the fascia.
Your second row of shingles should be cut to 10 inches wide.
You can frame a new roof over the old but it will need to be designed for the span and pitch.
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.
Installing the new roof.