Tear off all layers,inspect for bad sheathing and replace at like thickness,install 1 2 inch. Installation board used metal plate and screw. Install new rubber,install drip edging to all roof edges. Install new 4-inch. E-box drain. Install roof cement and fabric to all roof edges-5 8" exterior grade plywood and 5 8"spacers required on sheathing fire resistant sheathing required along with party walls within 40"or less.(may overlay existing roofing lumber with new approved material or repair.).