am about to replace a 3 panel (4.0 ft) each ) 12 ft total 18 year old sliding door in a coastal rental property I recnetly purchased. One panel opens but the rest are frozen . The rollers a shot (obsolete ) and it is time for new door unit . I measured the heights at various spots and there is about 5/8" sag in the middle . There is vinyl siding on the outside and drywall on the inside. I would like to maintain the opening for a new door unit . I wondered if could cut the sagged portion then remove some drywall and install a steel plate (f;itch plate) or plywood to eliminate future sagging ? I would appreciate any suggestions on how to address this situation . - Thanks