Replacing sliding patio door unit - sagging header repair / remedail bracing

I 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 recently purchased. One panel opens but the rest are frozen . The rollers and weather stripping are 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 (flitch plate) or plywood to eliminate future sagging ? I would appreciate any suggestions on how to address this situation . - Thanks

 

You could... but you could also replace the header with an engineered header that will be much stronger than your existing one.

Last time I had to buy some steel I about fell over at the price. So I can't imagine that the difference in cost would be a huge factor either way.

 

I agree with sleeper, it is better to replace the existing header than to try to fix the old one. And modern engineered beams can be quite strong.

 

Thanks for the replies so far. - I was hoping to add to the exiting header to avoid disturbing the impossible to match vinyl siding on the exterior . Further discussion on this is appreciated . - Thanks