If you have traditional site built roof framing your storage capacity depends on the design of the framing.
Garage roof trusses sagging.
The previous owner mistakenly decided to store various items across the trusses causing sag.
The sag is visible from on the apex of the roof from end to end bowed and between the ends of individual trusses sagging in the middle.
Sagging rafters can lead to serious problems with the entire roofing system.
Try to rope in a friend or two as the beams can be heavy and try to plan your renovation project for a sunny weekend.
Finally make sure stored items are securely locked in place.
If you notice that there is a visible dip in your roof chances are you have a problem with one or several rafters and you may soon be facing more issues like water leaks bowed out walls or your roof caving in on you.