Age and remaining life
A newer roof with localized damage is usually a repair candidate. A roof near the end of its life with widespread wear is often better replaced.
Extent of damage
Isolated missing shingles or a flashing leak can be repaired. Widespread granule loss, curling or multiple leaks point toward replacement.
Decking and ventilation
Soft or rotted decking and poor ventilation are problems a repair won't fix. They are corrected during replacement.