Start with what can still be bought for the door. Manufacturers support replacement panels for a model for a limited run of years; past that, a single bent section can force the full door question. Springs, cables, rollers, and openers are universal enough to repair at almost any age. Panels and door structure are where age becomes destiny.
Run the 50 percent rule, then the Norwalk questions. Is the door's model still supported for panels, because an unsupported door turns every future panel strike into a replacement. Is the structure sound beneath the corrosion, since shore doors rust from the bottom panel upward and the honest read is at the bottom rail, not the street view. And does the opening's size belong to a standard, because a pre-standard detached garage opening changes replacement arithmetic entirely. Age, support, structure, size: the verdict follows those four, not the salesperson's calendar.
Then do the arithmetic honestly. List every repair the door needs in the next year or two, not just today's failure: the second spring, the rusting bottom section, the opener at year fourteen. If that list totals half or more of a new door's installed price, replacement is usually the better money, and it resets the clock on every part at once, with modern safety sensors and insulation the old door never had.
Where repair clearly wins: a sound door under about twelve years old with one failed part. Where replacement clearly wins: multiple damaged sections, rust through in more than the bottom panel, or a door whose maker no longer exists. Everything between is a judgment call, and the right output of the visit is both prices in writing, with the panel support question answered in plain words.
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