Start with age and spread together. A single failed detail (a boot, a flashing run, a valley) on a roof with flexible, granule covered shingles is a repair at almost any age. Failures appearing in different places in the same year, on shingles that crack when lifted, are the roof announcing its end as a system.
The Spring version of the arithmetic has an HOA line the generic version lacks. In the master planned ZIPs, replacement is constrained by specification: shingle class, color family, and sometimes profile are set by the association, and a repair that ignores the spec can be undone at the owner's expense. Add the storm history: a roof patched after two named storms is spending its remaining life in installments. Age, spread of damage, and spec, in that order, make the honest call here.
Then the arithmetic: list the repairs the roof plausibly needs in the next two years, not just today's. If that list reaches a third to a half of a replacement price, replacement usually wins, and it resets every component at once: decking checked, underlayment, flashing, ventilation, and covering on one clock.
Where repair clearly wins: a sound roof under about 15 years with one cause. Where replacement clearly wins: brittle mats across slopes, multiple damaged sections, or documented storm damage on an older roof. Everything between deserves both prices in writing before anyone signs.
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