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.
Use Sugar Land's arithmetic: a repair at the market's small end against a replacement inside the $8,000 to $20,000 frame, weighed by the flashing evidence. Boats of pipe boots failing together, granule loss across every slope, and two leak entries in one season are the roof telling you its age class. One entry point, sound field, young boots: repair, documented, and revisit after the next storm season.
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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