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 the tract arithmetic. Carson's roofs were installed street by street, so the neighbors' experience is evidence, and the one third rule, replace when a third of the roof needs repair or the covering is at its age limit, sorts most cases. Then add the system questions this city adds: will the new covering inherit failed flashing and no ventilation, and does the deck support an overlay or demand a tear off. The honest proposal prices repair and replacement side by side with those answers in writing, because in a tract city the replacement decision is usually a when, and the arithmetic should say when.
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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