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.
In the historic districts, add fabric to the arithmetic. A slate roof with failed flashing is a repair program on a roof built to outlive its owner; replacing it with shingles is rarely the honest math even when the single repair bill looks large. The reverse holds for a brittle, layered asphalt roof: there, repeated detail repairs are the subscription, and replacement is the exit. The material verdict comes before the price verdict 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.
Prefer to talk later? Leave your details and an independent local pro will call you back. No charge to you, no obligation.
Watchung Roofing Co is a free referral service. Submitting this form connects you with independent local contractors who provide the work.