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 Northridge, the repair or replace roof guide questions are read against a local fact first: Northridge is a neighborhood of the City of Los Angeles in the San Fernando Valley, first settled as Zelzah and renamed in 1929; it has no city hall of its own, and this site serves its postal footprint, named honestly as such. That fact is what changes the order of the steps below for a Northridge household: what gets documented first, what can wait a day, and which call is the first call. The sequence itself is the standing one for this work; the local detail is what makes it this city's version of it.
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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