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 Alhambra, the repair or replace roof guide questions are read against a local fact first: Alhambra sits in the western San Gabriel Valley at the edge of the San Rafael Hills, one of the valley's oldest suburbs, developed from Benjamin Wilson's Ramona Acres ranch in the late 1800s. That fact is what changes the order of the steps below for a Alhambra 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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