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.
The Simi Valley version of the arithmetic starts with the covering question. On tile: the tile's life is long, the felt's is twenty to thirty years, so the decision is rarely replace the roof; it is relay the slopes whose felt has expired, priced against the years that buys. On shingle: the standard rule, a third of the roof in repair need or the surface at its age limit, plus the ventilation and flashing state the replacement would inherit. Both decisions deserve the same artifact, a slope by slope report with the felt or the surface photographed, because 'the roof' is not one age in this city. It is one age per slope.
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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