How the Arcadia climate wears a roof, sun first
The thing that ages an Arcadia roof is not what wears out a roof in a wetter climate. Here the dominant force is the sun. The San Gabriel Valley runs hot and bright for the better part of the year, and that relentless ultraviolet exposure is hard on every roofing material in a way that surprises people who assume a dry climate is an easy one. On a tile roof the tile itself shrugs off the sun, but the underlayment beneath it, the membrane that does the actual waterproofing, bakes in the trapped attic heat and grows brittle decades before the tile shows any sign of age. On a shingle roof the sun dries the asphalt, drives off the protective oils, and curls and cracks the shingles from above while a hot attic cooks them from below.
Then there is the rain, which arrives rarely but rarely arrives gently. Arcadia can go most of the year nearly dry and then take a series of soaking winter storms in a matter of weeks, and that pattern is its own kind of test. A roof that has been quietly degrading under the summer sun suddenly has to shed real volumes of water, and the weak points that went unnoticed all year give way at once. This is why so many Arcadia roof leaks announce themselves with the first heavy rain of the season, on a roof that looked perfectly fine in October. The damage was done by the sun months earlier, and the rain merely found it.