The pattern here is wind first, rain second. Santa Ana events between October and March lift and break the seal on tile and shingle roofs, often without any visible damage from the ground. Then the first real rain of the season finds every one of those gaps at once. That is why our roof-leak call volume spikes on the first significant storm rather than during the wind event itself.
Flashing is the second cause and it is remarkably consistent. Chimneys, skylights, vent stacks, and roof-to-wall transitions where a single-storey section meets a two-storey wall — these leak in the same spot every time it rains hard, and sealant applied over failed flashing lasts about one season.
Age is the third. A composition shingle roof in Southern California sun runs roughly 20 to 25 years. Concrete and clay tile last far longer, but the underlayment beneath them does not — usually 20 to 30 years — and the underlayment is the actual waterproof layer. A great many Fullerton tile roofs look perfect and are leaking because the felt underneath has failed.
Then there is debris. The mature eucalyptus, pine and pepper trees across Golden Hills, Chapman Woods, North Fullerton and the older streets drop constantly. Valleys and gutters pack with litter, water backs up under the roofing, and it goes in.