Sun first, then the weather that finds the weak spots
If you want to understand a Rancho Cucamonga roof, start with the light, because that is what does the most damage and gets the least credit. The city sits where the air is dry and the sun comes in strong, and that energy goes to work across a long, mostly cloudless year. On tile and concrete roofs the tile hardly registers it, but the underlayment underneath, the layer truly responsible for keeping water out, stiffens and grows brittle until it splits and quits sealing. On shingle the same sun pulls the granules loose, dries the asphalt, and curls the edges well before its rated age. The part that catches people off guard is that the roof loses most of its ground during the calm, pleasant stretches when nothing seems to be happening at all.
Once the sun has prepared the way, the weather shows up in concentrated hits and goes straight for the spots that are already compromised. The Santa Anas drop out of the canyons in fall and snap or slide the tiles the heat has weakened, while prying at shingle that has lost its grip. The winter storms are short and intense, and they pressure-test every flashing joint and worn underlayment seam in a single afternoon after months of drought. The dry brush along the upper neighborhoods turns the roof into a fire concern, where one ember in the wrong gap finishes a job no rainstorm ever could. We inspect with all of that in view, because a roof that looks perfect in June can be carrying three different weaknesses, each waiting on a different season.