Irrigation against the foundation
Ask where the water against a foundation comes from in most of the country and the answer is weather. Ask here and the honest answer, for much of the year, is a timer in the garage that was set once and has been running ever since.
A source that never stops
Landscape irrigation delivers water to the same ground on the same schedule through the entire dry season. Where a head is aimed at a wall, where a lateral line has cracked underground, or where the grade slopes back toward the house instead of away from it, that ground stays saturated for months at a stretch.
Underground line breaks are the version that does real damage, because there is nothing to see. The water goes into the soil, the system still appears to work, and the only evidence is a section of landscaping that is greener than the rest and a water bill that stepped up and stayed there.
What saturated soil does to a building
Against a slab edge it works at the perimeter, where the wall assembly meets the concrete and where the moisture barrier — if there is one — is at its most vulnerable. Under a raised foundation it turns the crawl space into a damp room under the house. On the hillside properties in Glendale, Culver City and the Whittier hills it adds load behind retaining walls that were built for a rainy season that mostly does not come, and pushes water toward garages cut into the slope.
The pattern that gives it away is seasonal in reverse: the problem is worst in summer, when irrigation runs most, and improves in winter, when it is switched off. Anyone reporting a damp wall that gets better when it rains is describing an irrigation problem.
Where it matters most on this site
Simi Valley and Camarillo, where lots are larger and irrigation coverage is extensive. The Rancho district in Burbank, where lines run underground to barns and paddocks and a break simply disappears. The agricultural edge of Camarillo. And the older single-storey tracts everywhere, where beds were planted against the house decades ago and the grade has settled since.