Why LA homes get mold without rain

Plumbing leaks are the biggest source

With little rain for much of the year, the water that feeds mold in LA usually comes from pipes. A supply line that drips under a vanity, a wax ring that seeps behind a toilet or a pinhole leak inside a wall can run for months before anyone notices. By then, the cabinet base or the drywall behind it may have been wet the whole time.

The marine layer and coastal humidity

In late spring and early summer, the marine layer (“May Gray” and “June Gloom”) keeps coastal neighborhoods cool and humid. That moisture settles into the coolest, least ventilated parts of a house: closets on exterior walls, north-facing rooms and spaces behind furniture pushed against outside walls.

Bathrooms and ventilation

Bathrooms without an exhaust fan, or with a fan too weak to clear the steam, stay damp for hours after every shower. Some older fans vent into the attic instead of outside, which moves the moisture somewhere you can’t see it.

Raised foundations and crawlspaces

Many older LA homes sit on raised foundations. The crawlspace underneath collects moisture from the ground, from plumbing leaks and from poor drainage, and few owners ever look down there. A musty smell strongest near the floor is often the first sign. We cover these in our crawlspace and attic inspection.

When it does rain

Winter storms find every weak point at once. Roof leaks, failing window flashing and poor drainage on hillside lots show up quickly after heavy rain, often as a ceiling stain days later.

How to keep moisture in check

The EPA’s guidance applies directly:¹

• Keep indoor humidity below 60%, ideally between 30% and 50%.
• Run bathroom and kitchen exhaust fans, and make sure they vent outside.
• Fix leaks promptly, and dry anything that gets wet within 24–48 hours.
• Keep air moving in closets on exterior walls.

If you’re not sure where moisture is coming from, a moisture and humidity assessment maps it room by room.