Where mold hides in a Los Angeles home

Almost every call starts the same way: somebody found a patch, or smelled something, and wants to know how bad it is. The patch is rarely the interesting part. What an inspector is doing in the first fifteen minutes is working out which of a small number of failure modes is running, because in this housing stock it is nearly always one of them.

1. The attic above the bathroom

The most predictable find in the county. Somebody added an exhaust fan to a house that never had one, ran the duct as far as the joist bay, and stopped. Every shower puts a few minutes of steam onto cold roof sheathing. By the second winter there is a dark bloom directly above the bathroom, usually on the north-facing slope, which gets the least sun and dries slowest.

Check from inside the attic with a light, not from the roof. You are looking for discoloration on the underside of the sheathing and a flexible duct that ends in open air.

2. The crawl space, on bare soil

Most raised-foundation houses here sit over bare earth, and soil releases moisture continuously whether or not it has rained. In the foothill communities the ground can be genuinely damp in September. Air moves upward through a floor, so whatever is under there is upwind of every room above it. This is the answer to the classic musty-house-with-nothing-visible call.

3. The air handler and its condensate line

A cooling system is a dehumidifier by side effect and it produces a continuous stream of water for as long as it runs. That water is supposed to go down a drain line and outside. When the line blocks, and they block, it goes into the secondary pan, and when that rusts through it goes onto whatever is underneath: attic framing, closet floor, ceiling drywall.

A ceiling stain that appears in August, in a month with no rainfall, is not a roof leak. It is almost always this.

4. Behind plaster on the irrigated elevation

On slab houses especially, irrigation running daily against a stucco wall delivers more water to that elevation over a year than the rain does, and unlike the rain it lands in the same place every day. It shows up at the base of the interior wall on that side of the house. The diagnostic is free: turn that sprinkler zone off for a week and have the same spot metered again.

5. Under the kitchen and bathroom sinks

Unglamorous and extremely common. A supply connection weeping a few drops a day, into a particle-board cabinet base that acts as a sponge and hides it. People store things in there and do not look at the back corner for years. Empty the cabinet and put a hand flat on the base at the back.

6. The wall behind the washing machine

Rubber supply hoses perish, and the standpipe connection is behind a machine nobody moves. In converted garages, which this county has in enormous numbers, the laundry was frequently added to a structure with no insulation and a slab poured for a car, which makes it the single most reliable finding in a back unit.

7. Window sills and frames near the coast

Salt air shortens the life of the metal in a building envelope, so flashings and steel window frames fail sooner in Santa Monica or Long Beach than they would twenty miles inland. Once one admits water, the surrounding wall is working in air that will not help it dry. Look for rust staining running down the wall below a sill.

8. The closet on a cold exterior wall

The coldest, least ventilated surface in most houses, behind a door that stays shut. In a full household producing real humidity from cooking, laundry and bathing, with no mechanical extraction, this is where it condenses first. It is also where people are most surprised to find it.

What the nose is telling you

A musty smell with nothing visible anywhere is not a false alarm and it is not nothing. It is the most under-investigated presentation in this trade, precisely because there is no photograph to react to. In this housing stock it usually means the crawl space, the attic or a cavity, in that order.

If you have looked at all eight and found nothing, that is worth knowing too, and it is the point at which a meter and a thermal camera beat another hour of looking. The on-site inspection is free, the findings are written down, and a useful share of them end with no mold work at all.