That stain on the wall is not telling you much by itself
You've probably noticed a faint ring on the ceiling under a bathroom, a soft spot near a window sill, or a musty smell in a closet that backs up against an exterior wall. In a lot of Los Angeles homes, especially the older bungalows in places like Highland Park or the stucco builds in the Valley, these little signs show up and then nothing happens for months. No leak, no flooding, just a mark that stays the same color and a smell that comes and goes with the weather. That's usually when people go buy a moisture meter, point it at the wall, and get a number that doesn't mean anything to them.
A number on a cheap meter is not a diagnosis. It's a starting point, and only if you know what you're comparing it to.
Why LA houses hold moisture differently than you'd think
This is a dry climate most of the year, which is exactly why moisture problems here get missed. Homeowners assume mold needs a flood or a humid swamp climate like Florida, so a little dampness gets ignored. But LA's housing stock works against that assumption. A huge share of homes were built between the 1920s and 1960s, with minimal vapor barriers, old plumbing runs behind plaster walls, and crawl spaces that were never sealed properly.
Then the rainy season hits, usually December through March, and it's often a handful of intense atmospheric river storms rather than steady rain. Water gets driven sideways into stucco cracks, pools on flat roofs common in mid-century homes, or backs up where hillside lots in places like Silver Lake or the Hollywood Hills shed runoff against a foundation. The house dries out fast once the sun comes back, which hides the problem instead of fixing it. Moisture gets trapped in wall cavities and under flooring long after the surface looks fine.
More on this from Reading List Reading A Moisture Meter In A Los Angeles Home Without Guessing.
What a pin meter actually reads, and what it doesn't
A standard pin-type moisture meter measures electrical resistance between two small probes pushed into the material. Wood, drywall, and plaster all have different baseline readings, so a number like 18 means something different on a drywall patch than it does on a baseboard. Without knowing the material's dry baseline, you're just staring at a digit.
A pinless meter, the kind that scans without puncturing, reads moisture in a zone below the surface using electromagnetic signals. It's useful for scanning a wide wall quickly without poking holes everywhere, but it can be thrown off by metal lath behind old plaster, which is extremely common in pre-1960s LA construction. If you get a high reading on a plaster wall with lath, that could be the lath, not water.
What you can check yourself before calling anyone
Start by comparing a suspect area to a known-dry area of the same material elsewhere in the house, same type of wall, same orientation if possible. A reading that's notably higher than the baseline tells you more than the raw number does. Check it again a few days after a storm and once more two weeks later. If the number drops back toward baseline, that's surface moisture drying out normally. If it stays elevated, something is holding water in there.
Look outside at the same spot. Check for cracked stucco, a downspout dumping water against the foundation, or a roof penetration nearby like a vent pipe or chimney flashing. In a lot of cases the moisture source is obvious once you walk the exterior with the interior reading in mind.
Smell matters too. A musty smell that's stronger in one room, especially combined with a visible stain, is worth taking seriously even if the meter reading looks borderline.
Where this stops being a DIY project
If the reading stays elevated after a few weeks of dry weather, if there's a musty smell with no clear exterior source, or if you see any surface discoloration that's spreading rather than static, that's the point to bring in someone who does mold inspections for a living. A home inspector uses calibrated meters plus thermal imaging to find moisture that hasn't reached the surface yet, and they know how to read around things like metal lath, radiant floor heating, or the double-wall construction common in some Spanish Revival homes from the 1920s.
There's also the question of what's actually growing in there. A moisture meter tells you water is present. It says nothing about whether mold has started, what kind, or whether it's reached a level that matters for your health or your walls. That requires sampling and lab analysis, which isn't something a hardware store meter gets you close to.
If you've already found mold you can see, testing the moisture level is really just confirming what you already know. At that point the job is remediation, not detection, and that's worth getting a professional opinion on before you start cutting into walls yourself.