Mold, Plainly, With the Federal Numbers Attached
Nobody enjoys this conversation, so here it is with the useful parts first and no drama attached to any of it.
The EPA sets a size line that decides who should do the work. Under about 10 square feet, which is roughly a three foot by three foot patch, it says most homeowners can handle the cleanup themselves. Over that, it points you to its remediation guidance and to a contractor with real experience in mold. That line is the single most useful thing on this page, because almost every patch of growth we open up is well under it.
And you probably do not need a test. EPA again, in its own words: if visible growth is present, sampling is usually unnecessary. It also states plainly that no federal limits exist for mold in indoor air, so a test result has no standard to be measured against. If somebody is selling you a test before they have looked at the wall, that is worth a second thought.
What mold looks like, so you are not guessing
Most people picture black, and most of what we find is not. On the back of board it usually shows as a fuzzy or powdery patch, and the color runs anywhere from white and gray through green to almost black. It often follows a shape rather than a blot: a run down a stud bay, a ring around a fixing, a line where the water stopped.
There is usually a smell before there is anything to see. If a room smells earthy or musty and you cannot find the source, that is the classic sign of something growing behind the wall where nobody can look at it. The word most people reach for is mildew, which in ordinary use means the same thing on a surface you can see.
Killing it is not the job. Removing it is.
Two federal findings sit behind this and they are worth reading together. Drywall is porous, and the EPA says mold can grow into the spaces in porous materials, which makes it difficult or impossible to remove completely. It also says dead mold can still cause reactions in some people, so killing it is not enough on its own.
Which is why the bleach bottle under your sink is not the answer to a moldy wall, and the EPA does not recommend biocides as a routine part of cleanup. On board, the reliable fix is to take that piece out and put a new piece in.
Where we stop, and who takes it from there
We are drywall. We open it, we dry it, we replace what has to go, and we tell you exactly what we saw. If what is back there is beyond the EPA's line, you get told that at the estimate. Same if the source turns out to be a plumbing or a roofing problem. Either way you leave the conversation knowing which trade you actually need. If anyone in the house has health concerns, that is a conversation for a doctor, and the EPA says the same.