How large is large
Under about ten square feet, EPA says most people handle it themselves.
EPA puts the line at roughly 10 sq ft: below it, in most cases you can clean it up yourself. Above it the work becomes containment, negative air and controlled removal, because disturbing a larger area without those moves the spores through the rest of the house rather than out of it.
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What ten square feet actually looks like
About a patch three feet by three feet, or a strip a foot high along ten feet of wall. Most bathroom ceiling patches and window-reveal growth are well under it. A wall behind a leaking shower or the base of a room that flooded is usually well over.
Measure the visible area rather than estimating it. People consistently guess low on a ceiling and high on a wall.
Why area is the wrong thing to measure alone
Visible growth is the part that reached a surface. Where the moisture came from inside a wall or under a floor, what you can see is an indicator rather than the extent. A small patch fed by a slow leak in a cavity can sit above a far larger area of growth that will not be visible until the wall is opened.
The question that settles it is where the water came from. Condensation on a cold surface grows on the surface; a leak inside a structure grows inside it.
If the source is not obvious, that is itself a reason to have someone look before you start scrubbing.
What changes above the line
The work becomes a controlled removal: the area is sealed off, air is filtered and kept at negative pressure so it flows inward, porous material that cannot be cleaned through is cut out and bagged, and the remaining surfaces are HEPA vacuumed and wiped rather than sprayed.
EPA publishes the detailed version of this for schools and commercial buildings, and the method is the same in a house at a smaller scale.
Bleach is not the answer on porous material. It wets the substrate and leaves the roots, which is why the patch returns in the same place.
Fix the water first, in every case
Cleaning without stopping the moisture returns the growth on its original timetable. That is true at every size, and it is the single most common reason a small job becomes a large one. Find the source, stop it, dry the material, then clean.
Where the source is not obvious after a proper look, treat that as part of the job rather than as a detail to sort out later.
Who decides, and who does it
The same patch leads to different actions depending on whose building it is.
- 01You own and live here. Measure it, find the water, and decide honestly which side of the line you are on. Below it, a weekend and a HEPA vacuum. Above it, someone with containment.
- 02You own it and rent it out. Deal with the moisture source first and document the date you were told. Health complaints move faster than building work does.
- 03You are selling. A cleaned patch with no record looks worse to a buyer than a documented remediation. Keep the invoice and any verification.
- 04You rent here. Report it in writing with a photograph and a rough size. Cleaning it yourself with bleach is what the landlord will later say you did wrong.
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What to ask before you agree to anything
Ask these of whoever turns up. A contractor who answers them plainly is the one you want; a contractor who will not is telling you something.
- 01How did you establish the size, and did you open anything to check?
- 02Where is the water coming from, and is stopping it part of your price?
- 03Are you containing the area, and what pressure are you running?
- 04What is being cut out and how far past the visible growth are you going?
- 05Do you HEPA vacuum and wipe, or do you spray a biocide? I would like to know which.
- 06Does this state license mold work, and if so what do you hold?
Have this ready before you call
Three facts move the first minute past the formalities: what happened, when it started, and whether water is still moving. These add the detail that decides what gets loaded on the van.
- A photograph of the patch with something for scale
- Roughly how many square feet, measured rather than guessed
- Where you think the water came from, if you know
- When it was first noticed
- Whether the surface is painted plaster, drywall, timber or tile grout
- Whether anyone in the household has asthma or is immunocompromised
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Where these figures come from
Every number on this page is published by someone other than us, and each link goes to their own text rather than to a summary of it.