The 10-square-foot rule
The EPA’s practical dividing line: under about 10 square feet (roughly a 3×3 patch) on a hard, non-porous surface, you can handle it yourself — detergent and water, gloves, N95, ventilation, and dry it thoroughly. Skip the bleach; on porous materials it mostly adds water, and the roots survive.
Above that, or anywhere mold is inside wall cavities, under flooring, or in the HVAC system, it becomes a containment job. Professionals seal the area under negative pressure precisely so that disturbing the growth doesn’t distribute spores through the rest of your house — which is exactly what a homeowner with a scrub brush does.
Never run the HVAC if you suspect mold in the ducts. That’s a distribution system.
Fix the water, or you’ll pay twice
Mold is a symptom. It needs moisture, and it will come back within weeks if the source is still there — a slow supply leak, a roof problem, poor bathroom ventilation, condensation on cold walls, or a humidifier running too high. Any remediation quote that doesn’t ask where the water is coming from is a quote to repeat the job.
Where it’s growing tells you which trade fixes the cause
A remediation company removes the mold. A different trade removes the water, and that one is decided by location. Go look at where it’s actually growing.
Bathroom ceiling, or the grout and silicone around the shower → ventilation, not a leak. You need an extractor fan that runs long enough and vents outside, not into the attic. That’s an HVAC or electrical contractor. If a fan is already fitted, hold a tissue to it — if it doesn’t hold, the fan is decorative.
On an exterior wall, behind furniture or in a corner, worse in winter → condensation. Cold wall surface meeting humid indoor air. The fix is insulation, an air gap behind furniture, and humidity control — an insulation contractor, not a plumber, and not a remediator.
Around window frames and sills → condensation on the glazing or a failed seal. Window contractor.
Under a sink, behind a toilet, at the base of a wall backing onto a bathroom → a live plumbing leak. Plumber first, always, before anyone quotes remediation.
Dark staining on attic sheathing or the underside of the roof deck → roof ventilation, or a roof leak. Roofer. Fixr puts attic mold remediation at $1,500 to $9,000.
Inside ducts or on supply registers, smell strongest when the system runs → HVAC, and turn the system off now; it is a distribution network. Fixr puts HVAC mold remediation at $3,000 to $10,000, the most expensive location on the list, which is the argument for not running it.
Crawl space, with a musty smell that shows up throughout the house → ground moisture rising through the floor. Encapsulation and a vapour barrier; a crawl space or foundation contractor. Fixr puts crawl space mold at $500 to $2,000.
Basement walls or the floor slab → groundwater through the foundation. Waterproofing contractor after the remediation. Fixr puts basement mold at $1,500 to $4,000.
Everywhere at once, days or weeks after a flood or burst pipe → restoration company, urgently, and this is the version most likely to be a live insurance claim. Fixr puts mold inside walls and drywall at $1,000 to $12,000.
Timing separates the two causes people most often confuse:
- Worse in winter, on cold surfaces, no water source anywhere near it → condensation. Heating and ventilation, not a leak hunt.
- Worse through humid summer months, in a basement or crawl space → ambient humidity. A dehumidifier held around 50% relative humidity solves more of these than any contractor.
- Appeared within a couple of weeks of a specific water event → that event caused it. Restoration, and tell your insurer while the connection is still obvious.
- Keeps returning in the same spot after you clean it → the source is still live. Stop cleaning and start looking; you are treating the symptom on a loop.
For overall scale: Fixr puts mold remediation at $1,500 to $9,000 with a national average of $3,500, or $10 to $30 per square foot, and an independent inspection at $450 to $800.
Who to call first, in order
- The trade that owns the water source — plumber, roofer, HVAC, waterproofer — per the section above.
- An independent mold inspector, if the growth is hidden, disputed, or you need documentation for a landlord or an insurer. Skip this if you can see it plainly and it’s small.
- A remediation company to contain and remove.
- The inspector again, for a post-remediation clearance test. The same firm should not mark its own homework.
- The rebuild — drywall, paint, flooring — only after clearance.
What each wrong order costs you:
- Remediating before the leak is fixed is the expensive one. You pay full price for a job that reappears in the same spot within weeks, and the second quote is rarely cheaper.
- Hiring one company to test and to remove hands the pricing decision to the party being paid. Get the inspection from someone with nothing to sell you.
- Letting the remediator run its own clearance test leaves you with no independent evidence the job worked — which matters most in exactly the situations where you need evidence.
- Closing walls or painting before clearance hides the problem rather than ending it. Paint, including “mold-resistant” paint, does not kill what is already growing.
- Scrubbing it yourself first on anything beyond a small hard-surface patch does two things at once: it aerosolises spores into the rest of the house, and it destroys the documentation you’d want in a landlord or insurance dispute. Photograph before you touch it.
- Running the HVAC while you work out what to do turns a local problem into a whole-house one.
Who will tell you this isn’t their job
Mold sits between more trades than almost anything else in a house, and the refusals are predictable:
- Home inspectors generally won’t test for mold. It is a separate discipline and, in a number of states, a separately licensed one. You want a mold assessor or industrial hygienist.
- Assessors and testing labs won’t remove it, and the better ones decline on principle — the separation is the point of hiring them.
- Remediation companies won’t fix the leak. They’re not plumbers or roofers, and many will decline to quote at all until the source is dealt with. That refusal is a good sign, not an obstacle.
- Duct cleaners are not remediators. A duct cleaning is not a remediation, and no reputable duct cleaner will claim otherwise.
- HVAC contractors won’t remediate. They replace or clean affected components after the remediation, not instead of it.
- Pest control will say no. Mold is not a pest, and this is a call people make surprisingly often. The one adjacent thing pest control does handle is the moisture-loving insects that show up alongside mold — different problem, different invoice.
- Painters and general handymen should decline to paint over it. If one offers, that is the contractor to avoid.
- Your landlord’s maintenance guy with a spray bottle of bleach is not remediation. Bleach on porous material adds water and leaves the roots; the growth returns.
Insurance: what usually decides it
Coverage follows the cause, not the mold. Mold that traces to a sudden, accidental, covered event — a burst pipe, an appliance failure — is generally the version insurers will engage with. Mold that traces to long-running humidity, a slow leak, or a maintenance issue is normally excluded, and many policies cap mold specifically even when they cover the underlying event. Ask your insurer what your mold limit is; it is often far below the numbers above.
The mechanism behind most denials is timing rather than the mold itself. Once a leak is characterised as gradual — something a reasonable owner would have noticed — the resulting mold is characterised the same way, and “I didn’t realise it had been leaking” reads as confirmation rather than mitigation. Practical consequences: report the water event the day you find it, photograph the growth before anyone disturbs it, keep the plumber’s or roofer’s invoice showing when the source was fixed, and describe what you found and when you found it rather than speculating about how long it had been there.
If you rent
Put it in writing — email or text, so it’s timestamped — with photos. Most states treat significant mold as a habitability issue, making remediation the landlord’s obligation and cost. If they don’t act, your local health or code-enforcement department will often inspect, and that inspection report is what gives you leverage. Keep your own copies of everything; verbal complaints are worth nothing in a dispute.