Few things worry a homeowner more than finding black spots on a basement wall or behind a bathroom vanity. “Black mould” gets a lot of scary headlines, and it is easy to find conflicting advice online about how dangerous it is and how to deal with it.
This guide gives Ontario homeowners the plain facts: what black mould is, what the health risks are, when you can clean it yourself, and what safe professional removal looks like.
What is black mould?
“Black mould” is a common name, not a scientific one. People usually mean Stachybotrys chartarum, a greenish-black mould that grows on materials that have stayed wet for a long time, especially drywall paper, ceiling tiles and wood. But many other kinds of mould are also dark in colour, and you cannot tell which type you have just by looking at it.
That matters less than it sounds. Health Canada’s advice is the same for all indoor mould: it should not be growing in your home, it should be cleaned up or removed, and the moisture problem that caused it must be fixed.
Where black mould grows in Ontario homes
- Basement drywall and framing after a flood, sewer backup or foundation leak
- Behind and under bathroom vanities, tubs and showers with slow leaks
- Around windows with regular winter condensation
- In attics with poor ventilation or roof leaks
- Under carpet and pad that got wet and was never properly dried
- Inside walls behind leaking dishwashers, fridges and washing machines
In almost every case, the mould is a sign of water that was not dried fast enough. Our guide on how to prevent mould after water damage explains how to stop it before it starts.
Health risks of black mould
According to Health Canada, living with mould can cause or worsen health problems, especially for people with asthma, allergies or other breathing conditions. Common effects include:
- Eye, nose and throat irritation
- Coughing, wheezing and phlegm
- Worsening asthma symptoms
- Allergic reactions such as a runny nose, sneezing or skin irritation
Some people are more sensitive than others. Babies and young children, older adults, pregnant people, and people with a weakened immune system or a lung condition should avoid areas with mould and should not take part in cleaning it up.
You may read claims online that black mould is “toxic” and causes severe illness. Some moulds, including Stachybotrys, can produce substances called mycotoxins, but the health effects of everyday home exposure are still being studied. The practical answer does not change: any mould growing indoors should be removed safely, and anyone with health concerns should speak with their doctor.
Can you clean black mould yourself?
Health Canada groups mould by the size of the affected area:
- Small: less than about one square metre, roughly the size of a small bath mat, in up to three patches. Most homeowners can clean this themselves with care.
- Moderate: more than three patches each smaller than one square metre, or patches between about one and three square metres. You may be able to clean it, but a professional assessment is a good idea.
- Extensive: a single patch larger than about three square metres. This should be handled by trained professionals.
Call a professional no matter the size if the mould keeps coming back, if it is inside walls, ceilings or ductwork, if it followed a sewer backup or flood, or if anyone in the home is sensitive to mould.
Cleaning a small area safely
- Wear an N95 mask, rubber gloves and safety goggles.
- Keep children and pets out of the room and open a window if you can.
- Scrub hard surfaces with water and unscented dish soap or detergent, then wipe with a damp cloth and dry the area quickly.
- Throw away mouldy materials that cannot be cleaned, such as cardboard, ceiling tiles or small pieces of carpet, sealed in plastic bags.
- Find and fix the moisture problem, or the mould will come back.
Do not use bleach to kill mould on drywall or wood. It does not soak into porous materials, it can damage surfaces, and mixing it with other cleaners can create dangerous fumes. Do not dry-brush or sand mould either, because it spreads spores through the house.
What professional black mould removal looks like
A proper mould remediation job is about more than wiping the visible spots. Here is how our crews handle it:
- Inspection. We find the full extent of the growth and the moisture source using moisture meters and thermal imaging, and check hidden areas like wall cavities.
- Containment. We seal the work area with plastic sheeting and use negative air machines with HEPA filters so spores do not spread to the rest of the home.
- Protection. Technicians wear proper respirators and protective suits.
- Removal. Mouldy drywall, insulation, carpet and other porous materials are cut out and bagged inside the containment. Framing and other structural wood is cleaned and treated.
- HEPA cleaning. Surfaces are HEPA vacuumed and wiped to remove spores and dust.
- Drying. The area is dried with dehumidifiers and air movers until moisture readings are normal.
- Verification. We confirm the area is clean and dry before walls are closed up and rebuilt.
- Fixing the cause. We help you understand what caused the moisture, from a leak to poor ventilation, so it does not happen again.
Black mould and insurance
Many Ontario home policies cover mould only when it comes from a sudden, covered event like a burst pipe, and some cap the amount. Mould from long-term leaks, humidity or poor maintenance is often excluded. That is why it helps to act fast after any water damage and keep records of the cleanup. Learn how we help with insurance claims.
How to keep black mould from coming back
- Dry any leak or flood within 24 to 48 hours. See our guide to your first 24 hours after water damage.
- Keep indoor humidity below 50 percent, and run a dehumidifier in the basement in summer.
- Use bathroom and kitchen exhaust fans, and let them run after showers.
- Fix slow leaks under sinks, around tubs and at windows right away.
- Keep gutters clean and make sure the ground slopes away from the foundation.
- Do not store cardboard boxes directly on a basement floor.
Frequently asked questions
Is black mould more dangerous than other mould?
Colour alone does not tell you how harmful a mould is. Health Canada treats all indoor mould growth as something to remove. What matters most is how much there is, where it is, and who lives in the home.
Can I stay in my home during mould removal?
For contained jobs in one area, many families stay home while work is done behind sealed containment. For large jobs, or if someone in the home is sensitive to mould, it may be better to stay elsewhere until cleaning is finished.
How long does professional mould removal take?
Small, contained jobs are often done in one to three days. Larger jobs involving several rooms, drying and rebuilding can take a week or more.
Will painting over mould fix it?
No. Paint, even mould-resistant paint, does not remove mould that is already growing. It will usually come back through the paint, and the moisture problem underneath will continue.
Do I need a mould test?
If you can see or smell mould, a test is usually not needed to decide it should be removed. Testing can help find hidden mould or confirm an area is clean after remediation.
Found black mould? Call now
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