Black mold in a basement is a dark mould growing on a damp surface, most often drywall paper, wood or stored cardboard. Colour does not identify the species, so you cannot tell by sight whether it is the species Stachybotrys. Health Canada advises that you clean only small patches (under 1 m2) on hard surfaces. For a larger area, hire a professional. Then fix the water source.
What “black mold” actually means
“Black mold” is a common name, not a scientific one. It usually points to Stachybotrys chartarum, a dark greenish black mould that grows on materials that stay wet for a long time. Typical places to find it are paper-faced drywall, wood, wallpaper and ceiling tiles that have been wet from leaks or water damage.
Many other moulds are also dark. Cladosporium and Aspergillus species can look black or very dark green. You cannot name a species by eye. Only a lab analysis of a sample can do that.
That matters less than people think. The practical advice is the same for any mould:
- Find the moisture.
- Remove the growth safely.
- Dry the space.
- Stop the water from returning.
Some of the health literature on Stachybotrys is disputed. A peer-reviewed review of the evidence reports that studies linking it to serious illness often have method flaws, so the findings are inconclusive. Do not read that as “harmless”. Damp, mouldy homes are linked to respiratory symptoms in general, and Health Canada’s own list of effects is described below.
Health effects Health Canada lists
Health Canada’s mould infographic lists these effects: irritation of the nose, throat and eyes, sneezing, difficulty breathing and worsened asthma. It advises that you see a doctor or health professional if you have these symptoms and suspect mould. It does not single out black mould.
Why mould grows in Ontario basements
Mould needs moisture, a food source and time. A basement supplies all three. Drywall paper, wood framing, carpet underlay and cardboard boxes are all food. The moisture comes from several places:
- Groundwater and clay soil. Much of southern Ontario, including the Toronto and Oakville area, sits on clay-rich soil. Clay drains slowly, so water can sit against foundation walls after heavy rain or spring melt.
- Foundation cracks and window wells. Water enters through cracks, cold joints, and the joint between the wall and floor.
- Plumbing and appliance leaks. A slow leak behind a finished wall can feed mould for months before anyone sees it.
- Humid summer air. Warm air meets cool concrete and condenses. This is common in unfinished basements in July and August.
- Flooding. Materials that stay wet for a long time are at high risk, so dry them fast.
If you see mould, look for the source before you look for a product. Check outside grading, eavestroughs and downspouts, window wells, and any plumbing near the mould.
Health Canada guidance: area size and when to hire
Health Canada gives a clear rule on its mould infographic. Clean small patches (less than 1 m2) of mould on non-porous surfaces with soap and water. If the area is larger, hire a professional. Wear a mask, goggles and gloves when you clean.
One square metre is about the size of a large pizza box laid flat, or a patch roughly 1 m by 1 m. Two details in that guidance matter in a basement.
First, the rule names non-porous surfaces: glass, metal, leather, plastic and vinyl. A basement has few of these. Most of what grows mould is porous: drywall, cardboard, paper and untreated wood. Health Canada’s list of porous surfaces includes exactly those. Mould on a porous material is often rooted in the material, so surface cleaning does not remove it.
Second, provincial health sources set other limits for larger patches. The Alberta health guidance uses a middle tier for 1 to 3 m2 and says to hire a professional above 3 m2. Use the Health Canada 1 m2 line as your cautious rule, since it is the national one.
A simple decision guide
| Situation | Suggested action |
|---|---|
| Under 1 m2, on tile, sealed concrete, glass, metal or vinyl | Clean with soap and water. Wear a mask, goggles and gloves. Fix the moisture. |
| Under 1 m2, on drywall, wood or cardboard | Throw out cardboard. For drywall or wood, get advice, since cleaning the surface may not be enough. |
| Over 1 m2 on any surface | Hire a professional, per Health Canada. |
| Mould returns after cleaning | Treat it as a water problem. Get a leak assessment. |
| Mould near a furnace, duct or air handler | Hire a professional. Spores can spread through the house. |
| Someone in the home has asthma, a weak immune system, or is pregnant or an infant | Hire a professional and avoid doing the work yourself. |
| Sewage or flood water was involved | Hire a professional. Do not clean it yourself. |
This table is a general guide, not medical or legal advice. If you are unsure, a professional inspection costs less than a failed cleanup.
What professional remediation involves
Methods differ from company to company, so ask each one for a written scope. A typical job has these stages.
- Inspection. The company finds the extent of the growth and the moisture source. It may use a moisture meter or a thermal camera. Some companies offer air or surface sampling. Ask why a company recommends sampling and what the results will change.
- Containment. Plastic sheeting seals the work area. A negative air machine with a HEPA filter keeps the air flowing into the work area, so spores do not drift into the rest of the home.
- Removal. Workers remove porous materials that are contaminated, such as drywall, insulation and carpet. They bag them before carrying them out. Hard surfaces such as framing are cleaned, often with HEPA vacuuming and wiping, and sometimes sanded.
- Drying. Dehumidifiers and air movers dry the space until the materials meet a target moisture level.
- Verification. Some jobs end with a visual check. Others end with a clearance test by a separate party. Ask which one your quote includes.
- Rebuild. Most mould companies remove materials but do not rebuild the wall. Check whether the quote includes replacement drywall, insulation and paint, or only the tear-out.
A company that offers both mould work and waterproofing is not automatically better. What matters is that the water source gets fixed, by someone, in writing.
What drives the price
Costs vary a lot, and I did not find a reliable Ontario price table from an authoritative source, so no dollar figures are given here. These factors move the price most:
- The area of contaminated material.
- Whether the drywall is finished, painted or behind built-in cabinets.
- How hard the work area is to seal and reach.
- Whether the job includes a clearance test.
- Whether rebuild work is included.
- Whether a leak repair or waterproofing job is needed afterward.
Ask for at least two written quotes. Compare scope line by line, not just the total.
Mould and waterproofing: two jobs, one cause
Removing mould treats the result. Waterproofing or foundation repair treats the cause. If liquid water enters through the foundation, mould will return. If the source is a plumbing leak, a plumber fixes it, not a waterproofing company.
Sort the cause first:
- Water along one wall after rain suggests a crack, grading or a downspout problem.
- Damp across the whole floor suggests groundwater or a drain problem.
- Damp only in humid weather suggests condensation, and a dehumidifier or better ventilation may help.
If you suspect the foundation, see the basement waterproofing companies in Toronto board for the Toronto ranking, or the basement waterproofing companies in Oakville board for Oakville. Our directory does not do repair work. It lists companies by Google rating and review count.
What our boards show for Toronto, Ottawa and Oakville
These numbers come from our own boards for October 2026. They describe the review data on Google. They are not endorsements and not a quality guarantee.
| Board | Companies ranked | Median rating | Median reviews | Total reviews | Under 20 reviews |
|---|---|---|---|---|---|
| Mold removal companies in Toronto | 19 | 4.9 | 76 | 2,044 | 4 |
| Mold removal companies in Ottawa | 20 | 4.75 | 43 | 1,295 | 4 |
| Mold removal companies in Oakville | 20 | 5.0 | 52 | 1,410 | 4 |
What this means for choosing:
- Ratings are bunched. The median rating is between 4.75 and 5.0 on all three boards. A rating alone does not separate companies. Review count and recency help more.
- Review depth differs by town. Toronto’s median is 76 reviews per company. Ottawa’s is 43. A rating of 4.9 on 300 reviews carries more weight than 5.0 on 6 reviews.
- Some companies have thin data. Four companies on each board have fewer than 20 reviews. That is not a sign of poor work. It means there is less public evidence to read.
- Reviews are not credentials. Check a company’s insurance, written scope and warranty yourself. Read the actual review text for mentions of mould, containment and follow-up.
The top five on each board are shown on the board pages. For example, the Toronto board lists Paul Davis Toronto at 4.8 with 498 reviews, and the Ottawa board lists Mold Busters Ottawa at 4.6 with 249 reviews. These are data points from Google, shown in rank order by our method, not recommendations. For other towns, see the mold removal trade hub, or boards such as mold removal companies in Mississauga and mold removal companies in Hamilton.
Safe steps while you wait for help
If you find mould and plan to hire a professional, a few actions limit the harm:
- Do not disturb it. Sanding, scrubbing dry mould or sweeping can release spores.
- Do not run a fan across it. Keep the basement door closed if you can.
- Stop any active water source. Turn off the supply to a leaking pipe.
- Move people with asthma or a weak immune system out of the area.
- Lower humidity with a dehumidifier. Keep the basement air dry.
- Photograph the area and note the date. Insurers and contractors may ask for it.
- Check your home insurance policy. Many policies limit mould cover, so read yours.
Frequently asked questions
Is all black mold in a basement toxic?
No. Many dark moulds are black, and colour does not tell you the species or the risk. Stachybotrys is the species most people mean, but only lab testing can name it. Treat any mould as a moisture problem and a health concern, whatever the colour.
How big can a patch be before I should hire a professional?
Health Canada says to clean small patches of less than 1 m2 on non-porous surfaces with soap and water, and to hire a professional if the area is larger. Mould on porous materials such as drywall usually means removal, not cleaning. Mould that keeps returning also calls for professional help.
Will bleach kill black mold in a basement?
Bleach does not fix the cause, which is moisture. It also cannot reach into porous materials such as drywall or untreated wood. Health Canada’s guidance for small patches on hard surfaces is soap and water, with a mask, goggles and gloves.
Should I fix the water problem before or after mould removal?
Find and stop the water source first, or plan both jobs together. If water keeps entering through the wall, floor or window well, mould will grow back on the cleaned area. A waterproofing company can assess leaks, and a mould company handles the contaminated materials.