How long before mould grows after a leak or flood?
The two days after a leak decide whether you are drying a house or cutting mould out of it. What Health Canada says, what to do with the time, and where the line sits between a cleaning job and a specialist one.
Water on the floor now? Call 647-697-7435, answered 24 hours
Health Canada says mould will begin to grow in an area with excessive moisture within 48 hours. After a leak or flood, that is the deadline: get the water out and the wet materials drying inside two days. Mould can start where you cannot see it, behind drywall and under flooring, so a dry-looking surface is not proof.
Forty-eight hours is a ceiling, not a grace period. Drying that starts on day one has a much better chance of finishing before anything grows than drying that starts at hour forty.
What does Health Canada say about mould after water damage?
Two of its guides say the same thing from opposite ends. The guide to addressing moisture and mould says mould will begin to grow in an area with excessive moisture within 48 hours, and to act quickly once you find damp. The flood clean-up guidance says a house and its furnishings are less likely to grow mould if they are dried within 48 hours, and that rapid drying is important to prevent it.
It is also clear about why it matters. Health Canada says people living in homes with mould and damp are more likely than others to have eye, nose and throat irritation, coughing and wheezing.
What should you do in the first 48 hours?
- Make the power safe. Health Canada's first point is to avoid electric shock: cut power to the flooded area, and ask your electrical utility for help if you need it.
- Stop the source. Close the main valve, or the fixture valve if you know which one it is.
- Work out what kind of water it is. Clean water from a supply pipe, or sewage from a floor drain or toilet. Health Canada says sewage carries a real risk of infection, and children, pregnant women and people with respiratory problems should never handle it.
- Photograph everything before it moves, for the insurer.
- Get the water out. A wet-dry shop vacuum for a small area. Extraction equipment for anything bigger.
- Take out what will not dry. Health Canada lists insulation, drywall, carpets, particleboard furniture, mattresses and pillows among the items to discard if they cannot be dried.
- Dry what is left. Open doors and windows and run fans if the weather is dry and mild. If it is not, or drying is slow, run a dehumidifier.
- Watch the air. Health Canada says to have a working carbon monoxide detector while drying out, and not to use outdoor combustion equipment indoors.
Steps 1 to 4 are covered in more detail in the first hour of a water leak, and for a basement in the flooded basement guide.
Once the emergency is over, Health Canada recommends keeping indoor relative humidity between 30 and 50 per cent, with a dehumidifier where needed.
How do you know if mould has already started?
Smell first, then sight. Health Canada's list of signs includes musty or earthy odours, stains or discolouration on floors, walls and window panes, peeling paint, wrinkled wallpaper and warped wood. It also says mould growth can be hidden behind walls or above ceiling tiles.
A smell in a room that took on water and was never properly dried usually means something behind the surface is still wet. That calls for moisture readings, not air freshener.
When is it no longer a cleaning job?
When the mould is large, inside the material, or on something porous. Health Canada sorts visible mould by size:
| Area | Health Canada's definition | Who should deal with it |
|---|---|---|
| Small | One square metre or less, no more than three patches | You can clean it yourself |
| Medium | More than three patches, each under one square metre, under three square metres in total | Usually cleanable with precautions, professional assessment recommended |
| Large | A single patch larger than three square metres | A qualified professional should assess and clean it up |
Size is not the only test:
- Mould under paint means the drywall needs to be removed and replaced, according to Health Canada. Painting over mould does not kill it.
- Mouldy carpet and underpad should be cut out and discarded, with the floor beneath cleaned and dried. See can wet carpet be saved.
- Soft, porous items that stayed wet, such as mattresses, bedding, soft furnishings and stuffed toys, go out too.
- Bleach is not the answer. Health Canada says not to use it to clean up mould. Unscented soap, then drying, for small areas on hard surfaces.
Health Canada also says infants, children, seniors, pregnant people and anyone with asthma or another respiratory condition should stay out of the area while mould is cleaned, and pets too. Anyone cleaning a small patch should wear goggles, a well-fitting N95 respirator and household gloves.
What does AllCleaner do, and what does it not do?
We deal with the water before it becomes mould: extraction, moisture mapping of walls, subfloor and cavities, drying with air movers and dehumidifiers until the readings reach target, and antimicrobial treatment on affected materials. That is the work that keeps a leak inside the 48 hour window.
We do not advertise mould remediation as a service. If we open a wall and find a large area of growth, we will tell you, and Health Canada's advice for that is a qualified professional who assesses and cleans it up.
Water damage work is priced on our published bands: a single small area $1,500 to $3,500, a whole room flood $4,000 to $7,500, a full basement $5,000 to $25,000, with the price confirmed before work starts. How long water damage restoration takes explains the drying stage, and water damage cleaning and restoration covers the service. If it happened today, call the emergency line, answered 24 hours on 647-697-7435, across Toronto and the GTA.
Short answers.
- Should I use bleach on mould after a flood?
- No. Health Canada's guide to moisture and mould says not to use bleach to clean up mould. For small areas on hard surfaces it suggests scrubbing with an unscented soap solution and drying quickly, and for walls a damp cloth with baking soda or a little unscented soap, without soaking the drywall.
- How big an area of mould can I clean up myself?
- Health Canada calls an area small if it covers one square metre or less, in no more than three patches, and says you can clean that yourself. Medium areas, under three square metres in total, can usually be cleaned with precautions. For a single patch larger than three square metres it recommends a qualified professional.
- Can mould grow behind a wall that looks dry?
- Yes. Health Canada says mould growth can be hidden behind walls or above ceiling tiles. A wall that took on water can look and feel dry on the painted face while the back of the drywall and the framing stay wet. Moisture readings, a musty smell or staining are the usual clues.
