Mould is the problem that shows up after the water is gone. A leak gets cleaned up, the floor looks dry, and a few weeks later there is a musty smell in the basement or dark spots creeping up the bottom of the drywall. In most cases, that mould started growing in the first two days after the water got in.
The good news is that mould after water damage is almost always preventable. This guide explains how mould gets started, what you can do in the first 48 hours, how professionals dry a home properly, and how to tell if mould has already taken hold.
Why mould grows after water damage
Mould spores are everywhere, indoors and out. They float in the air and settle on every surface in your home. They stay harmless until they find three things together: moisture, a food source and time.
A water leak supplies all three. Drywall paper, wood framing, carpet backing, cardboard boxes and the dust on almost any surface are food for mould. Once those materials stay wet, mould can begin to grow within 24 to 48 hours. Ontario homes add another factor. Basements are often cool and humid, and in winter the furnace keeps upper floors warm enough for mould to grow year round.
That is why the goal after any leak is simple: get materials dry, and get them dry fast.
Step 1: Stop the water and remove what is standing
Mould prevention starts with stopping the source. Shut off the main water valve for a plumbing leak, and fix or tarp the problem for a roof or window leak. Then remove standing water as quickly as you safely can with a wet vac, mops and towels.
If the water came from a sewer backup, a floor drain or outside flooding, treat it as contaminated. Do not handle it without protective gear, and call a professional. Our sewer backup cleanup and basement flooding cleanup services cover these situations.
Step 2: Get wet belongings out of the area
Move rugs, furniture, boxes, books and clothing out of the wet area. Soaked cardboard boxes are one of the most common places mould starts in a basement, so empty them and throw away the wet cardboard. Lay wet items out in a dry, warm room with good air flow.
Items you care about, such as photos, documents, electronics and upholstered furniture, can often be saved if they are dried and cleaned properly and quickly. A professional contents pack-out can take them off site for cleaning and drying.
Step 3: Remove materials that cannot be dried
Some materials hold water so well that drying them in place is not realistic. These usually include:
- Carpet pad (the foam under the carpet)
- Soaked insulation
- Particleboard and MDF, such as some cabinet bases and baseboards, once they swell
- Drywall that has been wet for a long time or touched contaminated water
- Ceiling tiles that are stained or sagging
Removing these early lets air reach the structure behind them. Before you cut or throw anything away, take photos for your insurance claim and check with your adjuster.
Step 4: Dry the air and the structure
This is where most do-it-yourself cleanups fall short. Opening windows and running a household fan helps the surface, but water soaks deep into drywall, wood framing and concrete. The surface can feel dry while the inside is still wet enough to grow mould.
To dry a home properly you need to do two things at once: move air across wet surfaces and pull moisture out of the air. Professionals use commercial dehumidifiers and high-speed air movers sized to the room, and they close the space off so the equipment works efficiently. For small leaks, a good household dehumidifier plus fans can help, as long as you empty or drain the dehumidifier often.
Keep indoor humidity low while drying. Health Canada recommends keeping indoor humidity below 50 percent, and around 30 percent in winter to avoid condensation on windows.
Step 5: Measure, do not guess
You cannot tell whether a wall is dry by touching it. Restoration technicians use moisture meters and thermal cameras to find water hiding behind drywall, under flooring and inside wall cavities. They take readings every day and keep drying until materials reach normal levels for that type of material.
These readings also matter for insurance. A drying log shows your insurer that the home was dried to a proper standard, which helps if a mould problem is questioned later. Read more about how we support insurance claims.
Step 6: Clean and sanitize surfaces
Once the area is dry, clean hard surfaces that were wet with a household detergent and water, then dry them again. For water from a clean source such as a supply line, this is usually enough. For water that came from outside, a drain or a sewer, surfaces need proper cleaning and disinfection by a trained crew.
Avoid the old advice of pouring bleach over everything. Bleach does not soak into porous materials like wood and drywall, and mixing it with other cleaners can create dangerous fumes.
Warning signs that mould has already started
- A musty or earthy smell, especially in the basement or a closed room
- Black, green, grey or white spots on drywall, baseboards, wood or grout
- Paint that bubbles, peels or stains after a leak
- Warped baseboards or flooring that keeps feeling damp
- Condensation that keeps coming back on windows or pipes
- People in the home noticing more stuffiness, coughing or allergy symptoms indoors
Health Canada’s guidance treats small areas of mould, less than about one square metre, as something a homeowner can usually clean. Larger areas, mould that keeps coming back, or mould inside walls and ceilings should be handled by professionals. Our mould remediation team contains the area, removes the mould safely and fixes the moisture problem that caused it. If you are worried about the type of mould you see, read our guide on black mould health risks and safe removal.
Common mistakes that lead to mould
- Waiting a few days to see if the area dries on its own
- Putting carpet back down over a wet subfloor or pad
- Painting over water stains before the drywall is fully dry
- Leaving wet cardboard boxes in the basement
- Turning off the dehumidifier as soon as the surface feels dry
- Closing up walls without checking moisture inside them
How long does drying take?
With professional equipment, most homes dry in three to five days after the water source is stopped and standing water is removed. Hardwood floors, plaster walls, concrete and large losses can take longer. Every day that materials stay wet adds to the risk of mould, which is why the first 48 hours matter most. Our guide to your first 24 hours after water damage walks through what to do on day one, and if a frozen pipe is the cause, see burst pipes in winter.
Frequently asked questions
How soon after water damage does mould grow?
Mould can begin to grow within 24 to 48 hours on wet materials such as drywall, wood and carpet. You may not see it or smell it for a week or more, which is why fast drying matters even when nothing looks wrong.
Will a dehumidifier alone prevent mould?
A dehumidifier helps, but on its own it may not dry wet walls, subfloors or wall cavities quickly enough. Air movement, removing materials that cannot be dried, and moisture readings are usually needed as well.
Does insurance cover mould after water damage?
Many Ontario policies cover mould only when it results from a sudden and covered water loss, and some limit the amount. Mould from long-term leaks or humidity is often excluded. Ask your broker what your policy includes.
Can I stay in my home while it dries?
Usually, yes, when the water was clean and the wet area can be closed off. With sewage, visible mould or large affected areas, it may be better to stay elsewhere until cleaning is done.
Should I test for mould?
If you can see or smell mould, testing is usually not needed to decide that it should be removed. Testing can help when you suspect hidden mould, after remediation to confirm the area is clean, or when an insurer asks for it.
Worried about mould after a leak? Call now
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