White mould on wood is a fungus growing on damp timber. A small patch (under about 1 square metre) on a sealed or hard surface can usually be cleaned with soap and water once you have fixed the moisture behind it. Growth on porous or soaked wood, patches that keep returning, or anything larger than about 3 square metres calls for a professional. Below we explain how to tell mould from look-alikes, how to clean it, and when removal is the right call.
TL;DR
- White growth on wood is not always mould. Salt deposits (efflorescence) and old finish residue can look similar, so test before you treat.
- Health Canada advises cleaning small mould patches on wood with unscented soap and water, then drying fast. Bleach is not needed.
- Mould on wood means moisture is getting in. If you do not fix the source, it comes back, whatever you wipe on it.
- Soaked, porous or deeply stained wood should be removed and replaced, not scrubbed. Large or repeat growth belongs with a certified remediation crew.
Table of Contents
- Is it really white mould?
- Why white mould grows on wood
- Is white mould on wood dangerous?
- How to clean white mould off wood (small areas)
- When the wood needs to come out
- Find and fix the moisture source
- When to call a professional
- Perspective: what we see in Ottawa homes
- What professional mould remediation costs
- Stopping white mould from coming back
- Need a hand with mould on wood?
- FAQ
- Sources
Is it really white mould?
Before you reach for a spray bottle, confirm what you are looking at. Three things commonly get mistaken for each other on wood:
| What it is | What it looks like | Quick check |
|---|---|---|
| Mould | Fuzzy, cottony or powdery patches, often with a musty smell, sometimes spreading into spots of green, grey or black | Smells earthy, returns after wiping, grows where it is damp |
| Efflorescence | Crystalline or chalky white deposit, mostly on masonry and concrete, sometimes on wood touching them | Mould tends to stay affixed, salt can be distinguished; a dab of water often dissolves salt, mould smears |
| Finish bloom or dust | Hazy white film on varnished or oiled furniture | Wipes off cleanly and does not come back |
Mould itself comes in many colours. Health Canada notes that mould often looks like a stain and can show up in a variety of colours, so white is a description, not a diagnosis. Do not rely on colour to judge how serious it is. Treat any visible growth the same way: find the water, then deal with the material.
If you are unsure, look at where it is. White growth on a basement joist near the foundation wall, on rafters in the attic, or on the underside of a deck board points to moisture. A single hazy patch on a polished table after a humid week is a different story.
Why white mould grows on wood
Mould needs moisture, a food source and time. Wood supplies the food. Water supplies the rest. According to Ottawa Public Health, moulds usually grow in damp or humid places such as bathrooms, and also in porous insulation and condensation or drip pans.
In Ottawa homes, the usual culprits are:
- Basement and crawl space dampness. Foundation seepage, high humidity in summer and unsealed dirt floors keep joists and sill plates damp.
- Attic ventilation problems. Warm indoor air that leaks into a cold attic condenses on roof sheathing during our winters. White or grey growth on the underside of the sheathing is common in older houses.
- Roof, plumbing or window leaks. A slow leak can feed growth inside a wall for months before you see it.
- Flooding. Wood that stays wet after a spill or flood is at risk quickly. Practitioners stress that if wet materials are dried within 48 hours, growth is unlikely to begin. Past that, the odds shift.
- Stored wood and furniture. Firewood, lumber and furniture kept in damp garages or sheds.
The point worth repeating: mould is a symptom. The visible patch is often the only part of a moisture problem you can see.
Is white mould on wood dangerous?
Treat it as a health concern, not just a cosmetic one. Health Canada has concluded that indoor mould growth may pose a health hazard. There is no health-based exposure limit for indoor mould, so no amount is considered “safe” to leave in place.
Common symptoms are eye, nose and throat irritation, coughing and wheezing. People with asthma or other respiratory conditions, infants, seniors and people with weakened immune systems are at higher risk. If someone in your home is having symptoms you suspect are linked to mould, see your doctor or another healthcare professional. Do not wait for the cleanup to be finished.
White mould is no safer than any other colour. Do not make decisions based on whether it looks “mild”.
How to clean white mould off wood (small areas)
This applies to a small patch on solid, sealed or hard wood: Health Canada describes small as 1 to 3 patches, each under about 1 square metre, or roughly 10 square feet. If you are bigger than that, skip to the next section.
- Fix the moisture first. Stop the leak, run a dehumidifier, improve ventilation. Cleaning without this step is wasted effort.
- Protect yourself. Wear an N95-type respirator, gloves and eye protection. If anyone in the house has asthma or a weakened immune system, have someone else do it or hire it out.
- Contain the area. Close doors, shut off forced-air registers in that room and avoid dry brushing, which throws spores into the air.
- Clean with unscented soap and water. Health Canada says to use a simple soap and water solution and that bleach is not necessary. Scrub gently with a soft brush, and wipe with a damp cloth.
- Dry quickly and completely. Use fans and a dehumidifier. Wood that stays damp grows mould again.
- Inspect afterwards. If staining or fuzz remains, or the wood feels soft, it is past surface cleaning.
What about vinegar and bleach?
Many top search results suggest vinegar, borax or diluted bleach. Health Canada’s guidance for homes is simpler: soap and water, and bleach not required. Bleach is also mostly water, which soaks into porous wood and can feed the problem. Ottawa Public Health does publish bleach-and-water ratios for hard surfaces after floods, so if you are cleaning a non-porous surface in a flood-affected home, follow their flood guidance. Never mix bleach with other cleaners, ammonia in particular.
Why wiping is not enough
Cleaning or painting over mould on wood does not solve it. If the moisture source remains, growth returns through the new finish or deeper into the wood. And if mould is under paint or deep in porous wood, Health Canada says the material needs to be removed and replaced instead of cleaned.
When the wood needs to come out
Some wood cannot be cleaned to a standard that prevents regrowth. Plan on removal when:
- The wood was soaked for days or flooded.
- It feels soft, crumbly or spongy, which suggests rot as well as mould.
- Growth sits under paint, stain or sealant.
- The patch keeps returning after you have cleaned it and dried the area.
- It is unfinished, untreated framing with widespread surface growth.
Ottawa Public Health guidance emphasizes that porous materials like drywall, paper and untreated wood exposed to sustained dampness or flooding should be treated as waste. Saving them often leads to hidden, persistent mould that costs more later.
Removing construction material also has a waste angle in Ottawa. The City says wood from renovation or demolition is not collected with regular garbage, so you need a plan to haul it away. Mouldy material should be bagged before it leaves the work area to avoid spreading spores through the house.
Find and fix the moisture source
This is the step that decides whether you see white mould again next spring. Work through the likely sources in order:
- Check the outside. Gutters, downspouts that dump next to the foundation, and grading that slopes toward the house.
- Check the roof and attic. Look for stains on sheathing, blocked soffit vents, bathroom fans that vent into the attic rather than outdoors.
- Check plumbing. Under sinks, behind toilets, around the water heater and washing machine.
- Check the foundation. Damp patches on basement walls, efflorescence on concrete and a musty smell can all point to water getting through.
- Measure humidity. An inexpensive hygrometer tells you whether your basement or bathroom is sitting at damp levels for weeks at a time.
If water is coming through the foundation or basement, a durable fix usually means exterior or interior waterproofing rather than a dehumidifier alone. For a plain-language look at the link between sealing out water and keeping growth away, see how waterproofing prevents mould.
Health Canada and Ottawa Public Health also advise against routine air testing for mould. It does not identify the cause of growth or predict health effects, so money is better spent on the moisture inspection. Specialized sampling is rare, and used when hidden mould is suspected and an exhaustive inspection does not explain ongoing problems.
When to call a professional
Call in a certified remediation company when any of these apply:
- The affected area is larger than about 1 square metre across multiple patches, or over 3 square metres in total.
- Mould keeps coming back after cleaning.
- It followed a flood, sewage backup or long-term leak.
- It is in a crawl space, the attic, or inside walls or floors.
- Someone in the home has asthma, is immunocompromised, or is an infant or senior.
- You manage a rental or commercial building and need documentation.
On that last point, in Ontario the Occupational Health and Safety Act requires employers to take every precaution reasonable in the circumstances to protect workers, and the province publishes guidance on mould in workplace buildings. Property managers should keep records of the moisture inspection and each remediation step.
Professional work is different from a big DIY clean. A crew isolates the work area, uses negative air and removes affected materials in a controlled way. Trying a large job in an occupied house can push contamination into clean rooms through the HVAC system or on foot traffic.
Perspective: what we see in Ottawa homes
We work in older Ottawa-area houses, and white growth on wood is rarely the whole story. The homeowner calls about a few white patches on a joist or on attic sheathing. When we open the area up, we often find that the wood is damp deeper in, or that insulation next to it has been wet for a while.
Three things we tell people most often:
- Start with the cause. We would rather you spend an hour tracing a leak than buy another spray. A clean-looking joist above a seeping foundation wall will be white again within a season.
- Be honest about the size. A small, contained patch on solid wood is a reasonable weekend task with soap, water and good drying. A whole basement or attic with widespread growth is not.
- Ask for the paperwork. A proper job comes with an itemized quote and documentation afterwards. Our estimates are line by line, so you can see what each part of the work costs, and we can usually get an estimate to you within 24 hours.
We hold abatement licensing and have more than 25 years of local experience, and we also handle demolition and renovation under the same roof. That matters here: the same crew that removes mouldy framing can replace it, so you are not coordinating three contractors. Our Google rating is a perfect 5.0, and we are proud of it, but the thing we care about more is that the mould does not come back.
— Jason
What professional mould remediation costs
Every home is different, and a written estimate is the only reliable number. For orientation, our published ranges on the mould remediation page are:
| Job | Published range |
|---|---|
| Small contained area | $500 to $1,500 CAD |
| Single room or bathroom | $2,000 to $6,000 CAD |
| Basement or flood-related | Quoted after inspection |
If you also need hazardous material testing, lab analysis on our abatement page is listed at $300 to $600 CAD. Older Ottawa houses can have asbestos-containing material near damaged wood, such as vermiculite in the attic, so it is worth knowing before anything is torn out.
If you want to understand why larger jobs are handled differently, our guide on remediation versus removal explains the over-1-square-metre threshold. For the step after cleanup, see our note on the remediation certificate and holdback. Specific spaces have their own pages: basement mould remediation and crawl space mould removal.
Stopping white mould from coming back
Once the wood is clean, dry and, where needed, replaced, a few habits keep it that way:
- Keep indoor humidity in check, using a dehumidifier in the basement through summer.
- Run bathroom fans during and after showers, venting outdoors.
- Fix leaks the week you notice them, and dry anything wet within 48 hours.
- Keep firewood, lumber and boxes off damp floors and away from foundation walls.
- When replacing damaged wood, consider mould-resistant materials in damp areas.
Need a hand with mould on wood?
If your patch is large, keeps coming back or sits somewhere you cannot see, we offer free estimates for Ottawa-area homes and buildings. See our mould remediation service to request one. If it turns out the wood has to be replaced, we can handle that as well, from framing repair to a finished renovation.
FAQ
Is white mould on wood toxic?
There is no safe level of indoor mould according to Health Canada, and white mould is no exception. Symptoms include irritated eyes, nose and throat, coughing and wheezing, and people with asthma, infants, seniors and those who are immunocompromised are at higher risk. If you have symptoms, speak to a healthcare professional.
How do you kill white mould on wood?
For a small patch on solid wood, Health Canada recommends cleaning with unscented soap and water and drying the wood thoroughly, with no need for bleach. Fix the moisture source at the same time, or it will return. If the mould is under paint or deep in the grain, the wood should be replaced.
Can you just wipe off white mould?
Wiping alone is not enough. Dry wiping can spread spores, and without a proper wash and fast drying, growth tends to return. Wash gently with soap and water, dry completely and watch the spot for several weeks.
Is white fungus on wood always mould?
Not always. White crystals can be efflorescence, a mineral salt deposit, and hazy films on furniture can be finish residue. If it is fuzzy, smells musty and keeps coming back, treat it as mould.
Sources
- Mould, Canada.ca (Health Canada)
- Guide to addressing moisture and mould indoors, Canada.ca
- Guidance for indoor air quality professionals, Canada.ca
- Special items, City of Ottawa
- Alert: mould in workplace buildings, Ontario.ca
- How to Remove White Mold From Wood, MasterClass