Discovering mould during demolition is common — water damage in walls, behind tubs, in basements, on the underside of subfloors. The right response depends on how much mould, what surface, and what caused it. Small visible mould (under 10 sq ft) on hard non-porous surfaces is DIYable. Anything more, on porous materials, or with unknown source needs professional remediation.
This guide explains how to assess mould safely, when to clean it yourself, and when to stop and call a remediation company.
- Skill level
- Safety
- Time
- Assess · contain · remediate
- Steps
- 9
- Tools
- 5, listed below
- Materials
- 7, listed below
Before you start
Three things to settle:
1. Find and fix the moisture source first. Mould only grows where moisture is. Cleaning mould without fixing the leak/condensation guarantees it comes back. Find the source: roof leak, plumbing drip, condensation point, basement humidity, washing machine line. Fix that first; mould remediation second.
2. 10 sq ft is the rough DIY threshold. Health Canada and EPA both flag ~10 sq ft (about 1 sq m) as the rough boundary between DIY and professional work. Below that, on hard surfaces, with proper PPE — DIY. Above that, on drywall/insulation/wood — call a remediation contractor.
3. Allergies, asthma, or compromised immunity = always pro. Even small mould remediation releases spores. If anyone in the household has asthma, allergies, or compromised immunity, hire pros to handle it. The cost is worth not triggering a respiratory event.
What you'll need
Tools
- Spray bottle (for water/cleaner)
- Stiff bristle brush
- HEPA vacuum
- Bucket
- Disposable cloths and rags
Materials
- P100 respirator (or N95 minimum)
- Disposable gloves (nitrile)
- Safety goggles
- Disposable Tyvek coveralls (for larger jobs)
- Detergent/water solution OR diluted bleach (10:1 water:bleach) — but see notes
- Plastic sheeting (for containment)
- Heavy-duty contractor bags
Bleach is not always the right answer
Bleach kills surface mould on non-porous surfaces (tile, glass, hard plastic) but doesn't reach mould roots in porous materials (drywall, wood, fabric). On porous materials, bleach kills the visible spores but the roots regrow — and the chlorine contributes to corrosion.
For porous materials: remove and replace, don't try to clean. For hard non-porous: detergent + water OR a EPA-registered fungicide is often more effective than bleach.
Step-by-step instructions
Find and fix the moisture source
Look for: leaking pipe, roof leak, condensation, high basement humidity, exterior drainage issue, plumbing drip.
Fix the source before remediating. Don't skip this step.
If you can't find the source: hire a moisture inspection before remediating.
Assess the size and surface
Measure visible mould area. Under 10 sq ft on hard surface (tile, glass, sealed wood): DIY OK.
Over 10 sq ft, on drywall, insulation, framing, carpet, or upholstery: stop. Call a remediation contractor.
Hidden mould (smell of mould but no visible growth): inspection first; mould testing if needed.
Set up containment for DIY work
Even small mould remediation releases spores. Tape plastic over doorways into adjacent rooms.
Cover or seal HVAC vents in the work area.
Open a window for fresh air; don't run HVAC during remediation.
Wear PPE
P100 cartridge respirator (or N95 minimum for very small jobs).
Disposable gloves (replace if torn).
Safety goggles (sealed, not just glasses).
Disposable coveralls if doing more than a small wipe.
Remove porous materials, don't clean them
Mouldy drywall, insulation, carpet pad, ceiling tile: cut out and remove. The mould roots are inside the material, not just on the surface.
Cut out at least 6 inches into clean material around the visible mould (mould often extends past the visible boundary).
Bag mouldy materials in heavy-duty contractor bags, double-bagged. Tie shut inside the work area before carrying out.
Clean hard non-porous surfaces
Tile, glass, hard plastic, sealed wood: scrub with detergent + water using a stiff brush. Rinse with clean water.
Stubborn stains: EPA-registered fungicide (Concrobium, RMR-86 for severe) per label.
Bleach (10:1) acceptable on non-porous but not always best — detergent often works.
Dry the area thoroughly
Mould regrows on damp surfaces. After cleaning, run a fan or dehumidifier until surfaces are bone-dry.
Target relative humidity below 50% in the work area.
Wait 24-48 hours before closing up the wall.
HEPA vacuum and final clean
HEPA-vacuum the work area + surrounding 6 ft.
Wipe surfaces with a damp cloth (catches settled spores).
Discard PPE in the same waste bag as the mouldy materials.
Run HEPA air scrubber for 24 hours after work to clear airborne spores.
Replace removed materials
Replace cut-out drywall, insulation, etc. with fresh materials.
Confirm the moisture source is fixed before closing up the wall.
Consider mould-resistant drywall (Purple Board) in moisture-prone areas.
Common mistakes that cost you
- Cleaning mould without fixing the moisture source. Mould comes back within weeks. Fix the leak/humidity first.
- Bleaching mouldy drywall. Roots stay alive in porous material. Remove and replace mouldy drywall — don't try to clean it.
- Skipping PPE on small jobs. Even a 1 sq ft mould patch releases spores when disturbed. Always P100 + gloves + goggles minimum.
- DIY on large or hidden mould. Hidden mould (behind walls, in HVAC) needs professional assessment. Don't poke holes 'to check.'
- Mixing bleach with ammonia. Produces toxic chloramine gas. Many cleaners contain ammonia — never combine with bleach.
Frequently asked questions
How do I know if mould is dangerous (toxic)?
All mould can cause respiratory issues; 'toxic black mould' (Stachybotrys) is one of many that produce mycotoxins.
Visual identification of species is unreliable — laboratory testing required.
For health concerns, treat all mould the same: contain, remediate or remove, and wear PPE.
Should I get mould tested?
Generally no, unless: insurance requires it, you're selling/buying the home, or you have unexplained health symptoms.
Tests confirm presence and species but don't change the remediation: remove or clean, fix moisture source.
Lab testing of a sample is an added cost on top of the inspection.
Will my home insurance cover mould remediation?
Depends on the cause. Sudden water events (burst pipe, recent flood) usually covered; gradual leaks and chronic humidity usually not.
Read your policy. Many policies cap mould coverage with a fixed limit even when a covered event caused it.
Document everything with photos.
How much mould can a homeowner clean up themselves?
About 10 sq ft, roughly 1 sq m, is the rough boundary Health Canada and the EPA flag between DIY and professional mould work. Under that size, on hard non-porous surfaces such as tile, glass or sealed wood, and with proper PPE, a homeowner can clean it.
Mould over 10 sq ft, mould on drywall, insulation, framing or carpet, and hidden mould call for a remediation contractor. So does anyone in the home with asthma, allergies or compromised immunity.
Does bleach kill mould on drywall?
No. Bleach kills surface mould on non-porous surfaces like tile, glass and hard plastic, but it does not reach the roots in porous materials such as drywall, wood and fabric, so the mould regrows. Mouldy drywall should be cut out at least 6 inches into clean material and replaced.
On hard surfaces, detergent and water or an EPA-registered fungicide often works better than bleach. Never mix bleach with ammonia, which produces toxic chloramine gas.
When to call us instead
Hidden water damage and mould are common in basement and bathroom renovations. Our team coordinates moisture remediation as part of the renovation scope so it's solved properly the first time.
Book a free 60-minute site visit →Dream Touch Renovations · 33 Roydon Pl, Suite 210, Ottawa, ON K2E 1A3 · (613) 862-4555
Renovating homes across Ottawa, Kanata, Nepean, Barrhaven, Orléans and Stittsville.
Related Ottawa services
If you're renovating a basement or bathroom and worry about mould, our team handles assessment and remediation. Book a free 60-minute consultation.
Official references
- Asbestos in the home, Health Canada
- Hand Tools: General Hand Tool Operation, Canadian Centre for Occupational Health and Safety