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.

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.

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 →

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If you're renovating a basement or bathroom and worry about mould, our team handles assessment and remediation. Book a free 60-minute consultation.