The Mould Strategy I Trust: Find the Moisture First, Then Decide How Big the Job Really Is

Mould is one of those building issues that can turn emotional fast.

I understand why. Nobody wants to see dark staining on a wall, a bathroom ceiling, a window frame, or the back of a closet. A resident sees it and thinks, “How bad is this?” Management sees it and thinks about repairs, access, liability, and timelines. Maintenance sees it and thinks about the real question underneath everything else: where is the moisture coming from?

That is the part I trust most. I do not start with panic, and I do not start by guessing what type of mould it is. I start with moisture.

If the water problem is still there, the mould problem is not really fixed. It is only hidden for a while.

Quick source note

Health Canada says indoor mould growth may pose a health hazard and recommends controlling dampness indoors and cleaning up visible mould regardless of the type of mould present. It also says mould will usually come back unless the moisture source is identified and solved.

Source: Health Canada, Guide to addressing moisture and mould indoors

The City of Toronto says small areas of mould may be cleaned using detergent, water, quick drying, and protective equipment, but larger or extensive areas should be addressed by a professional trained in mould remediation. Toronto also says the underlying cause of water accumulation or prolonged high humidity must be corrected or mould will continue to grow or come back.

Source: City of Toronto, Mould

CMHC gives the same practical message: mould needs moisture. It points to musty smells, wet spots, dampness, and leak signs as things worth investigating, and says medium and large areas should be handled by professionally trained people.

Source: CMHC, Mould in Housing

This post is not medical advice, legal advice, or a professional remediation manual. It is how I think through a mould report from the building side.

First thing I ask: what changed?

When somebody reports mould, I want to know what changed before the stain showed up.

Sometimes the answer is obvious. A pipe leaked. A tub overflowed. A window has been sweating every morning. A bathroom fan stopped working. A closet is packed tight against an exterior wall. A ceiling stain appeared after heavy rain.

Other times, the answer is not obvious at all.

That is why I do not like treating mould as only a surface problem. The surface is the symptom. The cause might be behind the wall, above the ceiling, under a sink, around a window, in a fan, or in the way air is moving through the unit.

My first check is usually simple:

• Where is the mould or staining?
• Is the surface wet now?
• Is there a leak nearby?
• Is there condensation?
• Is there a musty smell?
• Is it one small patch or several areas?
• Is the same spot coming back after cleaning?
• Is there any recent flood, overflow, roof leak, plumbing leak, or ventilation problem?

Those questions matter because a bathroom ceiling mark and a wet wall under a riser are not the same kind of job.

Why I do not start with bleach

I know bleach is the thing people talk about first. I do not start there.

Health Canada says not to use bleach to clean up mould. The safer public guidance is to use proper precautions and clean appropriate small areas with an unscented soap or detergent solution, then dry the area quickly.

The bigger issue is that bleach does not answer the building question. Why did the area get damp enough for mould to grow? If nobody fixes that, the stain may return, and everyone ends up frustrated.

That is why my strategy is not “spray it and walk away.” It is:

1. Identify the moisture clue.
2. Decide how big the affected area is.
3. Decide whether it is safe and appropriate for basic cleaning.
4. Escalate when the size, location, material, or recurrence says it is bigger than a simple cleanup.
5. Document the closeout so there is a record.

That order keeps the job from becoming a guessing game.

Small, medium, and large are not the same job

Health Canada sorts mould by size for cleanup decisions.

A small area is three or fewer patches with a total area less than one square metre. Medium is more than three patches, or patches greater than one square metre but less than three square metres. A single patch larger than three square metres needs professional assessment and cleanup.

I like this because it gives the conversation a practical frame.

A little window sill mould from condensation is not the same as a large wall area after a leak. A few spots on caulking are not the same as staining that keeps returning through drywall. A small surface cleanup is not the same as hidden moisture in a wall cavity.

When I see a larger area, repeated mould, soft material, water damaged drywall, sewage related moisture, or signs the problem is hidden, I stop thinking about it as a quick clean. That is when the job needs management, a contractor, or a qualified remediation person.

The moisture source is the real repair

In a building, the moisture source is the real repair.

That might mean:

• fixing a plumbing leak
• repairing failed caulking
• checking a bathroom fan
• dealing with condensation around windows
• improving air movement
• drying wet materials quickly
• investigating roof, wall, or balcony water entry
• removing water damaged material when it cannot be cleaned properly
• checking whether storage or furniture is blocking airflow

Sometimes the solution is simple. Sometimes it is not. But I would rather be honest about that than keep cleaning the same patch every few weeks.

Health Canada says moisture problems are preventable, but the cause has to be identified and solved or mould will reappear. That is the line I keep coming back to.

What I document on my side

For mould calls, documentation matters.

I want a record that shows what was reported, what was seen, and what happened next. That protects the resident, the building, and the staff doing the work.

My notes usually need:

• date and time reported
• unit or area
• exact location in the room
• size of visible staining or mould
• photos before work starts
• moisture clues, such as leak, condensation, odour, damp material, or soft drywall
• whether the problem seems small, medium, large, recurring, or hidden
• temporary steps taken, if any
• repair assigned
• contractor or management escalation
• access attempts and notices if needed
• completion date
• follow up check
• photos after work is complete

The Landlord and Tenant Board says landlords must keep rental property in a good state of repair and meet health, safety, housing, and maintenance standards. It also tells tenants to ask for repairs in writing and keep a copy.

Source: LTB, Maintenance and Repairs

That is why written records are not just paperwork. They are part of the repair.

What residents can do early

Residents can help by reporting early.

A small stain, a musty smell, a wet baseboard, or a window that keeps collecting water is easier to deal with early than after weeks of moisture. I would rather get a small report too soon than a big report too late.

Good reports include:

• where the issue is
• when it started
• whether it is spreading
• whether the area feels damp
• whether there was a leak or overflow
• a clear photo
• whether the fan, window, or fixture seems connected

I also tell people not to paint over mouldy surfaces. Painting hides the sign, but it does not fix the moisture problem.

When I stop and escalate

There are times when I stop treating it as a basic maintenance cleanup.

I escalate when:

• the area is large
• mould keeps coming back
• there are many patches
• drywall, carpet, insulation, or other porous material is involved
• the source may be hidden
• the area smells strongly musty but the source is not visible
• there was flooding, sewage, or long standing wet material
• the resident reports health concerns
• staff do not have the right protective equipment or training
• the job needs opening walls, removing material, or contractor assessment

That is not overreacting. That is knowing where a simple response ends.

For health concerns, I do not diagnose. I document the building condition and tell the resident to speak with a health care professional about symptoms. Health Canada gives that same cautious direction.

How I think about closing the job

I do not like closing a mould job just because the stain looks cleaner.

Before I consider it handled, I want to know:

• Was the moisture source fixed?
• Was the affected area cleaned or repaired properly?
• Was damaged material removed if it could not be cleaned?
• Did the area dry quickly and completely?
• Was ventilation or humidity part of the problem?
• Is follow up needed?
• Is there a photo record?
• Does the resident know what to watch for?

A clean surface is good. A dry, repaired cause is better.

The bottom line

The mould strategy I trust is simple, but it takes discipline.

Find the moisture first. Size the problem honestly. Clean only what is appropriate to clean. Escalate when the job is bigger than a small surface issue. Keep records. Follow up.

That approach is calmer for the resident and better for the building.

It is also the same lesson I keep seeing in maintenance work: a small sign is often the building asking you to look closer before it becomes a bigger repair.