Zusashi Maintenance does not do mould remediation. We clean commercial buildings, and we wrote this because we get asked about mould constantly by clients whose buildings we are already in. If you need the work done we can point you at a contractor who does it — see commercial mould remediation — but this page is here to be useful whether or not you ever call us.
Mould is a water problem wearing a biology costume
This is the sentence to carry through the rest of the guide. Mould spores are already in every building, all the time, harmlessly. What turns them into growth is persistent moisture. Remove the moisture and the growth cannot re-establish; remove the growth and leave the moisture, and it comes back in the same place, usually within months.
That has a direct commercial consequence. A quote that describes removing growth in detail and says nothing about where the water is coming from is a quote for a job you will be buying again. When you read a proposal, look for the moisture investigation before you look at the price.
Is this a cleaning job or a remediation job?
This is the question every manager is really asking, and there is a reasonably clear answer. The Canadian Centre for Occupational Health and Safety publishes size thresholds that the trade works to:
| Extent of visible growth | Category | What it usually means |
|---|---|---|
| One to three patches, each under 1 m² (about 10 sq ft) | Small isolated area | Surface cleaning with proper precautions — CCOHS puts the minimum at an N-95 respirator plus gloves and eye protection |
| More than three patches, or patches of 1–3 m² (10–32 sq ft) | Medium area | The same work with stepped-up controls, and the point at which most operators want somebody who does this for a living |
| Over 3 m² (32 sq ft), or heavy contamination | Large area | CCOHS is explicit that large remediation projects should be done by trained professionals |
Size is not the only test, and this is where managers get caught out. Regardless of the area you can see, treat it as a remediation job if any of the following is true:
- It is inside a wall cavity, above a ceiling, or in insulation. The visible patch tells you nothing about the extent behind it, and you cannot scope what you cannot see.
- It is anywhere in the HVAC system. Ductwork distributes; a contained problem becomes a building-wide one.
- The material has been wet for more than a couple of days, or was wet, dried, and got wet again.
- People in the space have started reporting symptoms.
- It is growing back after somebody already cleaned it.
One instruction from the same source gets ignored more than any other: the HVAC system should be off during cleaning. Running it while a contaminated area is disturbed moves spores into parts of the building that had nothing wrong with them.
Where it actually turns up in a commercial building
The commercial version of this problem is quite predictable. In rough order of how often it lands on a manager's desk:
- Ceiling tiles with a brown halo, usually beneath a roof penetration, a rooftop unit, or a floor with plumbing above. The stain is always older than the day somebody reported it.
- Perimeter walls and window returns in offices, where warm interior air meets a cold surface all winter. No leak at all — just condensation doing the same job more slowly.
- Warehouse and cold storage envelopes, where a temperature difference across a wall, a dock door or a chiller partition produces condensation on a reliable schedule. See warehouse and industrial cleaning for the wider envelope picture.
- Behind millwork, under sinks, inside service risers — the slow weep nobody sees for a year because nothing is visible from the room.
- Condominium common elements, where a unit owner reports a smell and the cause sits in a shaft, a corridor wall or the envelope. See common elements cleaning.
- After a leak or a flood, where the space was dried quickly but not thoroughly, or was closed up while still damp. This is the most preventable one on the list, and it is why the first 48 hours after water damage matter so much.
- Under carpet on a slab, particularly below grade or where a carpet has been over-wetted during cleaning and not properly dried. See commercial carpet cleaning.
Finding the water
Before you commission anything, work out which of these you are dealing with, because the fix is completely different in each case:
| Source | How you tell |
|---|---|
| Roof or envelope leak | Appears or worsens after rain or a thaw. Usually a defined stain with a tide line. |
| Plumbing | Independent of weather, steady, and often directly below or beside a service run. |
| Condensation | Seasonal, on cold surfaces, often diffuse rather than a defined patch. Worst in winter on perimeter walls, and in summer on chilled surfaces. |
| Humidity with nowhere to go | Whole-room rather than local. Look for a process producing moisture and an extract system that is off, undersized or blocked. |
| Ground water or drainage | Low on walls, below grade, worse in spring. Check what the exterior grading and downspouts do. |
| A past event nobody closed out | No current source at all. Something got wet once and was never properly dried. |
There is no mould licence in Ontario — so check something real
This catches people out, so it is worth stating plainly. Ontario regulates asbestos work in detail: the work is classified Type 1, 2 or 3 according to the hazard, training is mandatory, and there are notification and procedural requirements before certain projects start. There is no equivalent licensing regime for mould. CCOHS talks about trained professionals, not licensed ones.
So a contractor advertising a mould remediation licence is describing something the province does not issue. What you can actually verify:
- Certification — ask what they hold, and ask to see the certificate rather than the logo on the van.
- A current liability certificate and WSIB clearance — the same two documents you should ask any contractor for, us included.
- References from jobs like yours. A residential specialist and a contractor who works occupied commercial buildings around a tenant's operating hours are not the same business.
- A written scope covering containment, materials, the water source, and what you receive at the end.
The asbestos overlap, which is the real hazard
Remediation means opening things up. In a building of any age, drywall, ceiling tiles, insulation, flooring and their adhesives can contain asbestos — and that is regulated, in a way mould is not. A competent contractor raises it before demolition begins and factors it into the sequence. If it never comes up in the conversation, raise it yourself; it is a reasonable question and the answer tells you a lot about who you are dealing with.
Who is responsible?
In a commercial tenancy this is decided by the lease, not by a general rule. Commercial leases are contracts between businesses and they allocate repair and maintenance obligations differently from one another, so read the repair covenant, the structure and roof clauses, and the HVAC responsibility before you assume anything or write to anybody.
Two things tend to hold regardless. The party who controls the building envelope usually controls the water source, which is why an envelope-driven problem rarely stays a tenant's problem for long. And an employer has a duty to take reasonable precautions for the health and safety of workers in the space they occupy — that duty does not pause because a landlord is slow to respond. In a condominium, the corporation carries the repair obligation for common elements, so a symptom appearing inside a unit can still have a corporation-level cause.
None of that is a substitute for reading your own lease or declaration, and where money or liability is genuinely in dispute it is a question for your lawyer rather than your cleaner.
What to document, and why the dates matter most
Here is the part managers underrate. In practice the thing that goes wrong with a mould complaint is almost never the mould. It is the gap between the day it was reported and the day somebody attended. That gap is what gets quoted back at you later, and it is invisible unless somebody wrote both dates down at the time.
A workable record has eight columns and fits on one page:
- Date reported — and who reported it.
- Location — unit, floor, room, wall. Specific enough that the next person finds it.
- What was observed — staining, visible growth, a smell, damp to the touch. Facts, not conclusions.
- Approximate size — which is what puts it in one of the three CCOHS categories above.
- Suspected moisture source — even a guess, because it forces the question to be asked.
- Action taken — cleaned, monitored, contractor attended, referred.
- Date closed — the other half of the response-time pair.
- Initials.
Read across a year, that sheet does something no individual entry can: it shows you the repeat locations. The same spot appearing three times is not a cleaning failure, it is a building defect that nobody has costed yet.
We publish a free printable mould and moisture log in exactly this format. It is free, there is no account, and you can print it or save it as a PDF.
What cleaning genuinely contributes
Cleaning does not solve a mould problem. It does two useful things around one:
- It stops most of them starting. Extract grilles that actually move air, drains and condensate trays that are not blocked, hard floors dried rather than left wet, carpets extracted and properly dried rather than over-wetted, and spills dealt with the same day. Unglamorous, and far cheaper than remediation.
- It puts the building back afterwards. Once a remediation contractor has finished and released the space, there is fine settled dust across every surface and ledge, and floors, carpets, washrooms and corridors to bring back into service. That is ordinary commercial cleaning. It is the same arrangement we already have on water damage on our property management cleaning work: a restoration contractor dries and clears the space, and we handle the clean that follows.
And one thing worth saying out loud, because it saves people money: sometimes the answer is that it is just a cleaning job. A small isolated patch with an obvious, fixed source is not a remediation project, and being told so is a legitimate outcome of the call.
Frequently asked questions
Is mould in a commercial building a cleaning job or a remediation job?
Size is the usual first test, and the Canadian Centre for Occupational Health and Safety publishes the thresholds the trade works to. One to three patches, each under one square metre (about 10 sq ft), is a small isolated area. More than three patches, or patches between one and three square metres (10 to 32 sq ft), is a medium job with stepped-up controls. Over three square metres (32 sq ft), or heavy contamination, should be handled by trained professionals. Size is not the only test though: mould inside a wall cavity, above a ceiling, in insulation or anywhere in the HVAC system is a remediation job regardless of the visible area, because the patch you can see tells you nothing about the extent behind it.
Why does mould keep coming back in the same place?
Because mould is a symptom and the water is the problem. Mould needs persistent moisture to grow, so if the source has not been found and fixed it returns to the same spot on the same schedule. This is the single most common reason a building has a mould problem year after year: somebody has been cleaning it repeatedly and nobody has fixed the water. A contractor who quotes to remove growth without asking where the moisture is coming from is quoting you for a repeat visit.
Do mould remediation contractors need a licence in Ontario?
No, and this surprises most people. Ontario regulates asbestos work in real detail — the work is classified Type 1, 2 or 3 by hazard, training is mandatory, and there are notification requirements before certain projects begin. There is no equivalent licensing regime for mould, and CCOHS refers to trained professionals rather than licensed ones. So what you are actually verifying is certification and insurance: relevant remediation certification, a current liability policy, WSIB clearance, and references from jobs like yours. Anyone advertising a mould remediation licence is describing something the province does not issue.
Who is responsible for mould in a leased commercial unit?
In a commercial tenancy it is decided by the lease, not by a general rule — commercial leases are contracts between businesses and they allocate repair and maintenance obligations differently from one another. Read the repair covenant, the structure and roof clauses, and the HVAC responsibility before assuming. Two things are usually true regardless: the party who controls the building envelope generally controls the water source, and an employer has a duty to take reasonable precautions for the health and safety of workers in the space, which does not disappear because a landlord is slow. In a condominium the corporation carries the repair duty for common elements, so a unit-level symptom can still be a corporation-level cause.
What should be in a mould remediation quote?
How the water source will be found and fixed, not just how the growth will be removed. How the work area will be contained and what happens to the rest of the building while it runs. What materials are being removed versus cleaned. Whether asbestos has been considered if the building has any age to it. What you receive at the end — a written record of what was removed and treated, and whether any post-remediation verification is included. And all of it in writing before anyone starts: a verbal scope on a mould job is how disputes begin.
Could there be asbestos as well?
In an older building, yes, and it changes the job. Remediation means opening things up — drywall, ceiling tiles, insulation, flooring — and materials of a certain age can contain asbestos. Unlike mould, asbestos is regulated in Ontario: work is classified by hazard as Type 1, 2 or 3, training is mandatory, and there are notification and procedural requirements. A competent remediation contractor raises this before demolition starts. If they do not raise it, ask.
Should the HVAC be running during cleaning?
No. CCOHS is explicit that the HVAC system should be turned off during cleaning, and it is the instruction most often ignored. Running it while a contaminated area is disturbed turns a contained problem into a distributed one, moving spores into ducts and other parts of the building that had nothing wrong with them. It is also why mould found inside the HVAC system itself is treated as a remediation job whatever its apparent size.
What should a property manager document?
The date it was reported, where, what was actually seen, roughly how large an area, what the suspected water source is, what was done and when, and the date it was closed out. The date reported and the date actioned are the two that matter most, because in practice the thing that goes wrong is not the mould — it is the weeks between a tenant reporting it and anyone attending. A dated sheet also exposes the pattern nobody notices in the moment: the same location appearing three times in a year is a building defect, not a cleaning failure.
This article is general information for people running commercial buildings in Ontario. It is not legal advice, not an assessment of any specific building, and not a substitute for a qualified contractor's inspection. Zusashi Maintenance does not perform, assess or test for mould remediation. Where a duty or a cost is genuinely in dispute, take advice on your own lease or declaration. The size thresholds and work practices described here are drawn from the Canadian Centre for Occupational Health and Safety's published guidance on mould in the workplace; the asbestos requirements are Ontario Ministry of Labour material. Verify both against the sources before relying on them.