ImmoMulti — a direct buyer of income properties on the North Shore — regularly works with plex owners facing a mold problem. This issue is not just about health: it directly touches your legal duty to provide a dwelling in good condition, your tenants' remedies before the Tribunal administratif du logement (TAL), and your disclosure obligation at sale, on pain of a hidden-defect claim. According to the Government of Québec, mold proliferates as soon as humidity is high — which is why controlling infiltration and ventilation matters before the problem becomes costly or blocks a sale.
Must a plex landlord provide a mold-free dwelling?
Yes. The landlord has the obligation to provide a dwelling in good condition and fit for habitation. According to the City of Montréal, the landlord must keep all building components in good condition — roof, walls, windows, floors, balconies, stairs — and visible mold is prohibited under health and safety by-laws. To prevent mold, surfaces must remain dry.
The starting point of any mold problem in a plex is legal: the tenant is entitled to a dwelling in good condition, and it is the landlord's job to provide it and keep it that way. The City of Montréal states this clearly: the landlord is "responsible for maintaining all building components in good condition: roofing, walls, windows, floors, balconies, stairs, etc." Yet these are precisely the components — a leaking roof, a poorly insulated wall, a condensing window — behind most mold in a multi-unit building.
Municipal health and safety by-laws go further: any presence of visible mold is prohibited, as is water or humidity accumulation that degrades the structure or finishes. For a plex owner, this means a dwelling affected by mold is not merely "one to watch": it can be found non-compliant with minimum health standards.
Source: City of Montréal — "Tenant: you have the right to a dwelling in good condition" and City of Montréal — Housing health, maintenance and safety.
What causes mold in a plex?
According to the Government of Québec, mold proliferates in the presence of elevated humidity. Common causes in a plex are water infiltration (broken pipe, flooding, sewer backup) and insufficient ventilation that lets humidity from showers, baths and cooking accumulate. Indoor humidity should stay between 30% and 55% to prevent condensation.
The Government of Québec is unequivocal: mold proliferation occurs in the presence of an elevated humidity level. Understanding the moisture source is therefore key — for the occupants' health and for the value of your income property. Two broad families of causes recur in a plex.
Water infiltration
It is important to "identify and fix all causes of water accumulation or infiltration, such as broken pipes, flooding or sewer backup," the Government of Québec notes. In an older multi-unit building, an end-of-life roof, a foundation taking on water or aging plumbing continuously feed the humidity that nourishes mold.
Insufficient ventilation
"Insufficient ventilation inevitably causes excess humidity resulting from showers, baths and cooking activities," the Government of Québec observes. Proper ventilation is recommended, especially during humidity-producing activities. In a plex where units are occupied full time, everyday humidity accumulates if range hoods and bathroom fans are missing or deficient.
The simple rule to remember: keep surfaces dry
Indoor humidity should stay low enough to prevent condensation — usually between 30% and 55%. Watch for early signs: musty odours, condensation on windows, black spots on walls. Government of Québec — Finding and eliminating mould from your home.
Source: Government of Québec — Finding and eliminating mould from your home and Government of Québec — Health problems caused by mould.
What remedies does a tenant have at the TAL?
According to the Tribunal administratif du logement, a tenant may seek to cancel the lease or obtain an order requiring the landlord to perform work when inaction risks rendering the dwelling unfit for habitation. The tenant may also claim a rent reduction, damages and, where the landlord is negligent, punitive damages. The landlord may not re-rent a dwelling that remains unfit for habitation.
For a plex owner, the stake is not only the tenant's health: it is the range of remedies that opens up at the TAL if the problem is left unresolved. The Tribunal administratif du logement classifies the presence of mold or fungi among situations of unfitness (insalubrité) that can render a dwelling in poor habitability, or even unfit for habitation.
Concretely, a tenant facing mold may, according to the TAL:
- Cancel the lease or obtain a work order where failure to act risks rendering the dwelling unfit for habitation — or where it has already become so;
- Obtain a rent reduction for the period during which enjoyment of the dwelling is diminished;
- Claim damages for the harm suffered;
- Seek punitive damages where the dwelling became unfit for habitation through the landlord's negligence.
A concrete trap for the landlord
According to the TAL, the landlord may not re-rent the dwelling while it remains unfit for habitation. An unresolved mold problem can therefore freeze a unit of your plex — lost income on top of the cost of the work and any compensation.
Source: Tribunal administratif du logement — Unfitness (Insalubrité).
How does the City intervene on health standards?
According to the City of Montréal, the tenant must first notify the landlord. If the landlord does not act quickly, the tenant can report the problem to the City, which will assess whether an inspection is warranted. If the landlord does not make the corrections, the City can require work, impose fines and register a notice of deterioration in the land register.
The municipal route adds to the TAL remedy. The City of Montréal invites the tenant to notify the landlord in writing first and to keep a record of the exchange. For problems affecting walls, ceilings, windows or equipment in poor condition, it references a window to act on the order of 10 business days. If the landlord delays, the tenant can request a municipal inspection.
The consequences for a negligent plex owner are serious. Municipal teams can inspect, require corrective work and impose fines. Above all, in case of inaction, the City can register a notice of deterioration in the land register — a public registration that follows the building and cools buyers the day you want to sell.
"To prevent mold, surfaces must remain dry. Pay attention to musty odours, condensation on windows, water infiltration or black spots on walls."
— City of Montréal, Housing health, maintenance and safetySource: City of Montréal — Request an inspection of a dwelling.
Is mold a hidden defect when selling a plex?
Yes, potentially. According to Éducaloi, mold in the walls can constitute a hidden defect when it was concealed at the time of sale and could not be discovered during a normal pre-purchase inspection. The seller is responsible for the hidden defect even if unaware of its existence, and has a duty to inform the buyer of hidden defects they are aware of.
This is where the mold question meets the sale of your plex directly. Éducaloi explicitly cites "mold in the walls" among examples of hidden defects that, concealed at the time of sale and not detectable by a normal pre-purchase inspection, engage the seller's responsibility. The legal warranty against hidden defects applies automatically in principle, whether the seller is a professional or a private individual.
Two principles are especially consequential for a multi-unit seller:
- The seller is liable even in good faith. According to Éducaloi, the seller "is responsible for the hidden defect even if they were unaware of its existence" at the time of sale. Ignoring the problem does not protect you.
- The duty to disclose what you know. The seller must inform the buyer of the defects they know about so the buyer can evaluate repair costs. A known and concealed mold problem exposes you to a claim after the sale.
Expect to have to disclose
An "apparent" defect — visible on simple examination — is not covered by the legal warranty. But mold hidden in a wall or masked by a recent finish typically falls under hidden defects. An honest seller's declaration and a pre-purchase inspection protect both parties. Consult a notary or lawyer for your situation.
It is possible to sell without legal warranty, "at the buyer's own risk" — but this exclusion must appear in the promise to purchase and the deed of sale, and it does not cover the deliberate concealment of a known problem. To go deeper into hidden defects from the seller's perspective, see our article Hidden defects: protecting the seller of your plex.
Source: Éducaloi — The hidden defect in a building and Éducaloi — Real estate: buying without the legal warranty.
How much does mold decontamination cost in a plex?
The cost depends directly on the extent of the contamination and the cause to be corrected. The Government of Québec distinguishes small surfaces, often cleanable, from large or recurring surfaces that require professional evaluation, correction of the moisture source, and sometimes removal of materials. Decontaminating without correcting the cause solves nothing durably.
There is no single price: the decontamination cost of a plex varies with the affected area, the cause and the materials to be removed. The Government of Québec, in its technical information on evaluating and correcting mold problems, insists on a tiered logic: small surfaces on one hand, large or recurring contamination on the other.
| Situation | Typical approach | What drives the cost up |
|---|---|---|
| Small localized surface | Cleaning and drying, once the source is corrected | Porous materials to replace |
| Large or recurring surface | Professional evaluation, material removal, source correction | Extent, difficult access, rebuild after work |
| Structural cause (roof, foundation) | Envelope repair before any decontamination | Major roofing or drainage work |
The Government of Québec's central message is financially decisive for a plex owner: correcting the moisture source is essential. Cleaning mold without repairing the infiltration or ventilation merely postpones — and often worsens — the problem. That is why you should have the moisture source assessed by a professional before requesting any decontamination quote.
ImmoMulti: direct buyer of income properties on the North Shore
If your plex is dealing with a mold problem and you would rather not undertake decontamination work before selling, we can make you a direct, commission-free and confidential offer — on properties as-is. Get a proposal within 48 hours.
Before deciding between repair and sale, cost out both scenarios. Our multiplex yield calculation guide helps you evaluate the impact of a major correction on the overall value of your income property, while our pre-sale inspection of a plex article details the points to document before putting your multi-unit building on the market.
Informational content only. Does not constitute legal or tax advice. Health, housing and hidden-defect rules are applied case by case. Consult a notary, lawyer or qualified professional for advice specific to your property and situation.
