Mould in an Ontario Rental: Landlord Repair Duty or Tenant-Caused Damage?
Ontario law never uses the word mould. Who pays depends on two general rules, a one-year clock, and what you wrote down at move-in.
Who is responsible for mould in an Ontario rental unit, the landlord or the tenant?
There is no mould-specific rule. The landlord carries the repair and maintenance standard: under s.20(1) of the Residential Tenancies Act, the landlord must keep the unit “in a good state of repair and fit for habitation” and comply with health, safety, housing and maintenance standards. A tenant is liable only for undue damage caused by the wilful or negligent conduct of the tenant, another occupant, or someone the tenant let in (s.34). Which of the two a particular patch of mould falls under is a factual question about cause, and either side has one year to take it to the Landlord and Tenant Board.
Source: Residential Tenancies Act, 2006, ss.20, 29, 34 and 89(1.1) (Ontario e-Laws, consolidation from September 21, 2026)
I’m Arthur Zhao, a real estate broker in Toronto. Picture the move-out inspection: the tenant has handed back the keys, you walk the unit with your move-out checklist, and in the bathroom you look up. Above the shower there is a spread of dark spots on the ceiling paint, and the window frame beside it has a black line along the bottom edge where condensation has been sitting.
The first question is about money: can I make the former tenant pay to fix this? The honest answer starts with what the Residential Tenancies Act actually lets you claim, and how high the bar is. After that, this article turns to the other timing, when the mould shows up while the tenant still lives there, and to the standards the unit is judged against in the first place.
After move-out: can the landlord claim the cost?
The tool is s.89 of the Act. A landlord may apply to the Landlord and Tenant Board for an order requiring a tenant or former tenant to pay the reasonable cost of repairing (or, where repair is not reasonable, replacing) damaged property. Two conditions sit inside that section:
- Who and when. The damage must have been caused while the tenant was in possession, by the tenant, another occupant, or a person the tenant permitted in the building.
- What kind. The tenant or those people must have “wilfully or negligently” caused
undue damage. That is the same threshold as s.34, which makes the tenant responsible for repairing undue damage caused by wilful or negligent conduct.
Timing matters. Under s.89(1.1), the application can be made while the tenant is still in possession, or no later than one year after the tenant or former tenant ceased to be in possession.
What the Act does not give you is a reserve of the tenant’s money to draw on. Section 105(1) says the only security deposit a landlord may collect is a rent deposit under s.106, and s.106(10) requires that deposit to be applied to the rent for the last rental period. By the time you are standing under that bathroom ceiling, the deposit has already paid the final month. The Act’s set-off rule in s.89(2) works the other way around: when the Board orders payment and termination of the tenancy, the Board sets the rent deposit off against what the tenant owes. That is a Board step, not a landlord’s self-help.
⚠️The deposit is not a damage deposit. Under RTA s.105(1), the only security deposit a landlord may collect is a rent deposit, and s.106(10) requires it to be applied to the rent for the last rental period. Holding it back to pay for mould repairs is not one of its permitted uses.
During the tenancy: what the tenant can ask the Board for
If the mould appears while the tenant is living there and the tenant believes the landlord has not kept the unit in repair, s.29(1), paragraph 1, lets a tenant or former tenant apply for an order determining that the landlord breached s.20(1). The Landlord and Tenant Board’s form for this is the T6 Tenant Application about Maintenance.
If the Board finds a breach, s.30(1) lists what it may do, including:
- terminate the tenancy;
- order an abatement of rent;
- authorize a repair the tenant has made or will make, and order the landlord to pay the cost;
- order the landlord to do specified repairs within a specified time;
- order the landlord to pay the reasonable cost of repairing or replacing the tenant’s own damaged property and other reasonable out-of-pocket expenses;
- make any other order it considers appropriate.
Two provisions shape how this plays out. First, s.30(2): in deciding the remedy, “the Board shall consider whether the tenant or former tenant advised the landlord of the alleged breaches before applying to the Board.” It is a factor in the remedy, written into the Act. Second, s.29(2): no application may be made more than one year after the day the alleged conduct occurred.
A landlord sometimes asks whether it matters that the tenant saw the problem at the viewing and rented anyway. Section 20(2) answers that: the landlord’s duty in s.20(1) applies “even if the tenant was aware of a state of non-repair or a contravention of a standard before entering into the tenancy agreement.”
Two procedures, side by side
The standards the unit is measured against
Section 20(1) ties the landlord’s duty to “health, safety, housing and maintenance standards.” In Toronto, the relevant set is Municipal Code Chapter 629, Property Standards. None of its sections mentions mould, but several govern the conditions mould grows out of:
- §629-4 A: no person shall rent or offer to rent property that does not conform with the chapter’s standards.
- §629-5 A: the owner must “repair, maintain and keep clean the property in accordance with the standards.”
- §629-16 A(3): every part of a building must be maintained so as “to prevent the entry of moisture that would contribute to damage, decay or deterioration.”
- §629-20 A and D: roofs must be weather-tight and free from leaks; eavestroughs and downpipes must be free from leaks and water-tight.
- §629-27 A: “Every wall and ceiling shall be maintained clean and free of holes, cracks and damaged and deteriorated surface material.”
- §629-39 A and C: adequate ventilation to all areas, with every ventilation system kept in good repair; every “sanitary convenience room” must have natural ventilation through an opening to the outside or mechanical ventilation capable of completely changing the air at least once per hour.
The occupant has duties under the same chapter. §629-6 requires every occupant to maintain the property, including sanitary facilities and fixtures, in a clean and sanitary condition (A and C), and to “co-operate with the landlord in complying with the requirements of this chapter” (E).
Apply that to the bathroom in the opening. A broken or missing exhaust fan sits on the owner’s side of §629-39. A working fan that was never switched on, or a ceiling that was never wiped, is closer to the occupant’s side of §629-6. That reading is my inference from the wording; the chapter does not assign mould to either party.
ℹ️RentSafeTO covers only part of Toronto’s rental stock. According to the City of Toronto (2026), the program applies to apartment buildings with three or more storeys and 10 or more units, and “units in a private home (basement or main floor apartment) are not part of the RentSafeTO program.” Those units are still covered: “The Property Standards Bylaw sets the standards for all properties in Toronto.”
Where the law is silent
A full-text search of the Residential Tenancies Act and of Chapter 629 turns up no use of the word “mould” or “mold.” Three consequences follow from that silence.
- No dedicated rule on cause. If mould comes from a roof leak, foundation moisture or a ventilation system that does not work, it falls under the landlord’s s.20(1) duty and the Chapter 629 standards above. If it spreads because a tenant did not clean, did not run a working fan, or did not report a leak, it may fall under s.33 (ordinary cleanliness) and s.34 (undue damage). That is an inference from the general provisions, not a written rule.
- No dedicated rule on proof. The Act has no provision allocating the burden of proving who caused mould.
- Case by case. Whether a particular patch counts as undue damage, and whether the conduct was negligent, is decided by the Board on the facts of that tenancy. Both sides may share in it.
💡 My own view: documentation cuts both ways. Because the law has no mould rule, the dispute reduces to one factual question: did this form during the tenant’s occupancy, and from what? Dated photos of the bathroom ceiling and window frames at move-in and at move-out, plus any written repair requests, are what let either side answer it. A landlord without a move-in record will struggle to show the spots were not already there; a tenant without a written report will struggle to show the landlord knew about the leak. The same record protects whoever kept it.
Outside Toronto
Chapter 629 is a Toronto by-law. Elsewhere in Ontario, s.224(1) of the Act sets the fallback: the province’s prescribed maintenance standards apply to a residential complex in a local municipality if (a) no municipal property standards by-law applies to it, or (b) one applies and prescribed circumstances apply. So the first step outside Toronto is to find out whether your municipality has a property standards by-law covering the building. Either way, the RTA framework above (ss.20, 29, 30, 33, 34, 62 and 89) is the same across the province.
What to do, depending on who you are
One action per line. Dates and section numbers come from the provisions cited above.
- ☐Photograph bathroom ceilings, window frames and exhaust fans, with dates, before each new tenant moves in.
- ☐Test that every bathroom fan actually runs and vents, and fix it before the unit is rented (§629-39).
- ☐Give 24 hours’ written notice stating the reason, day and a time between 8 a.m. and 8 p.m. before entering to inspect or repair (s.27).
- ☐Treat the last month’s rent deposit as rent for the last month, not as a repair fund (s.106(10)).
- ☐Calendar the one-year limit from the day the tenant left if you are considering an s.89 claim.
- ☐Report a leak, damp patch or broken fan to the landlord in writing and keep a copy (s.30(2)).
- ☐Run the bathroom fan and wipe down surfaces that collect condensation (§629-6 A and C).
- ☐Photograph the problem with dates each time you notice a change.
- ☐Calendar one year from the conduct you are complaining about if you may file a T6 (s.29(2)).
- ☐Do a joint walk-through with a signed condition record at move-in and move-out.
- ☐Keep every repair request and reply in writing, not only by phone.
- ☐Allow access for repairs on proper notice; Chapter 629 asks occupants to co-operate (§629-6 E).
- Residential Tenancies Act, 2006, ss.20, 26, 27, 29, 30, 33, 34, 62, 89, 105, 106 and 224 (Ontario e-Laws, consolidation from September 21, 2026)
- City of Toronto Municipal Code, Chapter 629, Property Standards (April 30, 2025)
- City of Toronto, Rental Housing Standards (RentSafeTO scope; T6 reference)
- Tribunals Ontario, Landlord and Tenant Board forms (T6: Tenant Application about Maintenance)
Repairs in an Ontario Rental: Landlord vs Tenant — Who Pays for What →Landlord Won’t Fix It? Ontario’s T6 Application, Step by Step →Your Tenant Wants to Paint or Drill: What an Ontario Landlord Can Refuse, and How Damage Gets Paid Without a Damage Deposit →First-Time Renter Guide →
Frequently Asked Questions
Can a tenant stop paying rent because of mould?
I found no provision in the Residential Tenancies Act that lets a tenant hold back rent over a repair problem; the provisions that allow withholding or deducting rent deal with other situations, such as a landlord not providing the standard lease (s.12) or not paying interest on a rent deposit (s.106(9)). The route the Act gives for repairs is a T6 application under s.29, and a rent abatement is one of the remedies the Board may order under s.30(1).
Does my landlord have to test for mould?
Neither the Residential Tenancies Act nor Toronto’s Chapter 629 mentions mould or mould testing. What the landlord must do is keep the unit in a good state of repair and fit for habitation (RTA s.20(1)) and, in Toronto, repair, maintain and keep the property clean to the Chapter 629 standards (§629-5 A), including keeping walls and ceilings free of damaged and deteriorated surface material (§629-27 A).
Can my landlord come in to look at the mould?
Yes, with notice. RTA s.27(1) allows entry on at least 24 hours’ written notice to carry out a repair, or to inspect whether the unit is in good repair and complies with maintenance standards if the inspection is reasonable; the notice must give the reason, the day and a time between 8 a.m. and 8 p.m. (s.27(3)). Without notice, entry is allowed in an emergency or with the tenant’s consent at the time of entry (s.26(1)).
My tenant already moved out. How long do I have to claim for the damage?
Under RTA s.89(1.1), a landlord can apply while the tenant is in possession or no later than one year after the tenant ceased to be in possession. The claim succeeds only if the tenant, an occupant or someone the tenant let in wilfully or negligently caused undue damage (s.89(1)(a)).
Is a basement apartment covered by RentSafeTO?
No. The City of Toronto says units in a private home, including a basement or main floor apartment, are not part of RentSafeTO, which applies to buildings with three or more storeys and 10 or more units. The Property Standards Bylaw (Chapter 629) still applies, and §629-4 B bars using any basement room as a habitable room unless it meets the chapter’s egress, light, ventilation and ceiling height requirements.
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