# Who is responsible for mould in an Ontario rental unit? Updated 2026-09-26. The landlord is responsible for the conditions that cause it, unless the tenant caused the damage. Section 20 of the Residential Tenancies Act makes the landlord responsible for keeping the rental unit in a good state of repair and fit for habitation. Section 34 makes the tenant responsible for undue damage caused by their wilful or negligent conduct, or by someone they let in. Which one applies depends on where the water came from. ## What Toronto's by-law requires Chapter 629 of the Toronto Municipal Code does not name mould. It sets the standards that the moisture behind mould usually breaks: - The building is kept in repair so it prevents moisture getting in that would cause damage, decay or deterioration (section 629-16). - Roofs are weather-tight and free from leaks, and eavestroughs and downpipes are water-tight (629-20). - Windows and exterior doors are weather-tight to prevent drafts or leakage (629-21). - Plumbing is kept free from leaks and defects (629-37). - Every area has adequate ventilation, every ventilation system is cleaned and kept working, and every washroom has an outside opening or a fan that changes the air at least once an hour (629-39). - Walls and ceilings are kept clean and free of damaged or deteriorated surface material (629-27). ## The tenant's side Section 629-6 requires the occupant to keep the unit clean and sanitary and to co-operate with the landlord in meeting the by-law. A bathroom fan that is never switched on is part of the picture. Whether damp came from how the unit is used or how the building is built is a question of fact, and the evidence decides it. ## Find the water before the stain Mould on a wall is a symptom. The source is a roof, a window, a pipe, the foundation, or condensation that a ventilation fan should be clearing. Cleaning the surface without fixing the source brings it back, and a record that only shows a cleaning does not show the cause was dealt with. Record where the moisture came from, the fix, and photos before and after. That record answers the tenant and supports a claim under section 34 when the cause was the tenant's. ## If the tenant applies to the Board A tenant can apply for an order that the landlord breached section 20, within one year of the conduct. Under section 30 the Board can order the repair, a rent abatement, payment for the tenant's damaged property and other reasonable costs, or end the tenancy. ## How Roundhouse handles it Roundhouse traces the water before anyone quotes the repair, and the findings, photos and invoice sit on one record. See [basement leaks and waterproofing](https://roundhousegroup.ca/services/waterproofing/), [plumbing repairs](https://roundhousegroup.ca/services/plumbing/) or [request work](https://roundhousegroup.ca/request-work/). ## Related pages - [Waterproofing](https://roundhousegroup.ca/services/waterproofing/) - [Plumbing and drains](https://roundhousegroup.ca/services/plumbing/) - [Roofing](https://roundhousegroup.ca/services/roofing/) ## Questions ### Is mould the landlord's responsibility in Ontario? The landlord is responsible for keeping the unit in a good state of repair and fit for habitation under section 20 of the Residential Tenancies Act. Where the moisture comes from a tenant's wilful or negligent conduct, section 34 makes the tenant responsible for the undue damage. The cause decides it. ### Does Toronto's property standards by-law mention mould? Chapter 629 does not name mould. It requires what keeps moisture out and air moving, such as weather-tight roofs and windows, leak-free plumbing, working ventilation, and walls and ceilings free of damaged or deteriorated surfaces. ### Can a tenant apply to the Landlord and Tenant Board over mould? Yes. A tenant can apply for an order that the landlord breached section 20, within one year of the conduct. The Board can order repairs, a rent abatement, or payment for the tenant's damaged property and reasonable costs. ### Does a bathroom in a Toronto rental need a fan? It needs ventilation. Chapter 629 accepts either an opening to the outside air of at least 0.09 square metre for each toilet, or mechanical ventilation that changes the air in the room at least once an hour. ## Sources - [Residential Tenancies Act, 2006, S.O. 2006, c. 17, sections 20, 29, 30 and 34](https://www.ontario.ca/laws/statute/06r17) - [Toronto Municipal Code, Chapter 629, Property Standards](https://www.toronto.ca/legdocs/municode/1184_629.pdf) We only use ECRA/ESA-Licensed Electrical Contractors.