ImmoMulti — direct buyer of income properties on the North Shore — sees a lot of buildings where the backwater valve (also called a check valve or backflow-prevention device) is missing or neglected. It is a small, discreet device, often in the basement, but its absence can now be costly for a plex owner: refused insurance coverage after a sewer backup, non-compliance with a municipal by-law, and a friction point when you sell. Conversely, a properly installed valve is often subsidized, protects your basement units, and reassures buyers. Here is what an owner should know before selling or renovating an income property.
What is a sewer backup and what does a backwater valve do?
A sewer backup happens when the overloaded municipal network can no longer drain water, which rises through the building's lowest drains. The backwater valve (check valve) is a one-way valve that lets water out but blocks its return into your plex — provided it is in good condition.
During heavy rain or rapid snowmelt, the municipal sewer network can fill faster than it drains. The pressure then forces wastewater and stormwater back up through the lowest outlets in your building: floor drains, toilets, showers, and washers in the basement. This is a sewer backup, and it is one of the messiest and most expensive claims a plex owner can face.
The backwater valve — or check valve — is a device installed on the main drain or on certain branches. It works as a one-way valve: your building's water can flow out to the network, but the moment the flow tries to reverse, a flap closes and blocks the backflow. As CAA-Québec notes, the device is only effective if it is properly installed and maintained.
Source: CAA-Québec — "Le clapet antiretour : un aller simple pour les eaux usées" (French).
Is a backwater valve mandatory in a plex in Québec?
There is no single province-wide requirement, but more and more municipalities require one by by-law — often for new construction, basement renovations, and buildings that have already had a backup. In Montréal, by-law 11-010 has required protective devices since July 1, 2011.
The rules vary from city to city. In Montréal, backwater-valve installation is governed by the by-law on protecting buildings against sewer backups (11-010): since July 1, 2011, these devices are required notably for new construction, basement renovations, and buildings that have already experienced a backup. The City emphasizes that the valve is "the key" to preventing backup and flooding.
A crucial point for owners: since the Act respecting municipal powers came into force on January 1, 2006, a municipality can exempt itself from liability for damage caused by a backup if its by-law requires the owner to install a protective device and the owner failed to do so. In plain terms: not installing the required valve can transfer the entire financial risk of a loss onto you.
On the North Shore — Terrebonne, Mascouche, Blainville, Boisbriand, Saint-Eustache, Deux-Montagnes — each city has its own rules and priority areas. Before selling or renovating, check the applicable by-law with your municipality's urban planning or public works department.
Watch out for buildings with basement units
A plex with one or more basement units is especially exposed to backups. In several cities, these buildings are prioritized by the valve requirement. A flooded basement unit also means lost rent and a tenant to rehouse.
Sources: Ville de Montréal — "Backwater valve: the key to preventing sewer backup and flooding" (French); Act respecting municipal powers (CQLR, c. C-47.1).
What your insurer requires (and won't cover without a valve)
Sewer backup protection is not part of the base policy: it comes through a separate endorsement (endorsement 16c – Water Damage, Ground Water), with its own limit. Some insurers require a backflow-prevention system, especially after a first claim, as a condition of coverage.
Many owners discover too late that their base policy does not cover sewer backup. According to explanations from InfoAssurance (a service of the property and casualty insurance industry) and the Insurance Bureau of Canada, this protection is generally obtained by adding a separate endorsement to the policy. Endorsement 16c – Water Damage (Ground Water) covers damage from sewer and septic backups as well as water infiltration through foundations, subject to the policy's exclusions. This endorsement has its own limit, to be agreed with the insurer.
The key point for a multi-unit owner: some insurers require the installation of a backflow-prevention system — a valve — notably following a claim, as a condition to renew the contract. And the device must be in good working order: a clogged valve that lets a backup through can compromise the payout.
| Situation | With a compliant backwater valve | Without a valve (or a defective one) |
|---|---|---|
| Sewer backup coverage | Insurable via endorsement 16c (ground water) | Possible refusal or claim exclusion |
| After a first claim | Easier renewal | Insurer may require a valve or cancel |
| Premium and deductible | Generally more favourable terms | Higher premium, increased deductible |
| Municipal liability | City exempt if you are compliant | Financial risk transferred to owner |
Sources: InfoAssurance — "How to protect yourself from sewer back-ups"; Insurance Bureau of Canada — Protection against water damage and flooding. Terms vary by insurer and policy.
Municipal subsidies: who pays for the valve?
Several municipalities offer subsidy programs to install backflow-prevention devices. Ville de Québec has one: the work must comply with regulations and standards and be carried out by a plumbing contractor holding the appropriate RBQ licence. These programs often target high-risk areas.
Good news for your budget: valve installation is often subsidized. Ville de Québec, for example, offers a subsidy program for installing residential backflow-prevention devices. To be eligible, the work must be performed in compliance with municipal regulations, all applicable standards and codes, and the manufacturer's recommendations, and be carried out by a plumbing contractor holding the appropriate licence from the Régie du bâtiment du Québec.
These programs are generally not open to everyone: they prioritize high-risk backup areas, identified based on topography, the age and capacity of the local sewer network, and the history of incidents. Each city has its own conditions, caps, and forms. Check with your municipality before having the work done, since the subsidy is often conditional on a prior application.
To maximize your chances of getting the subsidy
- Verify your area's eligibility with the city before starting;
- Have the valve installed by an RBQ-licensed plumbing contractor;
- Obtain the municipal permit if required;
- Keep invoices, photos, and the installation report for your insurance and sale file.
Source: Ville de Québec — Subsidy program for installing residential backflow-prevention devices (French).
Insurability and selling: why the valve matters for your North Shore plex
When selling, the buyer and their inspector check for a backwater valve, especially for a building with a basement or in a high-risk area. Its absence can prevent the buyer from insuring, reduce their financing, or serve as negotiation leverage. An uncorrected past backup can also raise a latent-defect issue.
The backwater valve is not just a compliance matter: it is a factor that directly affects the value and saleability of your income property. An informed buyer — especially a multi-unit investor — knows that a plex that cannot be insured against backup is harder to finance and resell. Their broker, inspector, and insurer will ask the question.
"The backwater valve is the key to preventing sewer backup and flooding."
— Ville de Montréal, homeowner information article (translated)In practice, the absence of a valve can: cause the buyer's insurance application to fail (and thus their offer conditional on insurance), lower the amount financed by their lender, or become an argument to negotiate the price down. Worse, an undisclosed and uncorrected history of backups can open the door to a latent-defect claim after the sale. As real-estate legal commentary notes, the state of a building's drainage system can become a serious issue between seller and buyer.
For an owner considering selling a plex on the North Shore, there are two paths: fix the situation (install a compliant valve, ideally subsidized) to maximize value, or sell the building as-is to a buyer who takes on that kind of work. That is exactly what ImmoMulti does.
Source: Assistco — Legal note on the importance of the backwater valve (French). For a specific case, consult a lawyer or notary.
What to do before selling or renovating
Check whether a valve is present and in good condition, review your city's by-law, apply for the subsidy if your area is eligible, use an RBQ-licensed contractor, and keep all documentation. These steps protect your insurance, your compliance, and your sale price.
Here is a simple checklist for a multi-unit owner:
- Locate the valve. A plumber can confirm its presence, type, and condition during a main-drain inspection.
- Read your municipality's by-law. Some cities make the valve mandatory; others require it after a first backup.
- Talk to your insurer. Confirm that backup is covered (endorsement 16c), the limit, and any valve-related conditions.
- Apply for the subsidy. If your area is eligible, apply before the work.
- Document everything. Invoices, permits, photos, report: this file reassures the buyer and avoids disputes.
If the scope of work or the overall condition of your building gives you pause, know that ImmoMulti buys plexes and multi-unit properties on the North Shore as-is — valve or not, deferred work or not — with a direct offer within 48 hours, no broker, no commission. To go further, see our guide on the North Shore real-estate market in 2026 and our overview of the 2027 building code and plex value.
Informational content only. By-laws, insurance terms, and subsidy programs are subject to change by municipalities, insurers, and the Québec government. Consult a licensed plumbing contractor, your insurer, a lawyer, or a notary for advice specific to your building.
