Backflow stop valve: the key to preventing sewer back-up and flooding

Last updated May 28, 2025

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Would you like to prevent basement flooding during major rainstorms? Have you had a sewer back-up event? By installing stop valves and creating a catch basin, you can prevent this kind of damage. Learn how to plan preventive work and details about funding.

Prevention is important

Each year, violent storms and torrential rain are becoming more intense and happening more often. Enormous quantities of rain falling and accumulating in the streets create more pressure than the municipal sewer network can handle. If your plumbing lines don’t have backflow stop valves or if they are incorrectly installed, wastewater can back up and flood your basement and garage.

Stop valve installation

Are stop valves installed in your home? If so, are they accessible? Are they in working order?

If your property has already flooded, or see sewer back-up, installation of a stop valve on each plumbing fixture is essential. And it’s mandatory according to municipal regulations. Since 2011, stop valves must be installed in all new buildings in Montréal.

Understanding how they work

A stop valve must be installed on the drain pipe of each plumbing fixture or group of plumbing fixtures in the basement. This means toilets, washbasins, shower stalls, washing machines, garage floor drains. Easy access to the stop valves is necessary for maintenance work.

Maintenance

Ideally, twice a year check that valves are in working order and not blocked.

Common problems

Some stop valves are installed at the wrong location or are worn out and no longer function. Rats may have chewed on them. On some properties, stop valves are hidden and access has been blocked since a floor has been laid, meaning their condition is unknown. It’s important to have a professional verify that your property is protected.

Creating a catch basin

Rainwater falling on your lot or the foundation drain can also cause damage in your basement. For better stormwater discharge, a sufficiently large catch basin must be created in your residence (minimum dimensions: 600 mm × 450 mm × 450 mm, or 24 in. x 18 in. x 18 in.). Include a pump to prevent overflow. Since major storms often lead to power outages, think about equipping your pump with a battery system. A stop valve will also prevent wastewater back-up.

Work grant

Plumbing contractor

Contact a plumbing contractor that is a member of the Corporation des maîtres mécaniciens en tuyauterie du Québec (CMMTQ) for advice on the type of stop valve to use, and for installation and maintenance guidance. A plumbing contractor can also help you with the creation of a catch basin and all its necessary equipment. Once the work is finished, ask your contractor to give you a certificate of compliance with your bill.

Financial assistance

If you own a house or a building with 5 or fewer dwellings in Montréal, you can apply for a grant under the RénoPlex program for work to protect your building against flooding that costs more than $500. To qualify, the value of your residence must be less than or equal to $3,476 per sq. metre (learn how to calculate property value by square metre). Here are some examples of grant amounts:

Installation of a normally closed back flow stop valve: $80
Installation of a normally closed back flow stop valve, under an existing slab: $560 per valve
Installation of a catch basin and a pump system: $1500

View Part 3 of the RénoPlex program to learn what types of work projects are eligible for subsidies

Full diagnosis and renovations

Problems related to the installation of gutters, flat roofs and countersloping garage entrances can also be corrected with the right work. View our article to learn how to determine what problem affects your home and to plan renovations to help prevent major damage.

ArticleProtecting your building from heavy rains

Contact your local borough to find out what regulations apply in your area and what permits are required.