When the city sewer backs up during a heavy storm, a backwater valve closes and keeps sewage from coming up through your basement floor drain, toilet or shower. We install, repair and clean backwater valves for London homes, and help you with the paperwork for the City's basement flooding grant.
Your home's main drain carries wastewater out to the city sewer. During an intense storm, that sewer can fill faster than it drains, and the water looks for the lowest way out. In many homes, that's the basement floor drain.
A backwater valve is installed on the main drain under your basement floor. Normally its flap stays open so wastewater flows out freely. When sewage starts flowing the wrong way, the flap floats shut and seals the line until the sewer drains down again.
While the valve is closed, nothing can drain out of the house either, so it's important not to run water during a backup.
The names sound alike, but they protect different things.
Backwater valve: on your sewer drain. Keeps sewage out of your basement.
Backflow preventer: on your water supply. Keeps contaminated water out of the drinking water system. See backflow testing and repair.
Water or sewage came up through the floor drain during a storm. Without protection, it's likely to happen again.
A basement bathroom, laundry or floor drain sits lower than the sewer in the street, so it's the first place a backup shows up.
Flooring, drywall and furniture make a backup far more expensive to clean up than a bare concrete floor.
Some insurers ask whether a backwater valve is installed when you add or renew sewer backup coverage.
Not sure? We'll check your drain layout and tell you honestly whether a valve makes sense for your home. Book an assessment.
Looking for backwater valve installation near me? Most installations take a day. Here's what to expect.
We run a camera through your main drain to find the right spot and check the pipe is in good shape.
We confirm whether your foundation drains connect to the sewer, since that changes the plan.
You get a written quote, and we arrange any plumbing permit and inspection required.
We cut a small section of the basement floor, install a normally-open valve on the main drain downstream of all your fixtures, and fit an access cover.
We patch the concrete, test the valve and leave the area clean.
We show you where the valve is and how to check it, and give you an itemized invoice for your records or a grant claim.
In many older homes, the weeping tiles around the foundation drain straight into the sewer line. If a backwater valve closes on that line, groundwater from the weeping tiles has nowhere to go and can push up through the basement floor.
That's why the weeping tiles are usually disconnected from the sewer and redirected to a sump pit at the same time. If your home needs it, we'll plan both together with our sump pump installation service.
Usually the case in newer homes. The backwater valve can go in on its own.
Common in older homes. The weeping tiles are disconnected and a sump pit and pump are added along with the valve.
Not sure which you have? We check during the camera inspection, before you get a quote.
A backwater valve only protects you if the flap can close. Grease, wipes and debris can hold it open, so it's worth checking once a year, ideally before spring storms.
Open the access cover, make sure the flap moves freely and clear anything caught in it. If the valve keeps getting fouled, the main drain may need cleaning. See our drain cleaning service. Need backwater valve repair near me? If the flap is stuck closed or no longer seals, we repair or replace the valve.
Book a Valve Check"After a sewer backup in our Byron basement, we decided to have a backwater valve installed. Nova Drain & Plumbing Services London inspected the main drain, explained where the valve should go and completed the installation. They tested the flap mechanism and showed us how to access it for future cleaning."
Gregory M.
Byron, London, ON
October 12, 2026
"We wanted to protect our finished basement from sewage backing up through the floor drain. The plumber inspected our existing drainage layout and recommended a suitable backwater valve location. The installation required access through the concrete floor, but the team explained each step and checked the drainage afterward."
Océane V.
Westmount, London, ON
September 24, 2026
"Our existing backwater valve was not closing properly. The plumber opened the access cover and found debris around the flap. He cleaned the valve, inspected the sealing surfaces and checked that the mechanism could move freely again. I appreciated that he explained the maintenance instead of immediately recommending replacement."
Elowen F.
Old North, London, ON
August 18, 2026
"We purchased an older house in Wortley Village and wanted the sewer system checked before renovating the basement. The technician performed a camera inspection and discussed backflow protection with us. We scheduled a valve installation, and the team made sure the access cover remained easy to reach afterward."
Rebecca H.
Wortley Village, London, ON
July 9, 2026
The City's Basement Flooding Grant Program can cover most of the eligible cost of a backwater valve for homes that have flooded before or are in an area prone to basement flooding. Approval comes from the City in writing, so check your eligibility with the City's Sewer Engineering division before work starts. We'll provide the itemized invoice the application needs.
No. It stops sewage coming back up through your drains. Groundwater coming through the foundation is handled by weeping tiles and a sump pump, so many homes need both.
Usually not. Under the Ontario Building Code, a backwater valve on a building drain can't serve more than one dwelling unit, so a separate basement unit typically needs its own valve. We'll check your layout during the assessment.
In most Ontario municipalities, installing a backwater valve needs a plumbing permit and inspection, and a permit is typically required for grant claims too. We arrange it as part of the job.
Most installations are done in a day, including cutting and patching the concrete. If the weeping tiles need to be disconnected and a sump pit added at the same time, it can take longer. We'll give you a timeline with the quote.
Check it at least once a year, ideally before spring. Open the access cover, make sure the flap moves freely and clear any debris. We can do the check for you if you'd rather not.
Our plumbers install, inspect and repair backwater valves across London.
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