Backwater Valves: What They Do and Who Needs One

A backwater valve is a one-way valve on your home's sewer line that lets wastewater flow out but closes if the municipal sewer backs up toward the house. In Ontario, any home with a floor drain, toilet, shower or laundry in the basement can benefit from one, and it matters most in older neighbourhoods and low-lying areas where sewers overload during heavy rain. It is among the few defences against sewage coming up through basement drains.

What a backwater valve does

Your home's drains all lead to one pipe, called the sewer lateral, that runs under the basement floor and out to the sewer main under the street. Normally everything flows downhill and away. During an intense storm or rapid melt, the main can fill beyond its capacity. When that happens, sewage looks for the lowest open outlet, and that is often a basement floor drain, shower or toilet.

A mainline backwater valve sits in the lateral, usually under the basement floor near where the pipe leaves the house. Inside is a flap that stays open in normal use so wastewater and sewer gases pass freely. When flow reverses, the rising water lifts the flap and closes it against a gasket, sealing the pipe. Once the main drains down, the flap drops open again.

Who needs a backwater valve in Ontario?

• Homes with any plumbing fixture or floor drain in the basement. If the lowest drain sits below street level, it is the first place a backup will surface.
• Homes in older areas served by combined sewers, where rainwater and sewage share one pipe. Parts of older London neighbourhoods were built this way.
• Homes that have had a sewer backup before, or where neighbours on the street have.
• Homes in low spots, at the bottom of a hill or near the end of a sewer run.
• Finished basements, basement apartments and basements with a furnace, water heater and electrical panel at floor level.
• Homes where the insurer asks for one as a condition of sewer backup coverage.

Many newer homes had a valve installed during construction. Look for a round or rectangular access cover set into the basement floor, often near the front wall or the floor drain, sometimes with a clear lid. If you are unsure, a plumber can confirm with a camera inspection of the drain.

What a backwater valve will not do

The valve only stops flow coming back from the sewer. It does not stop groundwater seeping through the foundation, surface water coming in through windows, or a blockage in your own pipe, such as tree roots or grease. Those have different fixes.

There is one more point every household needs to know. While the valve is closed, nothing can leave the house. Water from a shower, toilet, dishwasher or washing machine has nowhere to go and will come up through the lowest drain on your side of the valve. During a severe storm, hold off on using water until the rain has eased. Some valves can be fitted with an alarm that tells you when the flap has closed.

Installation, permits and cost help

Retrofitting a valve means breaking out a section of the basement floor, cutting the valve into the lateral, and pouring new concrete around an access box. It is a job for a licensed plumber and generally requires a permit and inspection from the municipality. The plumber also has to look at how the foundation drains are connected. If weeping tile ties into the lateral on the house side of the valve, it may need to be separated and sent to a sump pit, or the valve could trap that water in the basement during a storm.

Some Ontario municipalities, including the City of London, have offered grant programs that help homeowners with the cost of backwater valves and related flood protection work. Terms, eligibility and funding change, and programs often require approval before the work starts, so check with the city before you hire anyone. Ask your insurance broker too. Some insurers reduce the premium for sewer backup coverage, or require a valve to offer it at all.

Backwater valve maintenance

A valve that is stuck open with debris offers no protection. Check it at least once a year, and before the spring melt if you can.

1. Stop using water in the house while the cover is off.
2. Put on rubber gloves and eye protection, then remove the access lid.
3. Look for grease, wipes, grit or anything else sitting on the flap or in the body of the valve, and clear it out.
4. Move the flap by hand, or with a tool, to confirm it swings freely and seats flat.
5. Check the gasket or O-ring for cracks or wear and replace it if needed.
6. Flush with a bucket of water, then refit the lid and make sure it seals.

To find out where you stand, go to the basement and look for a valve access cover in the floor. If there is none and you have a drain or bathroom down there, call the city about current programs and ask a licensed plumber for an assessment.

Common questions