What a backwater valve does
Your home's main sewer line carries wastewater out to the municipal sewer under the street. During heavy rain or a spring thaw, that sewer can fill up and back up, and the water then flows the wrong way, back up your pipe and out of the lowest drain in the house, usually the basement floor drain.
A backwater valve is a one-way valve installed in your main sewer line. Normally it stays open so wastewater leaves the house as usual. When sewer water starts flowing back toward the house, a flap closes and blocks it.
How a backwater valve works
Most modern backwater valves are the normally open type. Inside is a flap with a float. Under normal conditions the flap rests open and wastewater flows straight through. When water backs up from the street, it lifts the float and pushes the flap closed against a seal, stopping the backflow. When the sewer level drops, the flap falls open again.
While the valve is closed, wastewater from your own house cannot leave either. During a storm backup, avoid running taps, flushing toilets, or using the washing machine or dishwasher, or water from your own fixtures can back up behind the closed valve.
Where is a backwater valve located?
A backwater valve is installed in the main sewer line where it leaves the house, usually in the basement floor near the front wall. It sits below the floor in a small pit or chamber with an access cover, often round or rectangular, so the valve can be opened, inspected and cleaned.
It is placed so that every drain you want to protect, such as the floor drain, basement toilet, shower and laundry, is upstream of the valve.
- Look in the basement floor near the front of the house for a removable cover.
- Check near the floor drain and the main cleanout.
- In a finished basement, the cover may be under carpet or behind an access panel.
- If you have a home inspection report, it may note whether a valve was found.
Do you need a backwater valve?
A backwater valve is worth considering if your basement is finished, if water or sewage has ever come up through a floor drain, if neighbours on your street have had sewer backups, or if your area has older combined sewers, as many older Toronto neighbourhoods do.
Some insurers ask whether a home has one, and some offer better terms for sewer backup coverage when it does. Ask your insurer what they require.
If your foundation drains connect to the sewer, a backwater valve is usually paired with a sump pump, so groundwater around the foundation has somewhere to go while the valve is closed.
Installation, permits and subsidies in Ontario
Installing a backwater valve means cutting the basement floor, exposing the main sewer line, and fitting the valve with an access chamber. It is a job for a licensed plumber, and in Ontario municipalities it typically needs a building permit and an inspection.
Several municipalities offer subsidies. The City of Toronto's Basement Flooding Protection Subsidy pays part of the cost of a backwater valve, a sump pump and disconnecting the foundation drain for eligible homes, and expects a permit, a licensed contractor and itemized invoices. Check your own municipality's program for its current amounts and rules.
The cost depends on where the main line runs, how much floor has to be opened, and whether a sump pump or foundation drain disconnection is needed too. Our basement flooding protection page explains how we do it in Toronto.
How to maintain a backwater valve
- Open the access cover and check the flap moves freely at least once a year, and before the spring thaw and summer storm season.
- Clear out any debris, grease or buildup around the flap and seal.
- Make sure the cover is accessible and not buried under flooring or storage.
- Follow the manufacturer's instructions, and have a plumber service it if the flap sticks or the seal is damaged.
A valve that is jammed open does nothing during a backup, and one jammed closed backs up your own drains. Knowing the warning signs of a sewer backup helps too.