What is a backwater valve and how does it work?

You’re not alone if you’ve never given a lot of thought to backwater valves—but you may want to get to know more about this little doohickey. This practical plumbing device can save you and your home from a disgusting and expensive disaster that no homeowner wants to run into: sewage backing up into your basement during a big rainstorm.

This article will explore what a backwater valve actually is, how it works, when it makes sense to install one — and how to qualify on municipal rebates on installation costs.

You’ll learn why this powerful piece of plumbing protection deserves a spot on your home‑maintenance must-have list before the next big storm hits.

What is a backwater valve?

Also sometimes known as a sewer backflow or backflow prevention valve, this device as a one‑way gate in your home’s main drain line that’s designed to stay open in normal conditions and quickly slam shut when water starts going in the wrong direction.

Installed on the pipe that carries wastewater from your home to your city’s sewer, it lets water from sinks and toilets exit a pipe, while preventing water from reversing course and pushing sewage back through the pipe into your basement.

“Backwater valves help protect homes,” explains Dan Sandink, director of research at the Institute for Catastrophic Loss Reduction (ICLR). “They allow water and sewage to flow out toward the public sewer system, but are designed to close during a ‘backflow’ event. Backflow events happen when public sewers are overwhelmed with water during extreme rain events. When the systems are overwhelmed, they can back up into basements.”

How do backwater valves work?

Think of a backwater valve as a water guard. Inside a backwater valve is a lightweight flap or gate. When operating for everyday use, the flap stays in an open position so that water from your plumbing system can flow freely out of your home and into the sewer system.

However, during a major rainstorm or severe ice melt, the city’s sewer system can become overwhelmed and water from the municipal system can start to move back into a home (hence the term “backwater”). When this reverse flow hits the backwater valve flap, the flap closes, creating a physical barrier so that the reversing water can’t come back through.

As soon as the municipal sewer pressure drops and flow direction returns to normal, gravity allows the flap to drop back down into the open position so your plumbing works normally again.

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Does your home need a backwater valve?

In fact, flooding is only expected to worsen due to issues like more extreme weather events due to climate change and aging city infrastructure. Currently, sewer backups are one of the leading causes of water damage in Canadian homes.

Homes with plumbing below street level (for example, basements with laundry rooms and finished bathrooms) are particularly at risk because an overloaded city sewer can push wastewater back through low-lying drains.

“Theoretically anyone who is connected to a municipal/public sewer system can be at risk during severe rainfall events,” says Sandink. However, he adds, homeowners who have experiences with sewer backflow flooding in the past are more likely to be at higher risk. Same with those in a low-lying area, or who have deeper basements.

“Some municipalities may be able to inform you if your area is prone to water backups,” he adds.

He also suggests that homeowners ask their insurers who may have more detailed information about their area’s risk profile and home build.

It's worth noting that your home may already be equipped with backwater valves. Canada has a national building code. The code requires that sewage backflow devices on main backwater pipes remain in an open position so that air and sewage gas can be freely exchanged.

Many provinces and municipalities have additional building codes that dictate when a valve is required, especially with new constructions or in high-risk areas. For example, in the town of Collingwood all new homes as of 2013 must have backwater valves.

Is it worth the cost?

Installing a backwater valve into an already existing home can be expensive and ranges from $1,000 to $5,000 depending on the complexity of the installation. But compare that price to what it could cost you if your basement is flooded with sewage—especially if you have a finished basement.

Luckily, you may not have to shoulder the cost on your own. Many regions and municipalities offer rebates and funding to help with the cost of installing a backwater valve as long as the work is done by a licensed plumber.

For example, Toronto’s Basement Flooding Protection Subsidy Program gives eligible property owners a subsidy of up to $3,400 to install flood prevention measures. In Edmonton, Alberta, homeowners can apply for the Backwater Valve Subsidy Program, which provides up to $800 for installing a backwater valve.

Installing a valve might save you money over the long run

Installing a backwater valve isn’t free, but it can certainly be a lot cheaper than dealing with even one serious sewer‑backup flood.

“Sewer backup and basement floods events can be devastating,” says Sandink. “Aside from the damage that is caused to your home — which can easily approach $50,000 or more for finished basements — people lose mementos that can’t be replaced.”

He also points out the health risks associated with basement flooding and sanitary sewer back-up.

For those who have insurance coverage for sewer back-up, he says, the claim will likely lead to higher rates in the future or even affect the coverage itself.

“It can be expensive to have flood protection installed – but unfortunately it may be necessary in many circumstances,” he says.  

In addition to protecting your basement and any valuable items, for many homeowners, a one‑time investment in a backwater valve is a manageable way to lower the odds of a catastrophic loss that could otherwise seriously damage both home and budget.

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