Backwater valves

Written by the Square One team

Reviewed by Dave Smythe

Updated June 12, 2024 | Published February 27, 2014

Whenever there’s a sudden downpour, there’s a risk that your basement could experience a water backup problem. It’s bad enough if clean rainwater backs up in your basement; try to imagine dirty sewage filling your basement and ruining everything it touches.

Fortunately, you can help prevent this by installing one simple device: a backwater valve.


Thumbnail of the Backwater Valves in Your Home video

What is a backwater valve?

A backwater valve (sometimes called a backflow valve or sewer backup valve) is a valve you can install on the main line that carries wastewater (sewage) from your house to the public sewer system. Backwater valves allow water or sewage to flow only one way — away from your house. Sudden heavy rainfall can overwhelm city sewer lines, causing water or sewage to flow back towards your home, known as a sewer backup. A backwater valve prevents sewage backups of this nature.

Backwater valves are required by some municipalities and recommended by others. Some municipalities offer subsidies to assist with the installation of backwater valves — check out the links at the end of this article.

The risk of water backup increases if there is a basement in your home, or if the ground floor is less than 30 cm above street level. If a new home has any fixtures located below street level, The National Plumbing Code requires a backwater valve.

Backwater valves are not to be confused with backflow preventers. Backflow preventers are devices installed on the freshwater lines which prevent water there from flowing in reverse. Same concept, but different device.

How much do backwater valves cost?

Backwater valves can be installed in the initial new construction or retrofitted into existing homes. Installing during the initial construction is naturally much cheaper, costing around $500. To retrofit a backwater valve, a section of the home’s foundation needs to be removed to access the main sewer line. The cost to retrofit a backwater valve ranges from $2,000 to $5,000.

How does a backwater valve work?

What a backwater valve looks like on the inside

Your home’s sewer system allows water and sewage to flow out of the house. A backwater valve stops water or sewage from flowing into your house should the main sewer line become overloaded.

Inside the valve is a small flap that is normally open, allowing water to exit your home and any sewer gases to vent. There is a small floatation device on each side of the flap. If water or sewage starts to flow backwards into the house, the floaters cause the flap to lift up and close, thus preventing anything from entering your home.

When the water stops coming back towards the house, gravity will allow the flap to fall into the open position again, allowing water and sewage to resume flowing out of the house.

When you retrofit a backwater valve onto your existing home (as opposed to installing it during construction), you’ll need a plumbing permit from your municipality. During installation, a plumber will have to cut a hole in the concrete floor, usually near the floor drain. Then, they’ll dig down to the main sewer line, cut out a portion, and replace it with the new valve.

The valves often have a clear top so you can see if it is operating properly. There’s also a lid that you can remove for cleaning. Without a properly functioning backwater valve, sewage could come into the basement through a floor drain, sinks, tubs, and toilets.

What should homeowners know about backwater valves?

Without a properly placed and installed backwater valve, sewage could come into the basement through a floor drain, sinks, tubs, and toilets.

Before you install a backwater valve in your home, talk to your municipal government to find out if any permits are required, as well as to find out what sort of equipment they recommend.

Check that your drain and downspouts are not connected to the weeping tiles and sanitary sewer. In most cases, if you disconnect the foundation drain, your home will require the installation of a sump pit and pump. Talk to a plumber and your local government department for advice on how to properly disconnect downspouts and foundation drains. Make sure eavestroughs, once disconnected from the sewer system, drain away from your home (but not directly toward your neighbour’s property).

Related articles

If your home doesn’t already have a backwater valve, you’ll probably have to hire pros to install it. A licensed plumber can both install the valve and can also obtain any necessary building permits. Municipal governments sometimes maintain lists of pre-approved plumbers.

Take a look at this video by the Insurance Bureau of Canada on how a backwater valve is installed, and how it works.

Whether its a new installation or an existing one, know where the backwater valve is in your home.

Dave Smythe, CEO of Out Of This World Plumbing, adds, “[homeowners] need to be aware that access will be needed to the valve once it is installed.

“People often forget this and cover them with flooring. This stops the ability for inspection and maintenance as well as losing access to the drains for clearing blockages if they occur. Drains can still block and cause a backup even with a backwater valve installed. Underground piping in the basement may require the expected installation location to change.”

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Backwater valve maintenance

You should check at least annually to remove any debris that could clog the valve and to make sure all moving parts have free movement. Most valves have an easy-to-remove cover, to allow quick cleaning. But be careful — some experts recommend running some hot soapy water down your sink first to make sure the system is fairly clean.

Always wear rubber gloves and use a long-handled brush to scrub around and under the flap. Most backwater valves will have manufacturer’s recommendations regarding maintenance.

You’ll also want to look at the O-ring around the lid to make sure it’s in good condition. If not, it should be replaced to ensure a proper seal. Also, check the floats at each side of the flap, and replace if necessary. If these are worn, they will lose their ability to float, and won’t be able to lift the flap when needed.

To keep everything flowing smoothly, don’t flush un-flushable things, including so-called “flushable wipes” or diaper liners. By disposing of these things in the garbage, rather than in the toilet, you’ll save your system from performing extra work, and will hopefully prevent the system from getting clogged.

If you’re unsure about the condition of the valve, or are not comfortable attempting it yourself, you can call a plumber to do an inspection.

Backwater valve repair and troubleshooting

If you have a properly positioned and installed backwater valve, you shouldn’t have many problems — as long as you have been maintaining it properly.

Things can get stuck inside the valve, preventing it from closing. Sharp items moving through can also damage the valve. Proper, regular maintenance can catch these issues before there is a serious problem.

Backwater valves are usually easily accessible. They have a clear top, so you should be able to see if water is flowing freely or if anything is stuck. You’ll find plenty instructional videos to help you DIY the maintenance. Or, you can call an expert if the thought of sticking your hand in a sewage pipe is not particularly appealing. If you do attempt to clear a block on your own, be sure to follow all the instructions carefully—especially wearing gloves.

If everything is clear, and the city sewer backs up during a major rainstorm, your backwater valve will close, which is exactly what you want to happen.

But remember:

When the valve is closed, water can’t flow out of your house either. There is a certain amount of storage space in your plumbing system to account for this. But, you may not want to shower and run your washing machine or dishwasher all at once during a major rainstorm or snowmelt event. Since none of your wastewater will be able to escape with the backwater valve closed, you could wind up flooding your own house.

How do you tell if your home has a backwater valve installed?

Since they’re often buried in the basement floor somewhere, it can be hard to tell if your home has a backwater valve unless you already know about it.

“To see if you have a backwater valve installed in your home,” says plumbing expert Dave Smythe, “they are usually located very close to where the drain piping leaves the home in the basement level. Most house drains exit at the front of the property but can be different for corner lot homes. The main water valve and water meter are usually in the same area as the piping runs in the same trench.

“Backwater valves may be hidden as they are normally installed in a box with a cover and could be obscured by carpets, boxes or any floor finishings that may be put down in the area. The box cover is usually about 10 by 14 inches in size and needs to be removed to expose the backwater valve itself.”

Another way to tell is to check your home inspection report (if you happen to have one).

What will your home insurance provider want to know?

Your home insurance provider may want to know if there is a backwater valve installed in your home, especially if it’s located in an area with a high risk of water backup.

The presence of a backwater valve could have an effect on the rate charged for your home insurance. Some home insurance providers may even require you to have one installed to be covered for any damage related to water backup.

Keeping your home dry and safe is important to you and your insurance provider. The cost incurred to install a backwater valve may be a lot less than the cost (and the headache) involved in repairing the damage to your home and valuable possessions caused by a water backup.

Commonly asked questions

What is the life expectancy of a backwater valve?

Your backwater valve should last many years with proper maintenance. Follow the manufacturer’s recommendation and have the valve checked and cleaned once a year. And as mentioned above, keeping your system free of unnecessary materials will also help to extend the life expectancy.

Are there any municipal or provincial subsidies for backwater valves?

In some areas, homeowners can obtain subsidies to assist them with installing backwater valves. We’ve listed some links below. If your area is not listed, check with your municipality.




New Brunswick


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About the expert: Dave Smythe

Dave is the CEO of Out Of This World Plumbing. He has over 20 years of experience in the plumbing trade, is a certified home inspector, and a gas and water well technician. Dave has also been a speaker at the Canadian Association of Home and Property Inspectors national conference.


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