Homeowners Guide to Backwater Valves

Flowing Water

7 things Durham Region residents need to know about backwater valves

If you own a home in Durham Region, below are 7 important things you need to know about backwater valves and your legal responsibilities.

All basements are at risk of flooding- whether due to a sudden downpour, a large amount of melting snow or other weather related factors. Having clean water backing up in your basement is bad enough. Now imagine the flood in your basement isn't rainwater, but rather dirty, contaminated sewage. Fortunately there is a way to help  prevent this disaster from happening- by installing a backwater valve.

Read below as we take you through the basics of what backwater valves are, how they work, reasons you might need one, how to tell if you already have one, a getting started checklist, trouble-shooting and maintenance tips.

1. What is a backwater valve?

A backwater valve (sometimes called a backflow or sewer backup valve) is a one-way valve that can be installed on your sewer line. It is designed with a flap that only allows water or sewage to flow one way- out of your house. The valve will automatically shut to prevent any leakage if the public sewer line becomes backed up.

A city's sewer lines can become overwhelmed by a sudden heavy rainfall or sanitation backup, causing water or sewage to flow back towards your home. Simply put, if there is a sewer system backup, a properly installed backwater valve will help prevent sewage from flowing back into your house.

Backwater valves are required by some municipalities, while recommended by others. Please see the end of this article with information specific to Durham Region's backflow prevention by-law.

Backwater valves can be installed in new construction, or can be retrofitted into existing homes. Installation during new construction is easier and generally costs under $500.

Retrofitting a backwater valve can range upwards of $1,000 to $3,000. Some municipalities offer a subsidy to help financially assist with the installation of backwater valves. Click here to read about Durham Region's "Basement Flooding Loan Program Resulting From a Sanitary Sewer Backup."

2. How does a backwater valve work?

Not to be confused with a backflow preventer, which prevents the backflow of clean water back into a water source, a backwater valve prevents the backup of sewage water into the drainpipe, which would otherwise flood your basement or wherever the drain pipe connects.

Inside of a backwater valve is a small flap. The flap is usually open, which allows water to exit your home and sewer gases to ventilate. On each side of the flap there sits a small flotation device. These floaters will cause the flap to lift and close if water or sewage starts to flow back into the house.

Once the backup is resolved and water or sewage are no longer flowing back to the house, gravity causes the flap to fall back into the open position. This allows the normal function of water and sewage to begin flowing out of the house once again.

If a backwater valve is being retrofitted into a home, a plumbing permit from your local municipality is required. A licensed plumber will then cut a hole into the concrete floor, usually near the floor drain. They will dig down to the main sewer line and replace a portion with the new valve.

The cost for this service should include the breaking up of concrete, installation of the backwater valve, refinishing of concrete and removal of all debris. Cost may increase based on unforeseen circumstances, such as severely degraded or damaged piping or blockages.

Once installed, the cover of the backwater valve must be accessible for servicing.

If a backwater valve isn't placed or installed correctly, sewage can come into the basement through a floor drain, sinks, tubs, and toilets. The importance of this is stressed in an article produced by "The Institute for Catastrophic Loss Reduction" called "Focus on Backwater Valves."

In it they state, "If placed in the wrong location relative to other plumbing fixtures on the sewer lateral, the valve could be bypassed and provide no protection. If the valve is placed in the wrong location, sewer backup pressure could build and crack the basement floor and lead to flooding.

Additionally, if the foundation drain is still connected to the sanitary sewer lateral (or to a combined lateral) downstream of the valve, sewage could be forced into the foundation drain and lead to structural damage, which may also lead to infiltration flooding and other problems."

3. Reasons you might need a backwater valve

There is an increased risk of water backup if a home's ground floor is less than a foot above street level. Below are additional circumstances that may require a backwater valve.

By-Laws and Regulations

The National Plumbing Code requires a backwater valve to be installed on all new buildings when fixtures are installed below the adjoining street (fixtures subject to backflow).

Depending on where you live, there might be municipal by-laws that require a backwater valve in your home. You can usually do an online search to easily identify if you live in one of these areas. You should also be able to find out if there are any subsidies or rebates being offered in your area for the installation of a backwater valve. Be sure to take advantage of backwater valve rebates your city might provide. We have provided a link near the end of this article with by-law and rebate information for Durham Region.

You Live in a Flood-Prone Area  

There are approximately 10.9 million private residences in Canada. The Insurance Bureau of Canada (IBC) estimates that just over one million of these homes (around 10%) are at high risk of flooding, and a portion are at very high risk of repeated flooding over the next 20 years.

For many Canadians, the equity in their home is a large part of their planned retirement savings. When our homes are at risk, our financial security is too. Homes that flood frequently may become less valuable, and could become subject to restrictions around insurance and even mortgages.

If your area experiences heavy rainfall or flash floods, a backwater valve is one of the best investments you can make. Even with newer or well-maintained municipal sewage systems, extreme weather can cause an overload. If that happens, your basement is at risk for being flooded with sewage backwater, causing an enormous amount of damage and clean-up.

You have a sump pump  

If your home has a sump pump below the water table, you're likely aware of the added risk that comes with extreme weather. Having a backwater valve Installed is a preventative measure that adds to the overall security of your home. A backwater valve also acts as a backup in the event your sump pump fails.

4. How do you know if you already have a backwater valve?

Backwater valves are usually located in the floor of your basement. They usually have a round or rectangular access panel that can be removed for maintenance.

5. Backwater valve checklist

To better understand the requirements and proper placement and installation of your backwater valve, review the following checklist:

  • Have a thorough plumbing inspection by a qualified professional completed- a crucial and highly recommended first step.
  • Talk to your municipal government or perform an online search to determine if there are any required permits, rules, regulations and/or equipment requirements.
  • Check to ensure your drain and downspouts are not connected to weeping tiles or sanitary sewer. In most instances, the disconnection of the foundation drain will require the installation of a sump pit and pump.
  • Consult with your local municipality to learn how to properly disconnect downspouts and foundation drains. Better yet, hire a professional plumber to do it for you.
  • A licensed plumber can acquire any necessary permits and install the backwater valve. Some municipalities maintain a list of pre-approved plumbers. Refer to Durham Region's list of pre-approved plumbers.
  • Ensure any disconnected eavestroughs drain away from your house, as well as your neighbour’s home.

6. What can go wrong with a backwater valve?

If your backwater has been correctly positioned and installed, there should be minimal issues. Proper maintenance is required, which will keep things running smoothly. Things can get stuck, preventing the valve from closing, and sharp objects can cause damage to the valve.

Proper, regular maintenance can catch these issues before there is a serious problem. Backwater valves are usually easily accessible. Normally equipped with a clear top, you should be able to see if water is flowing freely or if anything is stuck.

Some homes experience flooding from a sanitary or storm sewer backup, even with a backwater valve in place. In 2011, the City of Ottawa released a study of the effectiveness of backwater valves. It was the result of a July 2009 extreme rainfall event that resulted in approximately 1,500 basement flooding incidents in the west end of the city- or almost 8% of homes with backwater valves.

A review was undertaken by Ottawa to understand the situation better. It was determined that the main cause of failure of the mostly ‘normally closed’ backwater valves, concerned the valve cover. One third of the backwater valve covers inspected were not screwed down tightly enough or were cross-threaded.

One of the most common causes for backwater valve failure is nothing to do with the actual valve. It's the failure to disconnect foundation drains that are connected to the sanitary or combined sewer system.

If the foundation drain of a home is still connected to the sanitary or combined sewer lateral downstream of the valve, sewage could be forced back into the weeping tiles and lead to structural damage of the foundation. It could also cause infiltration flooding.

When a home's foundation drain is connected to the municipal sanitary or combined sewer system, the home’s foundation drains must be disconnected from that system and redirected to a sump pit, before installing a backwater valve. If this step is missed, flooding can occur as the backwater valve closes.

When the valve is closed, water cannot flow out of your house. There is a certain amount of storage space in your plumbing system to account for this, but you may not want to shower and do laundry simultaneously during a major storm, or extreme snow melting. Since none of this water will be able to escape once the backwater valve is closed, you could wind up flooding your own house.This is otherwise known as ‘self-flooding’.

7. Tips for maintaining your backwater valve

If a backwater valve is not properly maintained, sanitary waste and/or storm water can leak into a home during extreme rainfall or sanitary backup. Homeowners need to be educated about proper inspection and maintenance of the valves to ensure their effectiveness.

Most backwater valves come with a see- through top. This way, the valve can easily be inspected to see if there are any clogs from debris or if the valve is otherwise not functioning properly. It's always a good idea to schedule a qualified plumber to handle the annual inspection and maintenance. The preferred timing for maintenance is in the early spring before snow begins to melt.

Whether performing the inspection yourself or hiring a plumber, a checklist of what needs to be done includes:

  • Removal of the valve's round cap. Using a flashlight, check for any signs of debris.
  • Debris usually accumulates in the spring and summer, during the heaviest rain and flood season.
  • Make sure to wear gloves and protective eye wear. Sewage is toxic, and exposure to your skin or eyes could result in serious health issues.
  • Some experts recommend running hot soapy water down the sink first to help ensure the system is as clean as possible.
  • If there is debris in the valve, try to flush it with water. For difficult to remove debris, use a brush with a long handle to thoroughly scrub the valve.
  • Inspect and replace the O-ring if necessary.
  • Lubricate valve hinges.
  • Ensure the valve gate moves freely.
  • Properly reinstall the clean out plug when finished.
  • Make sure any removed covers or lids are properly tightened and put back in place.


Installing a backwater valve not only helps protect your home from a sewage backup, it's also the law in many municipalities. A sewage backup can cause disease and major property damage, resulting in lost time, money and a deterioration in health. A professional plumbing inspection should always be completed before installing a backwater valve in your home.

Contact Vaillancourt Plumbing & Drains today. Visit us online at www.vaillancourtplumbing.ca for all of your backwater valve needs in Durham Region.