Backwater Valve

Sanitary systems typically drain by gravity towards the wastewater treatment plants. When a large rainfall occurs, rainfall can enter the City's sanitary sewer in a number of ways, including direct sewer connections, infiltration, through manhole covers, etc. This rainfall can cause the sewer system to surcharge and back up into homes, resulting in basement flooding.

Backwater sanitary valves (also known as “check valves” or “backwater valves”) are mechanical devices that are designed to allow the flow of water in one direction away from your home. Used on a sanitary lateral, they can reduce the risk of sewage backup if installed properly and maintained adequately.

What is a Backwater Sanitary Valve and How Does it Work?

A backwater sanitary valve is a type of check valve that is designed to only allow flow in one direction. Different backwater sanitary valves work in different ways, but in general, the type of device that is used in sanitary sewer scenarios work like this:

  • The valve is normally in an open position, i.e. the "gate" (or "flap") is open.
  • When a backflow condition occurs, the water moves under the "gate" lifting it up. If the backflow condition worsens, the "gate" will close against the gasket, creating a seal that will stop the backflow from the sewer.
  • When the backflow condition ends, the "gate" will return to the open position, allowing normal outflow of sewage from the home to the City's sewer.

Graphic showing how the backwater valve works

Installing a Backwater Sanitary Valve

Installation should be completed by a licensed plumber and inspected by a municipal plumbing inspector. Here is what to expect when you install a backwater sanitary valve.

The location of the installation needs to be downstream of all fixtures to offer full protection. If the backwater valve is located upstream of any basement fixtures, there is a possibility of sanitary sewage exiting through that fixture under backflow conditions. The best way to confirm that the backwater sanitary valve is downstream of all fixtures is to camera the underground system to ensure that there are no fixtures tied in downstream of the location for the backwater valve.

There are a couple different types of backwater sanitary valves, and they are installed in different ways. The most common variety is called a Mainline Full-port Backwater valve, and it is installed by cutting a hole in the foundation floor inside the home above the main line sewer line to expose it and allow for the installation. There is another less common variety that can be installed somewhere on the sanitary lateral outside the house.

For more information, please review this video of the installation of a backwater sanitary valve at the "Climate Resilient Home."

Installing a backwater valve video still

The Basement Flooding Protection Subsidy Program includes incentives for the installation of Backwater Sanitary Valves.

Please learn more about backwater valves and the Basement Flooding Protection Subsidy Program.

Does a backwater sanitary valve guarantee that a backup will not occur?

No. Unfortunately, while they can offer a lot of protection, there is no guarantee that a sewage backup will not occur because of a number of factors:

  • Installation - The device needs to be installed correctly, including the location, orientation and position. If the plumber is not diligent in ensuring that the manufacturer's specification are followed, the device may not operate correctly. It is therefore worthwhile taking the time to understand the requirements of the device being installed and verify the contractor's workmanship. Some issues are as follows:
    • Location: The device must be located downstream of all sanitary fixtures, yet upstream of any connection from the foundation drain.
    • Orientation: Each device has directionality to it, in that it will only work if oriented correctly. Devices, therefore usually have clearly illustrated arrows on them.
    • Slope: The device likely has a minimum slope requirement to ensured for proper function.
  • Your foundation drainage complicates things - If your home's foundation drainage is collected by a sump and then discharged to the lawn, a backwater sanitary valve has a good chance of being suitable. If your foundation drain is connected to the sanitary sewer, the best scenario is to sever that connection. A connected foundation drain is problematic for two reasons:
    • If the connection is downstream of the backwater sanitary valve, during a sewage backup it will not backup into your home but will backup into the drainage materials around your foundation, including the weeping tile.
    • If the connection is upstream of the backwater valve, then in all likelihood, when the valve closes during a rainfall event, the drainage of groundwater around your home may back up into your basement, since it cannot get away.

This figure shows the proper location of a backwater sanitary valve and disconnected weeping tiles (courtesy of Institute for Catastrophic Loss Reduction [ICLR]).

A backwater sanitary valve installation showing it's correct position and disconnected weeping tile.

For these reasons, the basement flooding protection subsidy program includes incentives for both the backwater sanitary valve and sump pump with overflow.

  • Maintenance - Backwater sanitary valves are not maintenance-free items. They are mechanical devices in a dirty environment, requiring regular maintenance and cleaning to best ensure they will operate properly during a backflow condition. Manufacturer's recommendations for the type and frequency of maintenance should be followed.

Putting stuff down drains or flushing stuff down a toilet that shouldn't be there can cause materials to hang on to the "gate" or gasket, causing the "gate" not to seal properly under backflow conditions. To learn more about what not to put down the drain, check out the "Flushable wipes" and Fat, Oils and Grease pages on the City's website or the Wastewater: Where Does it Go? video.

  • Knowledge - Take a moment now to consider a closed backwater sanitary valve. During a condition where the "gate" is closed, backwater cannot get into your home. This is great and what you want, but unfortunately, during this condition, the opposite effect is also true, i.e. sewage from your home cannot get out. Your home's internal plumbing has limited capacity. If the gate is closed, and you happen to take a shower and run the laundry at the same time, this may rapidly fill up the space in your plumbing, and if it is too much for your plumbing to hold, it will start backing up into your own home at the lowest available fixture - usually a floor drain or a basement shower stall. Knowledge is key. During heavy rains is not a good time to use a lot of water within your home.

For more information on environmental initiatives:

Phone: For general information, call 311. For detailed inquiries, call 519-255-6100 ext. 6127.