What is sewer backflow protection?
Sewer backflow protection typically utilizes a backwater valve
(aka “backflow prevention device”) on drainage pipes to protect the building from sewer backup. The backwater valve is designed to automatically shut when water flows towards the property. The graphics below show two scenarios where a backwater valve can protect the building from a sewer backup.
Does my property need a backwater valve?
Most properties have been built so that an obstruction in the public sewer or sewer lateral between the curb and sewer main should not cause a sewage backup into the property. However, some properties require the extra protection of a backwater valve within the owner’s property. Typically, these properties have been built so that the drain of the lowest plumbing fixture (shower, floor drain, toilet, etc.) is lower than the air vent (aka “fresh air inlet”) of the sewer lateral.
The San Francisco Department of Building Inspection’s 2016 Plumbing Code, provides the following description for properties that are required to install a backwater valve:
SECTION Level - 710.1.1 Drainage of Fixtures Located Below the Fresh Air Inlet.
Where a fixture is installed with the flood rim level equal to or lower than the elevation of the fresh air inlet serving the house trap, the piping serving the fixture shall be protected by installing an approved type of backwater valve. Where a fixture is installed with the flood rim level higher than the elevation of the fresh air inlet serving the house trap, the fixture shall not discharge through such backwater valve.
Consider installing a backwater valve.
If your property appears to require a backwater valve, we strongly encourage you to consider installing one. The San Francisco Department of Building Inspection Plumbing Division will be able to provide the following:
- further guidance on whether your property should have a backwater valve and
- directions on how to properly install a backwater valve.
Please visit their website at https://sfdbi.org/backflow-preventer.
You may also be be eligible for SFPUC’s Flood Water Management Grant Assistance Program.
Be sure to maintain your backwater valve!
If you have a backwater valve, or decide to install one, please note that it must be checked to ensure that it is operating properly at all times. Debris in the drain line or other problems can easily damage or interfere with backwater valves. Follow the manufacturer's recommended maintenance instructions and schedule. It is best practice to conduct maintenance before every rainy season.