How a backflow occurs

"Backflow" happens when a blockage in the sewer causes wastewater to back up. The overflow typically happens at the sewer manhole upstream from the blockage. But sometimes the overflow point is inside a building — which can happen if the floor level where a fixture is installed is less than one foot higher in elevation than the next upstream manhole.

Is my property at risk of a backflow?

To determine the potential for a backup of wastewater into a building:

• Determine the floor elevation of the lowest plumbing fixture in the building.
• Determine the elevation of the nearest upstream manhole.
• If the floor elevation of the lowest plumbing fixture is less than one foot above the next upstream manhole cover of the public or private sewer, the building is subject to backflow.

A builder’s level, also called a dumpy level, can help determine the relative elevations to see if a building requires backwater valve protection. Or a licensed plumber can determine these elevations. 

Basement plumbing fixtures - The location of plumbing fixtures in a basement does not automatically trigger the need for a backwater valve. In many instances, plumbing fixtures in a basement are pumped vertically to a building drain above, and the pump outlet is already provided with a check valve and gate valve that protects against backflow. If the basement fixtures drain by gravity, backflow protection may be required.

When is a backwater valve required?

A backwater valve is a plumbing fixture that acts as one-way stopper. It allows wastewater to flow away yet prevents it from flowing back up your pipe. See Diagram 1.

DIAGRAM 1 - If a sewer blockage occurs, a properly working backwater valve can help prevent a backflow of wastewater into the home or building.

Most buildings in San José do not need a valve, but yours may. A building must have an approved type of backwater valve if the floor elevation of the lowest plumbing fixture in the building is less than one foot above the next upstream manhole cover of the public or private sewer system. See Diagram 2.

DIAGRAM 2 - If the manhole cover is less than one foot below the floor level of the building, an approved type of backwater valve is required.

A licensed plumber can verify if a backwater valve is necessary and can perform the installation. A plumbing permit and inspection are required for the installation of backwater valves. NOTE: A backwater valve shall not be installed if backflow protection is not necessary.

An accessory “sewer relief valve” may also be installed upstream of the backwater valve. A sewer relief valve is not required and shall not be installed in lieu of the backwater valve.

Steps for preventing a sewage backup

Property owners and tenants can prevent most blockages and backups by following these steps:

1. Put oil, fats, and grease in the garbage, not the drain - Most blockages are caused by oil, fats, and cooking grease that harden and create a plug in the sewer pipes. Hot water will not melt away this plug. Be sure to scrape fats, oils and grease from plates and pans into a covered container and put it in the trash. Or store it in the refrigerator until garbage collection day.
2. Use the garbage disposal sparingly - Compost plant foods if you can. Put other food scraps in the garbage.
3. Do not plant trees or large shrubs near sewer lines - Roots can get inside the pipes through any tiny crack. Over time, a root ball can form and clog the pipe.
4. If applicable, inspect the backwater valve bi-annually - If your property has a backwater valve, it will be located on your property somewhere along the sewer pipe or lateral that extends from the home/building to the main city sewer line in the street. Most often, it is located near the building. Property owners are responsible for maintaining adequate sewage flow to and through their sewage pipe to the sewer main in the street.