What is a backwater valve?

What is a backwater valve used for?

A backwater valve is used to keep sewage from coming back into a building through its own drains when the sewer line outside is blocked or overloaded. Santa Barbara, California and Portland, Oregon each describe that job, and Marysville, Ohio spells out what it protects: “A backwater valve may prevent a sewer backup from occurring on your property.” Source: City of Santa Barbara, California, Public Works, read 2026-10-30. “When installed correctly, the valves prevent sewage from re-entering the structure.” Source: City of Portland, Oregon, Permitting & Development, read 2026-10-30. “Valves prevent drainage or waste from backing up into a lower level or fixtures and causing a flooding condition.” Source: City of Marysville, Ohio, Division of Wastewater, read 2026-10-30.

How do city utilities define a backwater valve?

Every city and utility page we read describes the same thing: a device in the drain or sewer pipe that lets waste leave the building but not return. Ashley Valley, Utah says it in plain words, Marysville, Ohio says where it sits, and Portland, Oregon adds that it is not the same device as a drinking-water backflow preventer: In Ashley Valley, Utah, the district's Q&A says: “A backwater valve is a back flow prevention device that is designed to allow water or sewage to flow in only one direction, out of your house.” Source: Ashley Valley Water & Sewer Improvement District, Utah, read 2026-10-30. In Marysville, Ohio, the city says: “A backwater valve is a device installed in the building drain or sewer pipe where a sewer is subject to backflow.” Source: City of Marysville, Ohio, Division of Wastewater, read 2026-10-30. Portland, Oregon adds: “A backwater valve is not to be confused with a backflow prevention assembly.” Source: City of Portland, Oregon, Permitting & Development, read 2026-10-30.

How does a backwater valve work?

A flap or gate inside the valve swings open for the normal flow of wastewater out of the house and shuts when flow tries to reverse. Ashley Valley's diagram caption describes the flap, Philadelphia says the same in one line, and Portland quotes an Oregon code rule on how the valve must behave in low flows: “When water flows through the system (green arrows), the flapper opens allowing waste to flow from the home/building to the sewer.” Source: Ashley Valley Water & Sewer Improvement District, Utah, read 2026-10-30. “In the event of a backup (red arrows), the flapper will seal preventing sewage backup into your home/building.” Source: Ashley Valley Water & Sewer Improvement District, Utah, read 2026-10-30. In Philadelphia, Pennsylvania, the city's program page says: “Backwater valves let wastewater out of your home, but close to prevent water from coming back through your pipes.” Source: City of Philadelphia, Pennsylvania, Water Department, read 2026-10-30. In Oregon, the plumbing code text printed by Portland says: “Such backwater valves shall remain open during periods of low flows to avoid screening of solids and shall not restrict capacities or cause excessive turbulence during peak loads.” Source: City of Portland, Oregon, Permitting & Development, read 2026-10-30.

Where is a backwater valve located on a house?

The usual spot is on the sewer line where it enters the building, though it can sit anywhere along the line between the house and the street sewer, as long as it can be reached for service. Ashley Valley, Utah describes the spot, and Marysville, Ohio requires a place from which the valve can be reached for service: “The most general location for a backwater valve to be installed is where the sewer line enters the household or building.” Source: Ashley Valley Water & Sewer Improvement District, Utah, read 2026-10-30. “Backwater valves can be effective when installed anywhere on the sewer lateral in between the home and the city sewer line.” Source: Ashley Valley Water & Sewer Improvement District, Utah, read 2026-10-30. In Marysville, Ohio, the city says: “The valve shall be placed in a location that allows for future maintenance access.” Source: City of Marysville, Ohio, Division of Wastewater, read 2026-10-30.

Do I need a backwater valve?

It depends on how high your lowest drain sits compared with the next manhole up the street sewer. Where a fixture is below that manhole, local plumbing codes call for a valve. Ashley Valley, Utah explains the rule, Portland, Oregon quotes the code, and Marysville, Ohio applies it to new homes: “Plumbing code require backwater valves when a fixture is installed on a floor that is below the next upstream manhole.” Source: Ashley Valley Water & Sewer Improvement District, Utah, read 2026-10-30. “If the next upstream manhole is higher in elevation than your first floor or lowest fixture, you will have sewage back flowing into your home.” Source: Ashley Valley Water & Sewer Improvement District, Utah, read 2026-10-30. In Oregon, the plumbing code text printed by Portland says: “Fixtures on such floor level that are not below the next upstream manhole cover shall not be required to be protected by a backwater valve.” Source: City of Portland, Oregon, Permitting & Development, read 2026-10-30. “All new homes and businesses built within the City of Marysville, with public sewers and basements where the lowest drain opening is lower than the manhole cover, are required to install a backwater valve.” Source: City of Marysville, Ohio, Division of Wastewater, read 2026-10-30.

What types of backwater valve are there?

Ashley Valley, Utah sorts them into two kinds by what the flap does when nothing is flowing: normally open valves, whose hinged flapper rests open and floats shut on reverse flow, and normally closed valves, whose gate is pushed open by outgoing flow: For the normally open kind, Ashley Valley, Utah says: “It floats up and closes only when a reverse flow occurs.” Source: Ashley Valley Water & Sewer Improvement District, Utah, read 2026-10-30. For the normally closed kind, it says: “If a reverse flow occurs the gate is already in the normally closed position, stopping the reverse flow from entering your home.” Source: Ashley Valley Water & Sewer Improvement District, Utah, read 2026-10-30. “Since these valves are normally closed they may also help with odor (caused from a low flowing city main line that becomes septic) from entering your home.” Source: Ashley Valley Water & Sewer Improvement District, Utah, read 2026-10-30.

How often does a backwater valve need maintenance?

Santa Barbara, California recommends looking at the valve at least every six months, and more often after sewer blockages. Philadelphia, Pennsylvania adds that after a program warranty ends, upkeep falls to the resident: “It is recommended that you check and maintain the valve at least every 6 months.” Source: City of Santa Barbara, California, Public Works, read 2026-10-30. “If you have a history of sewer blockages, you should check more often.” Source: City of Santa Barbara, California, Public Works, read 2026-10-30. In Philadelphia, Pennsylvania, the program page says: “Residents are responsible for repairs or maintenance after the warranty expires.” Source: City of Philadelphia, Pennsylvania, Water Department, read 2026-10-30.

Sources

• Backwater Valves — City of Santa Barbara, California, Public Works; read 2026-10-30.
• Backwater Valves — City of Portland, Oregon, Permitting & Development; read 2026-10-30.
• Backwater Valve — City of Marysville, Ohio, Division of Wastewater; read 2026-10-30.
• Sewer Backwater Valve Q&A — Ashley Valley Water & Sewer Improvement District, Utah; read 2026-10-30.
• Basement Backup Protection Program — City of Philadelphia, Pennsylvania, Water Department; read 2026-10-30.