Can You Put a Check Valve on a Sewer Line?

You can put a check valve on a sewer line, and in many situations, you should, but you need the correct type of sewer check valve, the right location, and code approval. Property managers, homeowners, engineers, and plumbers all use these valves to reduce the risk of sewage backup from city sewers  or a septic tank into a building during heavy rain, flooding, or mainline blockages.

Heavy storms and surcharged municipal systems push sewage backward through the building drain and into lower fixtures such as basement toilets, floor drains, and tubs. That reverse flow creates property damage, contamination of finishes and contents, and serious health risks from bacteria and mold.

A properly selected sewer backwater valve or sewer check valve shuts automatically during reverse flow and keeps wastewater moving in only one direction when the system operates normally.

What Is a Check Valve and How Does It Work?

A check valve is a one-way valve that allows liquid to move forward but closes when pressure tries to push it backward. In sewer applications, the valve sits in line with the drainpipe and opens when sewage flows toward the city sewer or septic tank, then closes when backpressure from the main tries to force sewage back.

Swing check valve and flapper-style designs use a hinged disc or flap that swings open under normal flow  and swings shut when reverse flow pushes against it. A ball check valve uses a coated ball that lifts off its seat under forward pressure and reseats to stop reverse flow when the pressure drops or reverses.

Every design relies on a specific cracking pressure, which is the minimum pressure needed to unseat the internal mechanism and start flow  through the body of the valve. Sewer systems operate as gravity-based systems, so installers must orient sewer valves correctly and maintain proper slope so gravity, not pumps, drives the flow through the sewer line.

Water-main or potable-water check valves often protect clean supply water from contamination, while sewer-rated check valves and backwater valves handle solids, grease, and other debris in sewage. You should never substitute a valve designed for clean water in a raw  sewage  application because the internals and clearances are different.

Can You Legally Install a Check Valve on a Sewer Line?

Plumbing codes generally allow or require a sewer backwater valve when fixtures sit below the elevation of the next upstream sewer cover on the public sewer. The International Plumbing Code and Uniform Plumbing Code require backwater valves on drainage piping that serves fixtures located below the flood level rim elevation, which often includes basements and split-level lower floors.

Authorities consider a sewer check valve or sewer backwater valve a critical safeguard, primarily where the risk of backups from the city sewer or private main exists.

Many jurisdictions require a permit and an inspection for backwater valve installation, and inspectors verify location, accessibility, and labeling prior to approval. Local amendments vary, so every project team must consult with the local plumbing department or code official before installing any valve on the main building sewer.

When a Sewer Check Valve Makes Sense

A sewer check valve makes the most sense for buildings that are below street level, rely on basements with plumbing, or connect to combined sewer systems that carry stormwater and sewage. Those properties have a higher risk of sewage backup during storms because the city sewer can push wastewater backward into private laterals.

Older buildings without modern backflow protection or those that connect to a septic tank or lift station also benefit from properly chosen sewer valves. Property managers protect ground-level apartments, commercial basements, and storage areas that contain high-value equipment by installing a dedicated sewer backwater valve on the building drain.

Even a single event of sewer backup often costs far more than the price of the valve and installation.

Risks of Sewer Check Valves

Sewer check valves carry tradeoffs, and owners need realistic expectations. Solids and debris in sewage can catch on the flapper, ball, or swing mechanism and cause partial obstruction or prevent full closure, especially when users skip regular cleaning.

Any valve in a gravity sewer line adds some resistance to flow and can restrict high-volume discharge if the design or sizing doesn’t match the system’s demand. Manufacturers and installers recommend regular inspection, cleaning, and testing to keep the internal components moving freely. Installers also provide an access panel or riser over the valve  so maintenance crews can service it without breaking concrete or excavating the yard.

Types of Check Valves Used in Sewer Lines

Backwater Valves

A backwater valve is a sewer-specific check valve that prevents sewage from flowing back from the public sewer or septic line into the building. The valve usually uses a swing check design with a hinged flapper that hangs open during normal forward flow and automatically closes when reverse flow or backpressure develops.

Unlike general-purpose check valves used on a water main, backwater valves handle suspended solids, toilet paper, and other debris in sewage without jamming as easily. Contractors install these valves on the building’s sewer near the point where the line leaves the foundation, which protects all interior drains from surcharging city sewer lines or overloaded septic systems.

Residential and commercial buildings use backwater valves widely in flood-prone areas and neighborhoods that experience repeated basement sewage backup events.

Swing and Flapper-Style Sewer Check Valves

Swing and flapper-style sewer check valves work well on horizontal drain lines because gravity helps the flap return to the closed position when flow stops. During normal discharge, sewage pushes the flap upward and flows under it. When the flow stops or reverses, the flap drops back onto the seat and seals against reverse flow.

Manufacturers build these valves from corrosion-resistant materials such as PVC, ABS, and coated cast iron that withstand the chemical and biological load in sewage. Many sewer-rated swing check valves include a clear or removable top cover, so plumbers can inspect the internal parts and clean out debris without cutting the line. Engineers choose between designs based on system slope, expected flow rates, and compatibility with the existing drainpipe material.

Installation Considerations for Sewer Check Valves

Installers typically place the sewer check valve or sewer backwater valve on the main building drain just inside the foundation wall or as close as practical to the potential entry point of the backflow. That location ensures the valve protects all downstream fixtures and remains accessible for inspection and maintenance through a cleanout or access box.

The valve must match the diameter of the sewer line and sit in a nominally horizontal run with the proper slope, so gravity still carries the sewage to the city sewer or septic tank. Installers align the directional arrows on the body with the direction of normal flow and keep the access opening at the top.

Designers also check that the valve body and joints work with the chosen pipe material, whether cast iron, clay, or modern PVC pipe. For plastic systems, many contractors prefer solvent-welded connections rather than flexible couplings when the head pressure exceeds recommended limits.

Common Mistakes to Avoid

Several common mistakes reduce the effectiveness of a sewer check valve. Some projects use a valve rated for clean water instead of sewage, which increases the risk of clogging and failure because the internal workings can’t pass solids properly. Other installations bury the valve without an access riser or panel, which makes essential inspection and cleaning nearly impossible and leads to unnoticed failures.

Skipping permits or ignoring local plumbing code requirements can create liability issues and force costly rework if inspectors require relocation or replacement. Designers also avoid installing multiple valves in series on the same lateral without engineering review because unnecessary valves complicate maintenance and can trap debris.

Is a Sewer Check Valve Right for Your System?

A sewer check valve often justifies the investment when a building sits below street grade, connects to a combined or historically unreliable city sewer, or has already experienced a sewage backup. The valve improves long-term plumbing performance by preventing reverse flow events that erode piping, contaminate living spaces, and disrupt business operations.

If you decide to install a sewer backwater valve or related sewer valves, follow local plumbing codes, secure permits, and rely on qualified installers who understand gravity sewer design and cracking pressure requirements.

Property managers and contractors can source sewer-rated PVC check valves from Maxx Supply’s catalog of PCV check valves along with compatible PVC pipe. For projects that require more detailed selection guidance on swing check valve designs, engineers can review Maxx Supply’s swing check valve blog.