Section 311.1/Backwater Valves
IPC 311 requires backwater valves on drainage piping where fixtures are below the elevation of the next upstream manhole or where the building sewer is subject to backflow.
Section 311.1 requires a backwater valve on the building drain or building sewer where any fixture drain, branch, or stack is installed below the elevation of the manhole cover of the next upstream manhole in the public sewer. The backwater valve must be installed so that the valve is accessible for service and repair. Backwater valves must comply with ASME A112.14.1 or CSA B181.1. The valve must be a normally open type (allowing gravity flow under normal conditions) and must close automatically when backflow pressure is detected. Section 311.1.1 requires the backwater valve to be installed in an accessible location, typically in a valve pit or access box at grade. The valve must not be installed in a location where sewage backup could cause damage before the valve activates. Where a backwater valve is installed, a relief port or overflow must be considered for fixtures above the flood level that could be affected if the valve is closed and the building continues to generate flow above the valve.
Why this section exists
When the public sewer surcharges during heavy rain or a downstream blockage, sewage can flow backward through the building sewer and flood basement fixtures, floor drains, and lower-level spaces. The resulting damage includes contaminated finishes, destroyed contents, and public health hazards from raw sewage exposure. Backwater valves prevent this reverse flow by automatically closing when backpressure is detected. The requirement applies specifically to fixtures below the upstream manhole cover elevation because these fixtures are hydraulically below the potential flood level of the public sewer and are the most vulnerable to backflow.
What plan reviewers look for
Plan reviewers check the plumbing site plan for the building sewer elevation relative to the upstream public sewer manhole cover. If any fixtures are below the manhole cover elevation, they verify a backwater valve is shown on the building drain. They check the valve location for accessibility. They verify the valve type (normally open, automatic closure). They check whether fixtures above the backwater valve have an alternative drainage path in case the valve closes during active building use. For buildings with both above-grade and below-grade fixtures, they may check for separate drainage systems (one protected by the backwater valve, one discharging freely).