The maximum trap seal depth is 4-inches according to the International Residential Code (IRC P3201.2). A trap seal is the standing water in the U-shaped pipe under a sink that keeps sewer gas from traveling backwards through the drain piping and up through the sink into a home. It is measured as shown in the diagram below. Two problems are created when the trap seal exceeds 4-inches: drainage slows down, allowing more debris to collect in trap, and the trap is more likely to become clogged. 

    Installing a trap backwards is the most common way that a trap seal exceeds the 4-inch max. See our article Why is it a problem when a trap under a sink is installed backwards? for more on this. 

    Conversely, the minimum trap seal depth is 2-inches. A trap seal that is too shallow is more likely to be siphoned away during normal usage by the suction created downstream in the pipe, and is more susceptible to loss by evaporation.

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Here’s links to more articles about PLUMBING TRAPS:

• What is a P-trap?

• What are the code requirements for layout of drain piping under sinks? 

• What is a double trap in plumbing? 

• What types of plumbing traps are illegal by code? 

• What is bell trap at a plumbing drain? 

• What is an "S-Trap" under my sink? Why is it a problem? 

• How do I get rid of the sewer gas smell in my house?

• What is the minimum and maximum distance a P-trap can be below a sink? 

• What is a building trap?  

• Why is the European-style bottle trap not approved by the plumbing codes in the U.S.? 

• Can I make a trap under the sink from straight pipe and 90º elbows? 

• What is a drum trap at a plumbing drain?

• Can two sinks share a trap? 

Trap seal diagram - CodeCheck