Short definition
The trap seal is the column of standing water inside a fixture’s trap that blocks sewer gas from rising into the home. Measured between the trap dip (the bottom of the U) and the trap weir (the top of the outlet). UPC 1002.4 requires every fixture trap to hold a seal at least 2 inches and no more than 4 inches deep. Lose the seal and sewer gas enters; it’s that simple.
What it is
Every trap — P-trap, drum trap, integral toilet trap — works the same way: water sits in a low spot of the drain pipe and physically blocks gas from coming through. The seal is the water itself, not the fitting that holds it. The fitting just keeps the water in place.
The 2- to 4-inch depth range exists for two reasons:
- Below 2 inches — the seal is too easily lost. Evaporation, momentary siphonage, capillary wicking, and momentum from a sudden discharge can all carry a shallow seal away.
- Above 4 inches — the seal becomes a flow restriction. Drainage slows, solids accumulate, and the trap clogs more often.
The seal also has a structural pressure threshold: pneumatic pressure differentials beyond about ±400 Pa (per haq Ch.10.12) can break the seal even if the trap is otherwise intact.
Why it matters to a homeowner
If your home smells of sewer, a failed trap seal is by far the most likely cause. The fix depends on which mechanism caused the loss — pour water in to refill an evaporated trap, replace an S-trap that self-siphons, snake a vent that’s pushing pressure pulses through the seal, or repair a slip-joint that’s leaking water out. Each mechanism has a different fix, but all run through the same single concept: the seal isn’t holding.
How a trap seal can be lost
There are eight documented mechanisms. The most common ones in a residential setting:
- Self-siphonage — fixture’s own discharge pulls the seal out (during use).
- Induced siphonage — neighbor fixture’s discharge does it (during their use).
- Compression / back-pressure — stack-base hydraulic pulse blows the seal up into the fixture.
- Evaporation — slow drying out during disuse (vacation home, unused floor drain).
- Capillary action — hair or string draped over the trap weir wicks water out.
- Momentum — sudden large discharge dislodges the seal.
- Foaming — detergent foam soaks water out.
- Wavering out — wind across a vent terminal oscillates the trap level.
Common variants / not the same as
- Trap seal vs. trap. Seal is the water; trap is the fitting that holds it.
- Trap seal vs. air gap. Trap seal is a liquid barrier; air gap is an open atmospheric break (e.g., dishwasher airgap, indirect waste).
- Trap seal depth vs. trap dip / weir. Depth is the vertical span between dip and weir; dip and weir are the two reference points used to measure it.