SESKStuff Everyone Should Know
SESK: Stuff Everyone Should Know · sesk.info/home-yard-p-trap-water-seal

What normally keeps sewer gas from coming up through a sink drain?

The water seal in the P-trap blocks it.

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  1. Water sitting in the P-trap ✔
  2. The faucet aerator
  3. The sink overflow hole

Why: A P-trap holds a small water seal between the room and the drain system. If that seal disappears, sewer odors can enter.

Huh, didn't know that: The plumbing code says most trap seals must be at least 2 inches and no more than 4 inches deep.

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Why couldn't the plumber get a date? Because he was a real drip.
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References

Answer: Water sitting in the P-trap. A P-trap holds a small water seal between the room and the drain system. If that seal disappears, sewer odors can enter.

  1. Overview of Plumbing Trap Requirements, International Code Council www.iccsafe.org/building-safety-journal/bsj-technical/an-overview-of-plumbing-trap-requirements-under-the-international-plumbing-code
    Plumbing traps play a critical role in protecting building occupants from sewer gases, odors, and potential health hazards.
    Checked 2026-10-11.

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