Plumbing (C-36) · Lesson 4 of 8

Trap Seals and Trap Arms

"Trap Seals and Trap Arms" is lesson 4 of 8 in the Plumbing (C-36) Learning Hub. It opens with "What a Trap Seal Is" and works through 5 short steps, then a worked example you can check yourself against.

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Steps in this lesson

  1. What a Trap Seal Is
  2. The Seal Depth Rule: 2 to 4 Inches
  3. One Trap Per Fixture, One Trap Per Trap Arm
  4. The Trap Arm's Own Length Limit
  5. What's Prohibited Outright

The worked example in the course applies this lesson to a realistic job.

What you'll be able to do

  • Explain the main idea of "Trap Seals and Trap Arms" in plain language you can use on the exam.
  • Apply this step with confidence: "The Trap Arm's Own Length Limit".
  • Apply this step with confidence: "What's Prohibited Outright".
  • Test yourself on the worked example and the quick-check quiz inside the classroom.
TRAP SEALS AND TRAP ARMS The water plug that keeps sewer gas out (CPC 1005.1) SEAL 2"-4" FROM FIXTURE SECTION 1005.1 - TRAP SEAL DEPTH Minimum:2 inches Maximum:4 inches Too shallow: evaporates, sewer gas gets in Too deep: trap won't self-scour Deeper allowed only if AHJ requires it PROHIBITED OUTRIGHT • S-traps • Bell traps • Crown-vented traps • Any trap relying on a movable part (check valve, bladder) • Double-trapping one fixture • More than one trap on a trap arm • A trap installed without a vent • (except where the code specifically allows it) CC
The diagram for this lesson, exactly as it appears in the Learning Hub.

The first steps of this lesson

Step 1: What a Trap Seal Is

A trap seal is the standing water held inside a P-trap's bend. That water plug is what physically blocks sewer gas from traveling back up through the fixture into the room. It is not decorative plumbing shape - the water itself IS the barrier, and if it evaporates, siphons out, or gets pushed out, the barrier is gone even though the pipe looks fine.

Step 2: The Seal Depth Rule: 2 to 4 Inches

Per Section 1005.1, each fixture trap must have a liquid seal of not less than 2 inches and not more than 4 inches, except where the Authority Having Jurisdiction requires a deeper seal for a special condition. Too shallow and ordinary evaporation or a strong draft can break the seal; too deep and the trap won't self-scour properly.

Step 3: One Trap Per Fixture, One Trap Per Trap Arm

Each plumbing fixture must be separately trapped (Section 1001.2) - no double-trapping one fixture, and not more than one trap on a single trap arm. Exceptions exist for a set of up to three single-compartment sinks or lavatories that can share one trap if they're close together (waste outlets within 30 inches) and the trap sits centrally.

The remaining 2 steps, the worked example and the quiz are in the course. This is a preview. The full lesson, with the step-by-step explanations, a worked example and a quick-check quiz, is inside the classroom.

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