Spot the Violation

PlumbingTraps & ventingLevel 22018 UPC

Double-bowl sink with a disposer

Kitchen sink base with the doors open under a double-bowl stainless sink. A food waste disposer hangs from the left bowl; its discharge elbow feeds a white plastic U-shaped trap whose outlet runs right into a side-inlet tee on the right bowl's tailpiece. Below that tee the tailpiece drops into a second U-shaped trap, whose outlet runs to a trap adapter at the back wall. Chrome angle stops, a receptacle with the disposer cord, and a dish soap bottle sit in the cabinet.
Kitchen sink base, doors open

What's wrong here?

  1. Waste from the disposer bowl runs through two traps in series before it reaches the drain
  2. A kitchen sink trap and trap arm must be at least 2 in.
  3. Slip-joint tubular fittings cannot be used downstream of a food waste disposer
  4. Nothing. This installation is compliant
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Show the answer and the code reference

The answer

The disposer side is double trapped: its own trap discharges into the right bowl's tailpiece, which then runs through a second trap.

Correct choice: 1. Waste from the disposer bowl runs through two traps in series before it reaches the drain

UPC 1004.1 states that no fixture shall be double trapped. Here everything the disposer grinds has to pass through its own trap and then through the trap under the right bowl. Two water seals in series slow the drain, give disposer waste two low spots to settle in and clog, and push water and food back up into the right bowl when the disposer runs. Where the pipe between two traps is closed, the trapped air also causes air-locking and gurgling. A two-compartment sink needs only one trap, placed after the two wastes join and followed by a vented trap arm.

How to fix it

Remove the trap under the disposer and run the disposer discharge through a continuous waste (directional tee) into the single trap under the right bowl, keeping that trap's arm within the Table 1002.2 distance to its vent.

Code reference

  • UPC 2018 1004.1 — Prohibited Traps

    Lists traps that may not be used, including "S" traps, bell traps and crown-vented traps, and states that no fixture shall be double trapped.

    Drills citing 1004.1

Minnesota

MN adopts this rule as writtenConfidence: medium

Minnesota adopts the 2018 Uniform Plumbing Code with Minnesota amendments as the Minnesota Plumbing Code (Minn. Rules ch. 4714, effective Dec. 17, 2021). No Minnesota amendment to UPC 1004.1 is known; check ch. 4714 before relying on it.

About the Minnesota Plumbing Code

Why the other choices are wrong

  • A kitchen sink trap and trap arm must be at least 2 in. UPC Table 702.1 sizes a residential kitchen sink trap at 1-1/2 in., with or without a disposer, and these traps are 1-1/2 in.
  • Slip-joint tubular fittings cannot be used downstream of a food waste disposer. Tubular slip-joint traps and wastes are routinely used with disposers; the joints only need to stay accessible, as they are here.
  • Nothing. This installation is compliant. The left bowl's waste passes through the disposer trap and then the right-hand trap; UPC 1004.1 says no fixture shall be double trapped.
Code complianceLevel 2 · Journeyman#double-trap#kitchen-sink#disposer#trap

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