Spot the Violation

PlumbingFixtures & clearancesLevel 32018 UPC

Basement ceiling under the kitchen

Looking up at the open joist ceiling of an unfinished basement toward the poured concrete foundation wall, with spray foam at the rim joists. Directly below the kitchen sink, a white 1-1/2 in. tubular drain pipe with a slip-joint extension comes down through a hole in the subfloor between two 2x10 joists and drops into a tubular P-trap that hangs below the bottom of the joists. The trap arm runs sideways under a joist to a sanitary tee on a 2 in. PVC pipe that rises through the subfloor and continues down the wall. Red and blue PEX lines run up through the floor beside it, and a yellow cable is stapled along one joist. 2 IN PVC DWV SCH 40
Unfinished basement ceiling, directly below the kitchen sink

What's wrong here?

  1. The sink's trap is set below the floor joists, so the tailpiece from the sink outlet to the trap is far longer than 24 in.
  2. The trap arm is too long for a 1-1/2 in. trap
  3. A kitchen sink needs a 2 in. trap
  4. Nothing. This installation is compliant
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Show the answer and the code reference

The answer

The kitchen sink is trapped in the basement ceiling instead of under the sink, so the tailpiece between the sink outlet and the trap is well over the 24 in. maximum.

Correct choice: 1. The sink's trap is set below the floor joists, so the tailpiece from the sink outlet to the trap is far longer than 24 in.

UPC 1001.2 requires the vertical distance from a fixture outlet to its trap weir to be as short as practicable and caps the tailpiece at 24 in. A kitchen sink outlet sits about 2 ft above the finished floor, and this tailpiece then passes through the floor and a 2x10 joist bay before reaching a trap that hangs below the joists, so it is over 3 ft long. A long tailpiece is an unvented, unscoured run that builds up a film of grease and food waste that smells and narrows the pipe, and the long fall into the trap makes the seal easier to siphon. Hanging the trap below the floor is a common shortcut to avoid venting in the kitchen wall, and if this ceiling is finished later the trap and its slip joints will be concealed.

How to fix it

Move the P-trap up into the sink cabinet within 24 in. of the sink outlet, with its trap arm running into the wall to a properly vented sanitary tee (the 2 in. riser that already continues up through the floor into the wall can take the tee at cabinet height), and remove the tailpiece through the floor and the trap in the ceiling.

Code references

  • UPC 2018 1001.2 — Where Required

    Each plumbing fixture, except those with integral traps or as otherwise permitted, must be separately trapped by an approved liquid seal trap. The vertical distance between a fixture outlet and the trap weir must be as short as practicable, and the tailpiece from a fixture may not exceed 24 in. in length.

    Drills citing 1001.2

  • UPC 2018 Table 1002.2 — Horizontal Lengths of Trap Arms

    Sets the maximum distance from a trap weir to its vent by trap arm size, based on a slope of 1/4 in. per ft; a 1-1/2 in. trap arm may run up to 3 ft 6 in.

    Drills citing Table 1002.2

Minnesota

MN status unverifiedConfidence: medium

Minnesota adopts the 2018 Uniform Plumbing Code with Minnesota amendments as the Minnesota Plumbing Code (Minn. Rules ch. 4714, effective Dec. 17, 2021). Search excerpts suggest Minn. Rules 4714.1001 amends UPC 1001.2, reportedly keeping the 24 in. tailpiece limit and requiring each fixture's trap to be placed as near the fixture as possible, but the amended text could not be read. A trap hung in the basement ceiling under a kitchen sink fails the model 1001.2 either way; check ch. 4714 for the Minnesota wording.

About the Minnesota Plumbing Code

Why the other choices are wrong

  • The trap arm is too long for a 1-1/2 in. trap. Table 1002.2 allows a 1-1/2 in. trap arm up to 3 ft 6 in. from the trap weir to the vent; this one runs only about a foot to the vented tee.
  • A kitchen sink needs a 2 in. trap. Table 702.1 sizes a residential kitchen sink trap and trap arm at 1-1/2 in.; 2 in. is not required.
  • Nothing. This installation is compliant. The sink outlet is roughly two feet above the kitchen floor, so a trap hung below the joists puts the tailpiece at well over three feet, past the 24 in. limit.
Code complianceLevel 3 · Master#tailpiece#trap#kitchen sink#basement#trap location

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