Vanity drain in an older bath
What's wrong here?
- The trap outlet turns straight down through the floor, forming a prohibited S-trap
- The tailpiece between the basin and the trap is too long
- Slip-joint trap fittings may not be used inside a vanity cabinet
- Nothing. This installation is compliant
Show the answer and the code reference
The answer
The lavatory drains through an S-trap: the trap outlet turns and drops vertically through the floor instead of running out as a vented trap arm.
Correct choice: 1. The trap outlet turns straight down through the floor, forming a prohibited S-trap
UPC 1004.1 prohibits S-traps. When the outlet leg drops straight down, a full basin running out fills the drop and pulls the water out of the trap behind it (self-siphonage), leaving no seal against sewer gas. A trap is supposed to discharge into a trap arm that runs to a vent within the Table 1002.2 distance (UPC 1002.1, 1002.2). Nothing in this cabinet goes into the wall, and even if a vent is tied in somewhere below the floor, the outlet has already dropped below the trap weir before it reaches it.
How to fix it
Open the wall and bring a 1-1/2 in. drain and vent to a sanitary tee behind the vanity, then install a P-trap whose trap arm enters the wall within the Table 1002.2 distance. Where a wall vent cannot be run, an island-style or other approved venting method is needed, not an S-trap.
Code references
UPC 2018 1004.1 — Prohibited Traps
No trap that depends on moving parts for its seal may be used. "S" traps, bell traps and crown-vented traps are prohibited, and no fixture may be double trapped.
Drills citing 1004.1UPC 2018 1002.1 — Vent Pipes
Except as otherwise provided in the code, each fixture trap must be protected against siphonage and backpressure by a vent installed as the code requires.
Drills citing 1002.1
Minnesota
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 CodeWhy the other choices are wrong
- The tailpiece between the basin and the trap is too long. The UPC allows a tailpiece up to 24 in. long; this one is well under a foot.
- Slip-joint trap fittings may not be used inside a vanity cabinet. Slip joints are allowed at the trap inlet, the trap outlet and within the trap seal as long as they stay accessible, and an open vanity is accessible.
- Nothing. This installation is compliant. The outlet leg dropping straight down makes this an S-trap, which UPC 1004.1 prohibits because it self-siphons.