Vanity drain in an older bath
What's wrong here?
- The tailpiece between the basin and the trap is too long
- Slip-joint trap fittings may not be used inside a vanity cabinet
- The trap outlet turns straight down through the floor, forming a prohibited S-trap
- 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: 3. 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
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.