Basement laundry tub rough-in
What's wrong here?
- The dry vent turns horizontal below the laundry tub's flood-level rim instead of first rising 6 in. above it
- The trap arm is too long for a 1-1/2 in. trap
- A laundry tub needs a 2 in. trap
- Nothing. This installation is compliant
Show the answer and the code reference
The answer
The tub's vent rises only a few inches above the sanitary tee and then runs flat at about the height of the basin, below the fixture's flood-level rim.
Correct choice: 1. The dry vent turns horizontal below the laundry tub's flood-level rim instead of first rising 6 in. above it
UPC 905.3 requires each vent to rise vertically to at least 6 in. above the flood-level rim of the fixture it serves before it offsets horizontally. A dry vent run flat below the rim floods whenever the drain below it stops up: waste and lint settle in the level pipe, and once the vent is plugged the tub trap siphons. The model code only allows a lower vent where structural conditions prevent the rise, and then it must be built with drainage fittings graded to drain; here the wall is open, the run is dead level, and it uses short-turn vent elbows.
How to fix it
Extend the vent straight up from the sanitary tee to at least 6 in. above the tub rim (about 40 in. off the floor for a 34 in. tub) before turning toward the vent riser, and grade the horizontal run back toward the drain.
Code reference
UPC 2018 905.3 — Vent Pipe Rise
Unless prohibited by structural conditions, each vent shall rise vertically to not less than 6 in. above the flood-level rim of the fixture served before offsetting horizontally; vents less than 6 in. above the flood-level rim shall be installed with approved drainage fittings, material and grade to drain.
Drills citing 905.3
Minnesota
Minnesota adopts the 2018 Uniform Plumbing Code with Minnesota amendments as the Minnesota Plumbing Code (Minn. Rules ch. 4714, effective Dec. 17, 2021). Search-result excerpts suggest Minnesota amends UPC 905.3 (apparently in part 4714.0905), possibly dropping the model code's "unless prohibited by structural conditions" opening, but the amended text could not be read; check ch. 4714 before relying on the details. The violation stands under either version: no structural condition is visible, and the flat run is dead level and built with vent elbows.
About the Minnesota Plumbing CodeCitation under review. Minnesota status is unverified. Confirm in Minn. Rules ch. 4714 (apparently part 4714.0905) how Minnesota amends UPC 905.3, in particular whether the 'unless prohibited by structural conditions' opening and the last sentence (vents less than 6 in. above the flood-level rim installed with drainage fittings, material and grade to drain) survive. Also confirm the 2018 UPC heading 'Vent Pipe Rise' for 905.3. The answer holds under either version.
Why the other choices are wrong
- The trap arm is too long for a 1-1/2 in. trap. The arm runs only about 10 in. to the tee; Table 1002.2 allows 3 ft 6 in. for a 1-1/2 in. trap arm.
- A laundry tub needs a 2 in. trap. UPC Table 702.1 sizes a laundry tray trap at 1-1/2 in.; the 2 in. trap is for a clothes washer standpipe.
- Nothing. This installation is compliant. The vent goes flat roughly 10 in. below the tub rim. If the drain below the tee stops up, waste backs into that flat vent and the vent is lost; UPC 905.3 requires the vent to rise 6 in. above the flood-level rim before it offsets horizontally.