Unvented Tub Trap Arm in a Slab Remodel
1950s single-story house on a slab; the hall bath is being remodeled in place and the drain lines are buried in the slab. As found: the tub trap arm is 9 ft of 1-1/2 in. ABS running to a 2 in. drain that also takes the lav, and the lav has a 1-1/4 in. trap arm 3 ft long. A single 1-1/2 in. vent goes through the roof and ties into the 2 in. drain at the lav. The owner reports the tub drains slowly and the water closet gurgles when the lav empties. The crew's plan is to cut one 3 ft by 3 ft slab patch, re-pipe the tub with 2 in. pipe, and install an air admittance valve under the lav sink to fix the venting. The adopted code's trap-arm limits: 1-1/4 in. = 5 ft, 1-1/2 in. = 6 ft, 2 in. = 8 ft.
1. As found, which rules are violated, and how does each violation explain the reported symptoms?
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Violation 1 - tub trap arm too long.
- 1-1/2 in. trap arm, 9 ft developed. Limit is 6 ft.
- Past that length the arm cannot drain without pulling air through the trap. The arm stays full, the trap weir is submerged against a column of water, and the fixture drains by glugging air past the trap. That is the classic slow-draining tub.
Violation 2 - the tub trap has no vent of its own.
- The only vent in the group is at the lav. A vent must connect to the trap arm within the allowed distance for the arm size; the tub's 9 ft arm means the tub trap is effectively unvented.
- An unvented trap responds to pressure changes from other fixtures. When the lav empties, it sends a slug of water down the 2 in. drain past the WC connection, momentarily pressurizing that section. The WC trap seal burps - that is the gurgle.
Not a violation.
- The lav arm (1-1/4 in., 3 ft) is within the 5 ft limit, so the lav is not the source of the tub symptom.
Load check for context: WC 3 + tub 2 + lav 1 = 6 fixture units on the group. Confirm the existing 1-1/2 in. vent can carry 6 units at this developed length in your code's vent table before assuming the vent itself is undersized - the diagnosis here is distance, not size.
2. Does the crew's plan work? What do you direct them to do instead, using only the one 3 ft by 3 ft slab patch?
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Why the plan fails:
- The AAV under the lav vents the lav trap only. It does nothing for a trap 9 ft away at the tub.
- Re-piping the tub in 2 in. does not by itself fix it. The trap arm limit follows the trap/arm size, and a tub has a 1-1/2 in. outlet, so the trap stays 1-1/2 in. (6 ft limit) unless the fixture drain is genuinely enlarged. Even a true 2 in. arm only buys 8 ft - still short of 9 ft.
- An AAV cannot be buried in a slab or concealed in a wall. It must stay accessible and be installed upright, so it is not an option at the tub end.
Direct the crew to do one of these inside the patch:
- Relocate the tub trap so the trap arm to the existing 2 in. drain is 6 ft or less, keeping 1/4 in. per ft fall and no backfall; or
- Cut a 1-1/2 in. vent into the trap arm at 6 ft or less and take it up through the wall. It must rise vertically at least 6 in. above the flood level rim of the tub before it offsets horizontally, and it must tie into the existing vent system above the highest fixture drain it serves.
Option 2 is usually cheaper than moving the drain and is the correct answer when the tub stays where it is.
3. What do you inspect before the slab is closed, and what test do you require at rough-in?
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Before the pour:
- Trap arm length measured, not estimated, from trap weir to vent fitting: 6 ft maximum at 1-1/2 in.
- Fall on the fixture branch 1/4 in. per ft, verified with a level; no belly, no backfall, and the arm not offset vertically in a way that creates a second trap.
- Vent rises 6 in. above the tub flood level rim before any horizontal offset.
- Tub trap directly below the drain outlet, trap weir at least 2 in. below the drain opening.
- Vent tie-in elevation above the highest fixture drain on the system, so the tub vent cannot become a drain.
Test:
- Water test of the DWV rough-in (typically a 10 ft head held for the required period) or an air test at the pressure your code allows, with all openings plugged, before backfill.
At final:
- Fill the tub, pull the plug, and watch the trap. If the seal holds and the drain is quiet, the vent is working. Confirm the lav AAV, if used, is accessible, upright, and above the trap weir.