Basement Bathroom Trap Arm and Air Admittance Valve
A basement bathroom is being relocated. The lavatory and shower are moved 7 feet from a 2-inch vertical stack. The lavatory has a 1-1/2-inch trap and trap arm installed at 1/4 inch per foot; the shower has a 2-inch trap. The owner wants to install an air admittance valve under the lavatory inside a closed vanity cabinet to avoid running a new vent through the first floor. The existing stack extends through the roof and serves only this bathroom. The framing is complete and the drain is about to be covered.
1. Is the 1-1/2-inch lavatory trap arm at 7 feet compliant, and what options correct it?
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No. A 1-1/2-inch trap arm at 1/4 inch per foot is limited to 6 feet from the trap weir to the vent fitting. At 7 feet, the lavatory trap arm exceeds the limit and is not compliant. The shower's 2-inch trap arm has an 8-foot limit, so 7 feet would be acceptable for the shower if it is properly vented.
Options: enlarge the lavatory trap arm to 2 inches, which raises the limit to 8 feet, or add a vent connection within 6 feet of the lavatory trap. Simply covering the drain does not cure the excessive length. The trap arm must also maintain the required slope and must not have a fall greater than the trap weir allows.
2. Can an air admittance valve be installed under the lavatory inside a closed vanity cabinet?
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An air admittance valve must be listed, installed per its listing and the manufacturer's instructions, and remain accessible for service. A closed vanity cabinet is a poor location because the valve needs air and must be accessible. If the cabinet is sealed, the valve may not admit air properly and cannot be inspected or replaced without demolition.
An AAV is not a universal substitute for a required vent. It may be permitted for an individual fixture in accordance with the code, but it cannot be concealed in a way that makes it inaccessible. A better solution is to run a conventional vent or provide a vented cabinet with an accessible AAV. The AAV also does not correct the excessive trap arm length by itself.
3. If the lavatory is vented conventionally and the shower connects to the same stack, what venting issues must be checked before the drain is covered?
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The shower must have a vent connection within its allowable trap arm length, and the vent takeoff must be above the trap weir. If the lavatory is used to wet-vent the shower, the wet vent must be sized and arranged per the code, the fixtures must be on the same floor, and the trap arm lengths must still comply. The stack serving only this bathroom may be a wet vent or a dry vent depending on the arrangement, but the connections must be inspected at rough-in. Because the drain is about to be covered, the inspector should verify the vent takeoffs, trap arm lengths, and slope before concealment.