Garage ADU Bathroom Vent Tied Below the Flood-Level Rim
A 1968 two-story duplex is being converted into a main house plus an attached ADU. The garage slab will hold a new bathroom group-water closet, lavatory, and 36-inch shower-about 9 feet from an existing 3-inch cast-iron soil stack. The stack carries the second-floor bathroom and continues through the roof. The contractor proposes a 1½-inch vent from the lavatory running 8 feet horizontally with no grade and tying into the 3-inch stack 18 inches below the lavatory flood-level rim. The water closet would be wet-vented through the lavatory. The inspector red-tags the rough-in and cites an illegal horizontal vent and an improperly wet-vented water closet. The owner does not want to penetrate the roof again and asks whether an air-admittance valve can solve the problem. The existing second-floor fixtures drain slowly after the new work, and the lavatory gurgles when the toilet is flushed.
1. Evaluate the proposed venting arrangement under the plumbing code. Why is the inspector correct, and what vent path would you install?
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The proposed connection is not a vent. A vent must rise vertically at least 6 inches above the flood-level rim of the fixture it serves before any horizontal offset. Tying in 18 inches below the rim makes the 1½-inch line a branch drain or wet vent, not a dry vent, and the water closet cannot be wet-vented through a 1½-inch lavatory drain. Wet venting is limited by minimum size, fixture type, and developed length; a water closet generally needs a properly sized wet vent or its own vent, and a 3-inch soil stack carrying fixtures above is not automatically a dry vent for the new group. The ungraded 8-foot horizontal run also violates the requirement that horizontal vent pipe be graded and supported so it drains back and does not sag. The slow drainage and gurgling are consistent with a choked vent path. Install a 2-inch individual vent from the new group, rise above the flood-level rims before offsetting, and connect to a dry vent or extend through the roof. If a wet vent is used, size and limit it under the wet-vent rules and confirm the stack is acceptable for that use.
2. The owner proposes putting an air-admittance valve on the lavatory and abandoning the re-vent. Is that acceptable?
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No, not as a substitute for the water closet vent. An AAV is a one-way air valve; it admits air to relieve negative pressure but does not provide a positive-pressure relief path and cannot serve as the vent for a water closet. AAVs are also limited to individual fixtures, must be accessible, must be installed in the correct orientation, and are not permitted in concealed locations or where local amendments prohibit them. Using one on the lavatory might be acceptable only if the lavatory otherwise qualifies and the water closet has a separate compliant vent. It would not cure the illegal wet vent or the below-rim connection. The correct fix is to provide a dry vent for the bathroom group, or a code-compliant wet vent that includes the water closet, and to verify that the existing stack is not overloaded. The owner should be told that avoiding a roof penetration may require routing the vent through a side wall above the flood rims or using an approved alternate location, not an AAV on the lavatory alone.
3. What would you check before accepting the corrected rough-in and signing off on the drainage?
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Check the drain slope and the vent slope separately. The 2-inch branch drain should be graded at ¼ inch per foot, and the horizontal vent should be graded back toward the drain so condensation does not stand. Verify that the vent rises at least 6 inches above the highest flood-level rim it serves before any offset, that it is supported, and that it connects to a dry vent or stack vent in an approved manner. Confirm the water closet is not wet-vented through an undersized lavatory drain and that the fixture units on the stack do not exceed its capacity. Check that the new connection has not blocked or reduced the existing second-floor vent; the slow drainage upstairs suggests the tie-in may have created a wet vent or partial blockage. Flow-test the group, inspect the trap seals, and confirm no sags or back-graded sections. If an AAV is used on the lavatory, verify access and local approval.