Basement Bathroom Wet Vent Rejected
A 1950s house is being remodeled with a new basement bathroom directly below the first-floor bath. The existing 3-inch cast-iron soil stack is 9 feet from the new group and runs full-size through the roof. The contractor has installed a 1.5-inch lavatory drain that connects to a 3-inch water-closet branch; the shower drain ties into the lavatory drain downstream of the lavatory trap. There are no individual vents. The lavatory trap arm is 8 feet, and the shower trap arm is 7 feet of 2-inch pipe. The contractor says the lavatory wet-vents the water closet because the lavatory is upstream and the drain is open to the stack. The inspector rejects the wet vent and also questions the lavatory trap-arm length. The homeowner wants to avoid opening the first-floor walls, and the contractor proposes an air-admittance valve under the lavatory as a quick fix.
1. Is the proposed wet vent valid? Explain the fixture order, pipe size, and venting requirements.
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The wet vent is not valid as installed. A 1.5-in lavatory drain is too small to wet-vent a water closet; the wet-vent section must be at least 2 in, and the water closet must be downstream of the wet-vented fixtures. The shower cannot simply tie into the lavatory drain downstream of the lavatory trap and be considered vented; the arrangement must be a recognized wet vent, or each fixture must have its own vent. A workable layout is a 2-in line from the lavatory through the shower connection to the stack, with the water closet connecting downstream and the 2-in line continuing as the vent. The lavatory trap arm must be limited to 6 ft for 1.5-in pipe, or 8 ft if the lavatory drain is upsized to 2 in; the existing 8-ft 1.5-in arm is too long. The shower’s 7-ft 2-in trap arm is within the usual 8-ft limit, but only if the shower is properly vented.
2. Will an air-admittance valve solve the problem? Explain what an AAV can and cannot do here.
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An AAV does not fix this. AAVs are listed mechanical vents for individual fixtures and are subject to local code restrictions; they do not create a wet vent, and they are not a substitute for venting a water closet. Even where an AAV is allowed for a lavatory, the water closet still needs a code-recognized vent. The reliable correction is to run a 2-in vent from the lavatory-and-shower branch to the existing stack above the fixtures, or to install individual vents. Keep any AAV accessible if one is used elsewhere, but do not bury it in a wall or use it as the only vent for the bathroom group.
3. The shower trap arm is 7 feet of 2-inch pipe and connects downstream of the lavatory trap. Is the shower trap arm itself acceptable? What other defect must be corrected?
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The 7-ft shower trap arm is not the main defect; 2-in pipe is commonly allowed up to 8 ft from trap to vent. The defect is that the shower connects downstream of the lavatory trap without a proper vent. If the shower is wet-vented through a 2-in lavatory-and-shower line, that line must be sized and arranged as a wet vent; if not, give the shower an individual 2-in vent. Also check trap-arm slope: a 2-in trap arm at 1/4 in/ft over 7 ft falls 1.75 in, which is acceptable, but the trap weir must remain at the required distance above the vent connection. The lavatory’s 8-ft 1.5-in trap arm must be shortened or upsized to 2 in.