Venting a Basement Bathroom Group Through a Post-Tension Slab
An owner wants to finish a basement bathroom under a 1970s one-story house. The existing 3-inch cast-iron building drain runs under the slab, and a 3-inch stack rises at the rear wall. The new group is a 1.5-inch lavatory, a 1.5-inch bathtub, and a 3-inch water closet. The lavatory will be 9 feet from the stack, the tub 5 feet, and the water closet 4 feet. The owner proposes to drain the lavatory into a 1.5-inch line that connects to the water closet branch, using the lavatory as a wet vent for the water closet, and to run the tub with a 1.5-inch trap arm to a 2-inch branch. The slab is post-tensioned, so cutting more than one 4-inch-wide trench is expensive. The local authority allows air admittance valves only where a vent cannot be run to the roof. The contractor wants to know whether the layout is legal and what the least invasive compliant option is.
1. Evaluate the proposed wet vent and trap-arm lengths. What fails, and why?
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The layout fails for two independent reasons. First, the lavatory is 9 feet from the stack. A 1.5-inch trap arm is limited by trap-arm length tables - commonly 3 feet 6 inches in the UPC and 6 feet in the IPC at 1/4 inch per foot - so a 9-foot arm is too long either way. Second, a 1.5-inch lavatory drain cannot wet-vent a water closet. Wet-vent rules generally require the wet vent to be at least 2 inches and the water closet to be the most downstream fixture on that wet vent; a 1.5-inch lav drain is not a legal wet vent for a 3-inch water closet. The tub at 5 feet may be acceptable under some codes but not under the stricter 3-foot-6-inch limit, so it cannot be assumed compliant. The fact that the fixtures connect near each other does not create a vent.
2. Could air admittance valves solve the problem? Explain their limits here.
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An AAV might be allowed for the lavatory if the local code permits AAVs, but it would not cure the layout. An AAV is a one-way vent and must be accessible, installed above the fixture flood level, and used only where the code allows. It cannot serve as a wet vent for a water closet, and it does not shorten the trap arm. If the lavatory is given its own AAV but still drains 9 feet to the stack, the trap arm remains over length. If the lavatory is moved closer and given an AAV, the water closet still needs a vent. So an AAV may be a partial solution for the lavatory only; it is not a legal substitute for a vent on the water closet.
3. What is the least invasive compliant option, and what should the contractor verify before cutting?
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The least invasive compliant option is to keep the fixtures in one trench and run a 2-inch vent from the fixture group to the existing stack above the highest fixture, or through the first-floor wall and attic to a roof vent. Shorten the lavatory trap arm to within the table limit by relocating the lavatory or by putting the vent between the lavatory and the stack. Use a 2-inch drain for the lavatory if it is to wet-vent anything, and make the water closet the downstream fixture. The post-tension slab usually allows one carefully located trench; coordinate with the slab engineer before cutting. If the local code allows a 2-inch combination waste-and-vent for the lavatory and tub only, that may reduce cuts, but it cannot serve the water closet. Do not rely on an AAV for the water closet.