Basement Bath Wet Vent and Concealed Rough-In
A two-story single-family house is adding a basement bathroom under a concrete slab. The existing 3-in. PVC building drain runs below the slab; the proposed fixtures are a 3-in. water closet, a 2-in. lavatory drain, and a 1.5-in. shower drain. The lavatory trap is 7 ft developed length from a 1.5-in. dry vent stack. The shower trap arm is 9 ft from the same stack. The plumber proposes to connect the water closet into the lavatory drain and use the lavatory drain as a wet vent for the water closet, with the existing 1.5-in. vent continuing through the roof. The homeowner has already framed and insulated the ceiling; no permit is posted. The local inspector has asked for a rough-in inspection before the slab is patched. The jurisdiction has adopted the New Mexico Plumbing Code with local amendments.
1. Evaluate the proposed wet-vent arrangement and trap-arm lengths. What must change before rough-in?
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The shower trap arm is the clearest failure. A 1.5-in. trap arm is commonly limited to about 6 ft developed length from the trap weir to the vent; 9 ft exceeds that by about 3 ft. Increasing the shower drain to 2-in. would allow about 8 ft, but 9 ft still fails, so the practical fix is to add a vent within the allowed distance or relocate the trap/vent.\n\nThe lavatory trap arm at 7 ft is within the typical 8-ft limit for a 2-in. trap arm, but that does not automatically make the wet vent legal. A lavatory drain may wet-vent a water closet only when the fixtures are on the same floor, the wet-vented section is sized for the combined load, the fixture-unit limits are met, and the dry vent is adequate for the whole group. The existing 1.5-in. dry vent must be checked against the adopted table for the combined bathroom group; it may be undersized. The water closet must also connect downstream of the lavatory in the approved wet-vent arrangement.\n\nDo not conceal any of this work. Submit a corrected isometric, obtain the permit, complete the revised rough-in, and have it inspected and tested before patching the slab.
2. The inspector wants a drain, waste, and vent test before the slab is patched. Describe an acceptable test and the inspection sequence.
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First, the permit must be obtained and posted before concealed plumbing work proceeds. The rough-in inspection should occur after the DWV piping is complete but before insulation, framing, or slab patching hides it.\n\nFor the DWV test, a common accepted method is a water test with at least 10 ft of head at the lowest opening, or an air test at about 5 psi for 15 minutes where the authority approves air testing. The exact pressure, duration, and test medium can be modified by New Mexico or local amendments, so confirm with the AHJ. The test must include the building drain, branches, trap arms, and vent connections that will be concealed.\n\nDuring inspection, verify trap-arm lengths, vent connections, fitting orientation, support, and slope. Typical minimum slopes are 1/4 in./ft for 2-in. pipe and 1/8 in./ft for 3-in. pipe, but the adopted code controls. Only after the test holds and the rough-in is approved should the slab be patched.
3. Calculate the bathroom group fixture units and assess whether the existing 3-in. building drain is likely adequate.
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Using common drainage fixture-unit values, the lavatory is 1 DFU, the shower is 2 DFU, and the water closet is 3 DFU, for a bathroom group total of 6 DFU. A 3-in. building drain is generally more than adequate for 6 DFU, so the building drain size is not the main problem.\n\nThe branch sizing and venting are the real issues. The 2-in. lavatory drain is being asked to carry the water closet as well as the lavatory, and a wet-vented branch has fixture-unit and length limits. The shower branch is already over its trap-arm limit. Do not assume that because the 3-in. building drain is large enough, the 2-in. branch and 1.5-in. vent are also adequate. The corrected design must show a legal vent for the shower and a wet vent or separate vent for the water closet that satisfies the adopted code table.