Basement Bathroom Rough-In Fails Inspection
You are finishing a basement bathroom in a 1970s house: water closet, lavatory, and a 36 in by 60 in shower. The existing 3 in cast iron stack runs in a furred wall, and the 4 in building drain is under the slab. You propose a 3 in WC drain to the stack, a 2 in shower drain with a 6 ft trap arm, and a 1.5 in lavatory drain that connects to the shower line 18 in downstream of the shower trap. Because the ceiling is low, you run the 2 in horizontal branch at 1/8 in/ft. The lavatory is vented with an AAV under the sink cabinet. The shower and WC are wet-vented through the lavatory. The inspector red-tags it. After a temporary test, a WC flush causes the shower trap to gurgle and lose its seal, and a sewer odor appears.
1. Identify each defect in the proposed DWV layout and explain why the shower trap gurgles.
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Defects: the 2 in horizontal drain is at 1/8 in/ft, but 2 in and smaller pipe requires 1/4 in/ft, so the line is too flat and can hold solids and air. The 2 in shower trap arm is 6 ft, which is within the 8 ft maximum for a 2 in trap arm, but only if it is vented within that distance and sloped at 1/4 in/ft. The lavatory AAV is not a substitute for a proper vent where the lav is wet-venting a WC and shower; the AAV must be listed, sized, accessible, and permitted by the local code. The 1.5 in lav drain is questionable as a wet vent for a 3 in WC. When the WC flushes, it sends a pressure wave down the drain. Without a proper vent, the shower trap siphons and gurgles as air is pulled through the seal.
2. Redesign the drainage and venting with sizes, slopes, and maximum distances. State what must be accessible and where the vent must terminate.
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Run the WC in 3 in to the stack at 1/4 in/ft, or at 1/8 in/ft if the local code permits that slope for 3 in. Run the shower in 2 in at 1/4 in/ft with a 2 in vent within 8 ft of the trap. Run the lavatory in 1.5 in with a 1.5 in vent. Individual vents are the safest design. If wet venting is used, size and arrange it per the code; a 2 in lav drain may wet-vent the WC in some layouts, but verify the local rule. Tie the vents to the existing 3 in stack at least 6 in above the highest fixture or run a vent through the roof. The AAV may be used only where listed and must remain accessible; it cannot be concealed. Terminate the roof vent at least 6 in above the roof and 3 ft from openings.
3. Describe the test and inspection sequence before the walls and slab are closed.
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Before closing, perform a water or air test on the DWV system. Plug the building drain, fill the system with water to the top of the stack or 10 ft, and hold it for the code-required time; or use an air test if the local code permits. Inspect all joints, slope, trap arms, and vent connections. Verify the AAV is accessible and correctly sized. After the test, flush the WC and run the shower and lavatory to confirm the traps hold their seals and no gurgling occurs. Photograph the rough-in and call for inspection before insulation, drywall, or concrete placement.