Wet Venting a Relocated Bathroom Group in a 1926 Craftsman
A second-floor bathroom in a 1926 Craftsman is being remodeled. The existing 3-inch water closet drain runs 5 ft to a 3-inch vertical stack; the lavatory has a 2-inch drain, and the tub has a 1-1/2-inch trap arm. The remodel moves the water closet 4 ft, the lavatory 3 ft, and the tub 2 ft. The contractor proposes to use the lavatory drain as a wet vent for the water closet, keep the tub trap arm at 7 ft with 1/8 inch per foot slope, and terminate the existing 2-inch vent through the roof unchanged. The lavatory drain will be 2 inch and will connect to the stack above the water closet. The contractor says the bathroom is a 'bathroom group,' so wet venting is always allowed and the 1/8 slope gives more room under the floor. The framing is notched, and the local inspector has flagged the trap arm and vent.
1. Is the proposed wet vent for the water closet acceptable? Explain the conditions that must be met.
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It is not automatically acceptable. In a bathroom group, a lavatory drain may wet-vent a water closet only if it is sized and installed under the wet-vent rules: minimum 2 inch, limited developed length, same floor, no other fixtures upstream, and arranged so the vent path is maintained. The lavatory drain must not be reduced, and the water closet connection must be made so the lavatory trap arm remains the vent path. The plan must be checked against the wet-vent length and fixture-unit limits. A 2-inch lavatory drain can wet-vent a water closet only when the arrangement complies; the label 'bathroom group' does not waive the limits. If the length or arrangement is outside the table, provide a separate vent or revent for the water closet.
2. Evaluate the 7 ft tub trap arm at 1/8 inch per foot. What corrections are needed?
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The tub trap arm is deficient for two reasons. First, a 1-1/2-inch trap arm is normally limited to 6 ft from the trap weir to the vent; 7 ft exceeds that limit. Second, a 1-1/2-inch drain must slope at 1/4 inch per foot, not 1/8 inch per foot; 1/8 inch per foot is for 3-inch and larger drains. The arm also must not drop vertically before the vent. Correct by moving the tub trap within 6 ft of the vent and maintaining 1/4 inch per foot, or by installing a 2-inch trap and 2-inch trap arm if the tub outlet allows and the length is within 8 ft at 1/4 inch per foot. Another option is a separate vent or revent for the tub. The inspector will require a corrected isometric before cover.
3. What vent and testing requirements should be verified before the walls are closed?
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Verify that the existing 2-inch vent is adequate for the bathroom group and that its developed length is within the vent tables. The vent must terminate at the required height above the roof and away from openings, air intakes, and property lines. Check that the vent is continuous, properly supported, and not cut by the notched framing. The water closet wet vent must be 2 inch and within its length limit. The tub trap arm must be corrected as noted. Before cover, perform the DWV water or air test required by the Los Angeles Plumbing Code, with the test maintained for the required time and witnessed by the inspector. Also verify the stack, cleanouts, and trap primers.