Basement Bathroom Trap Arms and Vent Location
A basement bathroom rough-in has a 1-1/2 in. lavatory trap arm running 8 ft horizontally at 1/4 in. per ft to a 2 in. vent stack, and a 2 in. shower trap arm running 9 ft at 1/4 in. per ft to the same stack. A 3 in. water closet connects through a 6 ft closet bend. The inspector flags the lavatory and shower but passes the water closet. The owner wants to keep the layout and asks whether simply increasing the two trap arms to 2 in. and 3 in. would solve the problem.
1. Evaluate each trap arm against typical maximum trap-arm length limits at 1/4 in. per ft slope.
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At 1/4 in./ft, common trap-arm limits are about 6 ft for 1-1/2 in., 8 ft for 2 in., and 12 ft for 3 in. The lavatory arm is 8 ft, which exceeds the 6 ft limit. The shower arm is 9 ft, which exceeds the 8 ft limit. The 3 in. water closet arm at 6 ft is within the 12 ft limit, assuming the closet bend is the trap arm and the vent is properly located. The inspector's flags are consistent with those limits.
2. What is the consequence of leaving the lavatory arm at 8 ft, and what two compliant remedies would you propose?
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An overlong trap arm allows the trap seal to be siphoned or pressure-blown out when the fixture drains, admitting sewer gas. Two compliant remedies are: (1) relocate the vent connection or stack so each trap arm is within its limit - lavatory no more than about 6 ft and shower no more than about 8 ft; or (2) install an individual revent for each fixture within the allowed distance and connect it to the vent stack above the fixture flood-level rim. Simply upsizing the arm is not a reliable cure and may be prohibited because the trap arm must match the trap outlet and must not be reduced; verify local amendments.
3. If the lavatory and shower are wet-vented by the 2 in. stack, what additional conditions would you check before accepting the installation?
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A wet vent is only acceptable where the code allows it. Check that the fixtures are part of the same bathroom group on the same floor, that the wet vent is sized for the total fixture units it carries, that each fixture drain connects to the wet vent within the allowed length, and that the water closet is downstream of the wet-vented fixtures or otherwise arranged as the code permits. Also confirm the vent stack is continuous and does not become a drain above the highest connection.