Trap Arm Lengths and Slope on a Slab Remodel
A slab-on-grade rancher is being remodeled. The new bathroom group is 22 feet from the existing 3-inch vent stack. Proposed drains are: water closet with a 3-inch trap arm 9 feet long; lavatory with a 1.5-inch trap arm 7 feet long; shower with a 2-inch trap arm 6 feet long. The horizontal branch from the bathroom to the stack is 3-inch PVC and slopes 1/4 inch per foot over the 22-foot run. The lavatory trap arm will also run at 1/4 inch per foot. The inspector has flagged the lavatory only. The contractor argues that all trap arms are under 10 feet and that the 3-inch branch has more slope than required. The building drain is 4-inch PVC, and no new vent is planned for the bathroom group.
1. Check each trap arm against the maximum length allowed for its size. Which trap arms comply, and which fail?
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Using the plumbing code trap-arm limits, a 1.5-inch trap arm is limited to 6 feet, a 2-inch arm to 8 feet, and a 3-inch arm to 12 feet. The water closet at 9 feet is therefore within the 12-foot limit. The shower at 6 feet is within the 8-foot limit. The lavatory at 7 feet exceeds the 6-foot limit for a 1.5-inch trap arm, so the inspector is correct to flag it. The contractor’s “under 10 feet” rule is not the code rule; the limit changes with trap-arm size because the arm must vent before the trap seal is siphoned. The lavatory must be relocated closer to the vent, or a new vent must be installed within 6 feet of the trap weir.
2. Calculate the invert drop on the 22-foot branch if it is installed at 1/4 inch per foot. If the stack connection invert is 30 inches above the finished floor, what is the upstream invert at the bathroom?
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At 1/4 inch per foot, the drop is 22 feet × 0.25 inch per foot = 5.5 inches. Because the pipe slopes downward toward the stack, the upstream invert at the bathroom is higher than the stack connection. If the stack connection invert is 30 inches above finished floor, the upstream invert is 30 + 5.5 = 35.5 inches above finished floor. A 3-inch horizontal drain is allowed to slope at 1/8 inch per foot, so 1/4 inch per foot is more than the minimum and is acceptable for drainage. The extra slope does not correct the lavatory trap-arm length; venting and trap-arm distance are separate requirements.
3. If the contractor adds a vent for the lavatory, what size and location should be verified, and what else must be checked before the slab is closed?
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The new vent must connect to the lavatory trap arm within the 6-foot limit for a 1.5-inch trap arm, measured from the trap weir to the vent connection. Its size must meet the vent-sizing table for a 1.5-inch trap; commonly that is at least 1.25 inch, though 1.5 inch is often used. Verify that the vent rises vertically or at an approved angle and does not create a horizontal offset that could collect water below the flood level. Before the slab is closed, inspect the drain slope, trap-arm lengths, vent connection, pipe support, and the transition to the 4-inch building drain. Also confirm that the water closet and shower vents are correctly connected and that no future fixture will be wet-vented improperly.