Slope and Trap Arm Length for a Shower Drain
A remodeling project involves installing a new shower drain in a second-floor bathroom. The shower drain is 2 inches in diameter and connects to a 2-inch trap arm that runs horizontally 6 feet to a vertical stack. The trap arm has a 2-inch P-trap with a 2-inch water seal. The horizontal drain pipe from the trap to the stack has a slope of 1/4 inch per foot. The stack is a 3-inch soil stack. The local code requires that the trap arm length not exceed the developed length allowed by the code, which for a 2-inch trap arm is typically 5 feet unless the trap is vented. The code also requires that the slope of the trap arm be sufficient to drain the fixture but not so steep that it causes self-siphoning. The shower is on the second floor, and the stack continues to the roof as a vent.
1. Does the trap arm installation comply with the code? Explain.
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The trap arm length is 6 feet, which exceeds the typical code limit of 5 feet for a 2-inch trap arm without a vent. The code limits the trap arm length to prevent self-siphoning of the trap seal. Therefore, the installation is non-compliant. Additionally, the slope of 1/4 inch per foot is within the typical range (1/4 to 1/2 inch per foot) for a 2-inch pipe, so the slope is acceptable. However, the excessive length is a violation.
2. What options are available to bring the installation into compliance?
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Options include: (1) Relocate the stack or the shower drain to reduce the trap arm length to 5 feet or less. (2) Install a vent on the trap arm within the allowed distance (e.g., a circuit vent or individual vent) to increase the allowable trap arm length. (3) Increase the pipe size to 3 inches, which may allow a longer trap arm (check local code; some codes allow longer trap arms for larger pipes). (4) Install a vented trap or use a combination waste and vent system if permitted. The most practical solution is to add a vent on the trap arm within 5 feet of the trap.