Field Inspection: Trap Arm Distance and Vent Takeoff
During a rough-in inspection of a single-family house, the inspector finds a kitchen sink trap arm that runs 9 feet horizontally to a 2-inch vent, with the trap 15 inches below the sink. The sink is a double-bowl with a 1-1/2-inch trap arm. The vent rises vertically and connects to a 2-inch stack. The plumber says the sink drains fine and that the vent is "right there." The inspector must decide whether to approve the rough-in or require a correction.
1. Evaluate the trap arm distance and trap-to-vent relationship. Should the inspector approve?
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The inspector should check three things: trap arm length, trap arm slope, and the vent takeoff geometry.
Trap arm length: A 1-1/2-inch trap arm has a maximum developed length from the trap weir to the vent fitting, commonly 6 feet under many codes (3-1/2 feet under some older tables). A 9-foot run exceeds that limit. The plumber's "it drains fine" is not the standard; the standard is the code limit, which exists to prevent the trap from siphoning.
Slope: The trap arm must slope at 1/4 inch per foot toward the drain, and the fall must not exceed the trap's seal depth. At 9 feet, the fall would be 2.25 inches, which is within a 2-inch trap seal, but the length limit is still exceeded.
Vent takeoff: The vent must rise vertically or at an approved angle from the trap arm, and the connection must be above the trap weir. The inspector should verify the vent does not create a wet vent where prohibited.
Conclusion: The inspector should not approve. The correction is to add a vent closer to the trap (within 6 feet) or to increase the trap arm size if the code permits a longer run for a larger trap arm, and to re-inspect.
2. The plumber proposes adding an air admittance valve (AAV) at the sink instead of re-piping. What should the inspector consider before accepting?
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An AAV may be permitted only where the adopted code allows it, and only under specific conditions: it must be listed and labeled, installed in an accessible location, and there must be at least one vent through the roof serving the drainage system. The AAV must be sized for the fixture it serves and installed per the manufacturer's instructions, typically above the trap weir and with a minimum height above the flood level rim.
AAVs are often prohibited for certain fixtures or in certain occupancies, and some jurisdictions do not adopt the AAV provisions at all. The inspector should confirm the local amendment status before accepting the substitution.
If AAVs are allowed, the inspector should verify that the valve is accessible (not buried in a wall), that it is not installed in a location subject to flooding, and that the vent system still has a roof vent. The AAV solves the trap-arm distance problem only if the valve is installed within the allowable distance from the trap. The inspector should require a permit revision and re-inspection, not accept a verbal change.