Third-Floor Stack: Trap Arm, Vent Connection and Building Drain Slope
Rough-in inspection of a three-story mixed-use building: retail on grade, apartments on floors 2 and 3. Findings: on the third floor, a 1 1/2-inch kitchen sink trap arm runs 8 feet at 1/4 inch per foot before it reaches the 3-inch soil stack, and the sink vent is taken off the stack 2 feet above that connection. The third-floor bathroom group vents into the same stack at a point 4 feet below the second-floor bathroom group's drain connections. In the basement, the 4-inch building drain runs 60 feet to the building sewer at 1/16 inch per foot, and the invert at the sewer is only about 4 inches below the basement slab. The plumber's helper says the slope is fine because "4-inch pipe is allowed 1/8 inch." The approved plans show the 4-inch building drain at 1/8 inch per foot.
1. Evaluate the third-floor kitchen sink trap arm. Is it compliant, and what are the acceptable corrections?
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It is not compliant. The controlling principle is that a fixture drain must be vented before the trap arm falls more than one pipe diameter; otherwise the arm fills and siphons the trap. At 1/4 inch per foot over 8 feet, the arm drops 2 inches. The pipe is 1 1/2 inches, so the permitted fall is used up well before the vent connection, and the arm between the trap and the stack is effectively unvented. Confirm the actual limit against the trap-arm table in the adopted code, but the 2-inch drop on a 1 1/2-inch arm fails on the principle alone.
Acceptable corrections: move the vent fitting closer to the trap so the vent is taken within the permitted arm length and before the fall exceeds one pipe diameter; or increase the drain size after the trap to a 2-inch trap arm, which buys more allowable length, provided the trap itself and the fixture outlet are coordinated with an appropriate increaser and the vent remains properly sized. Whichever is chosen, the vent must rise vertically above the fixture flood level rim before it offsets, and the whole assembly must be re-inspected before cover.
2. Is the third-floor vent connection to the stack acceptable? Explain the rule, the risk, and the fix.
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No. A vent must connect so that it does not become a drain. Tying the third-floor vent into the stack 4 feet below the second-floor bathroom group's drainage connections means the segment of stack above that point is carrying the second floor's discharge while also serving as the third floor's vent. That segment is a wet vent whether or not anyone called it one, and it is subject to surcharge, foam and slug flow that can seal the third-floor vent and siphon traps.
The exception that permits wet venting is narrow: it generally covers bathroom groups on the same floor, requires the wet-vented portion to be sized larger than the drain it serves, limits the length and the fixtures allowed, and does not simply authorize venting one floor into another floor's drainage stack.
The fix is to carry the third-floor vent independently up to a point above all drainage connections on that stack, or to a separate vent stack carried to open air, sized for the fixture units it serves. Re-inspect the corrected vent before the walls close, and confirm the vent termination above the roof is correct.
3. The building drain slope: is 1/16 inch per foot acceptable? Explain the consequence and the available options.
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No. The code slope table requires the steeper slope up to a size break: 1/4 inch per foot for small pipe, 1/8 inch per foot for the 3- through 6-inch range, and 1/16 inch per foot only for 8-inch and larger. A 4-inch building drain at 1/16 inch per foot is below the minimum, and the approved plan at 1/8 inch per foot confirms the design intent. The helper's statement is a misreading of the table.
The consequence is practical, not just administrative: 60 feet at 1/8 inch per foot needs 7.5 inches of fall, and only about 4 inches is available at the sewer invert, so the as-built geometry cannot satisfy the code as drawn, and the flatter pipe will drop solids and clog.
Options to bring back to the designer and the code official: raise the basement floor or the drain elevation, lower the sewer tap or the lateral outside, shorten the run, install a lift station and discharge higher, or submit an alternative engineered design for approval. The inspector cannot waive the slope in the field; document the deficiency and require a revised design before the basement slab is poured.