Island Sink Twelve Feet From the Stack - Venting Systems and Vent Sizing
A kitchen remodel places the sink on a new island in the middle of a slab-on-grade floor, 12 feet from the nearest 2-inch vent stack. That stack already serves a bathroom group and is close to its fixture-unit capacity. The sink has a 1-1/2-inch trap and is rated 2 DFU. The local amendment prohibits air-admittance valves. The contractor proposes running the trap arm 12 feet to the stack and tying in with a sanitary tee, arguing that the slope will carry the waste and that 'a sink doesn't need much venting.' An existing 1-1/2-inch lavatory vent runs up a wall 6 feet away, but that lavatory is wet-vented by the water closet's branch.
1. Evaluate the contractor's proposal to run a 12-foot trap arm to the existing stack with a sanitary tee.
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Reject it - two separate code violations.
- Trap arm length. For a 1-1/2-inch trap, the trap arm is limited to about 6 feet of developed length at 1/4 in/ft slope, and the total fall on the arm cannot exceed the depth of the trap weir (a 1-1/2-inch trap gives roughly 1-1/2 inches of allowable fall). A 12-foot arm is roughly double the limit; the fixture would self-siphon, and the trap seal would be lost every time the sink drains.
- Fitting. A horizontal trap arm connecting to a vertical stack uses a wye or combination wye-and-eighth - never a sanitary tee for a horizontal-to-vertical connection in the direction of flow. A sanitary tee installed flat on a horizontal run also creates an illegal horizontal wet vent condition.
A sink absolutely needs venting: the vent equalizes pressure behind the slug of water so the trap seal survives. 'A sink doesn't need much venting' is exactly the reasoning that produces sewer gas complaints six months later.
2. Given that AAVs are prohibited, describe an acceptable venting method for this island sink and its key installation requirements.
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Use an island fixture vent (loop vent) - the recognized solution where a fixture is not within the trap-arm limit of a vertical vent.
How it is built:
- The sink trap arm connects to a horizontal drain that runs toward the connection point.
- A vent is taken off the drain immediately at the trap arm and rises above the flood level rim of the sink before any horizontal offset.
- The vent then turns and returns down to connect back into the same drain line downstream of the trap, forming a loop.
- Both legs are sized at not less than the trap arm size - 1-1/2 inch here - with the connection back into the drain made with a wye in the direction of flow.
- The foot vent must be sloped back toward the drain so it cannot fill with water, and it must be protected from frost where it is not in conditioned space.
- The loop must be accessible for inspection and testing before the cabinet goes in.
Because the loop vents back into the drain rather than to open air, this is only workable on a fixture with adequate drain downstream - confirm the run to the stack has capacity for the added 2 DFU at the required slope.
3. Can the nearby 1-1/2-inch lavatory vent be used to vent the island sink? Explain your reasoning and state what you would do instead.
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No. That lavatory vent is functioning as a wet vent for the bathroom group - it is carrying the waste flow of the lavatory on its way to the stack. A dry vent from another fixture may not be connected to a wet vent: joining the island sink into it would create a prohibited combined wet/dry vent arrangement, and the wet vent is already sized for its own fixture units.
The existing 2-inch stack is also a poor choice. It is near its DFU capacity, so adding the sink's vent load either overloads it or forces an unplanned upsize.
Correct approach: run a separate vent for the island loop through the roof or to a point of open air, sized for the fixture units it serves, or upsize and re-vent the existing stack with the AHJ's approval. Verify that the new penetration and termination meet the code's requirements for distance from operable windows and returns, and that it is increased one size where it passes through the roof if required locally. Do not solve this at the slab; solve it in the venting plan before the floor is poured.