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Vent Configuration for an Island Sink and a Wet Vent

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Vent Configuration for an Island Sink and a Wet Vent

You are roughing in a kitchen island sink and a nearby bathroom group in a single-family home. The island sink is 6 feet from the nearest wall. The bathroom has a water closet, a lavatory, and a bathtub. The lavatory is between the WC and the tub. The main stack is 12 feet away. The inspector has flagged that the island sink cannot be conventionally vented and that the bathroom may be wet-vented. You must design a venting arrangement that passes inspection and explain the wet-vent logic. The homeowner wants to minimize visible piping in the kitchen.

1. How do you vent the island sink without a conventional wall vent, and what are the code constraints?

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Use an island fixture vent (also called a loop vent or island vent) arrangement: the drain from the sink trap rises to a vent taken up to the required height above the flood level rim, loops back down and connects to the drain downstream of the trap, with a cleanout at the base. Alternatively, an air admittance valve (AAV) may be permitted if the local code allows it and it is accessible.

Constraints:

  • The island vent must rise above the flood level rim of the sink before turning down.
  • The vent must connect downstream of the trap and be sized per the fixture's dfu.
  • AAVs must be accessible, installed per manufacturer and code, and not used where prohibited.
  • The loop vent needs a cleanout for maintenance.

This keeps piping hidden in the island cabinet and satisfies the vent requirement.

2. Explain how the bathroom group can be wet-vented and what conditions must be met.

Read the worked answer

A wet vent uses the drain of one fixture to vent another. With the lavatory between the WC and the tub, the lavatory drain can wet-vent the WC and tub if:

  1. The fixtures are on the same floor and in the same bathroom group.
  2. The wet-vented branch is sized per code (commonly 2-inch for a WC wet vent).
  3. The lavatory is the most upstream fixture and its trap arm meets length and slope limits.
  4. The vertical drop and horizontal offsets meet code limits.
  5. The WC connects downstream of the lavatory.

The lavatory's own vent (or the stack) provides the vent path. Verify the trap arm lengths and the wet-vent diameter against the adopted code table; if any condition fails, a separate vent is required.