Where should the relief vent be placed when venting a bank of lavatories?

• A. In the attic above the roof
• B. On the roof near the main stack
• C. At the most downstream circuit vented fixture
• D. In the ceiling above the bank

Answer: (D)

When venting a bank of lavatories, the goal is to relieve pressure right where the traps for that group would be affected, preventing siphonage as multiple fixtures discharge. Placing the relief vent in the ceiling above the bank positions the vent closest to all the traps in that group, so air can break the suction across the entire bank and keep each trap sealed. If the relief vent were elsewhere—on the roof near the main stack, in the attic, or at the most downstream fixture—the negative pressure could still develop across other traps in the bank, or only protect a single fixture, leading to siphonage and potential sewer gas entry.