What is the maximum vertical distance allowed between the top of the fixture drain outlet and the horizontal centerline of the trap outlet?

What is the maximum vertical distance allowed between the top of the fixture drain outlet and the horizontal centerline of the trap outlet?

• A. 12 inches
• B. 15 inches
• C. 18 inches
• D. 20 inches

Answer: (B)

The key idea is keeping the trap close enough to the fixture outlet so the trap stays properly vented and protected from siphoning. The top of the fixture drain outlet should not be more than 15 inches above the horizontal centerline of the trap outlet. If the drop is larger, the vent may not effectively protect the trap seal, increasing the risk of siphoning and sewer gas escaping. So 15 inches is the maximum to maintain reliable drainage and trap function. Distances greater than that (for example) would not be allowed.

The key idea is keeping the trap close enough to the fixture outlet so the trap stays properly vented and protected from siphoning. The top of the fixture drain outlet should not be more than 15 inches above the horizontal centerline of the trap outlet. If the drop is larger, the vent may not effectively protect the trap seal, increasing the risk of siphoning and sewer gas escaping. So 15 inches is the maximum to maintain reliable drainage and trap function. Distances greater than that (for example) would not be allowed.