What is the maximum vertical distance between a fixture outlet and the trap weir?

• A. 12 inches
• B. 18 inches
• C. 24 inches
• D. 30 inches

Answer: (C)

The main idea is that the trap must stay within a short vertical reach of the fixture outlet to keep the trap water seal from being siphoned and to allow proper venting. If the fixture outlet is too far above the trap, the pushing water column during drainage can create negative pressure that pulls water out of the trap, breaking the seal and letting sewer gases escape. Limiting the vertical distance helps ensure the trap remains primed and that venting can effectively prevent siphonage. That’s why the maximum allowed distance is 24 inches.