A plumber is installing a wall-hung lavatory. The fixture outlet pipe (FOP) must be offset from the rough-in stub-out in the wall due to an unforeseen structural element. The local plumbing code specifies the maximum horizontal offset for the trap arm is 750 mm and the maximum vertical drop before the trap and re-vent is 600 mm. What is the most appropriate strategy to ensure code compliance while accommodating the offset?

Question: A plumber is installing a wall-hung lavatory. The fixture outlet pipe (FOP) must be offset from the rough-in stub-out in the wall due to an unforeseen structural element. The local plumbing code specifies the maximum horizontal offset for the trap arm is 750 mm and the maximum vertical drop before the trap and re-vent is 600 mm. What is the most appropriate strategy to ensure code compliance while accommodating the offset?

Answer options:

• Extend the trap arm horizontally beyond 750 mm to reach the stub-out. ✅ Install a P-trap immediately below the fixture and run a separate, dedicated re-vent pipe back to the main vent stack.
• Use a flexible trap connector to bridge the gap between the fixture and the stub-out.
• Increase the diameter of the drainage piping to compensate for the extended trap arm.

Correct answer: Install a P-trap immediately below the fixture and run a separate, dedicated re-vent pipe back to the main vent stack.

Explanation: When an offset makes it impossible to connect directly to a vented stub-out within standard trap arm limits, installing a P-trap immediately under the fixture and running a dedicated re-vent ensures proper venting and prevents trap siphonage. Extending the trap arm beyond 750 mm is a code violation. Flexible connectors are typically not allowed for trap arms. Increasing pipe diameter does not resolve venting issues caused by excessive trap arm length or inadequate re-venting.