A technician is troubleshooting a persistent odour issue in a bathroom despite the toilet and lavatory appearing to drain normally. Upon inspection, they find that the trap arm for the lavatory is extended significantly. What is the maximum developed length permitted for a 38 mm lavatory trap arm to prevent trap siphonage in most residential applications according to the NPC?

Question: A technician is troubleshooting a persistent odour issue in a bathroom despite the toilet and lavatory appearing to drain normally. Upon inspection, they find that the trap arm for the lavatory is extended significantly. What is the maximum developed length permitted for a 38 mm lavatory trap arm to prevent trap siphonage in most residential applications according to the NPC?

Answer options:

• 0.75 m ✅ 1.5 m
• 2.5 m
• 3.0 m

Correct answer: 1.5 m

Explanation: According to the National Plumbing Code of Canada, the maximum developed length for a 38 mm trap arm is 1.5 m. Exceeding this length can lead to insufficient venting effect, making the trap susceptible to self-siphonage and potential loss of its water seal, thus allowing sewer gases to enter the living space.