A plumber is installing a battery of 6 lavatories in a commercial washroom. All lavatories connect to a common 50 mm branch drain. What is the minimum size of the vent pipe required for an individual lavatory, assuming it is dry-vented?

Question: A plumber is installing a battery of 6 lavatories in a commercial washroom. All lavatories connect to a common 50 mm branch drain. What is the minimum size of the vent pipe required for an individual lavatory, assuming it is dry-vented?

Answer options:

• 25 mm
• 32 mm
• 38 mm ✅ 50 mm

Correct answer: 50 mm

Explanation: According to the National Plumbing Code of Canada, the minimum size of an individual vent pipe for a fixture is not less than half the size of the drainage pipe it serves, but never less than 32 mm. However, for a single lavatory, the vent must also be at least the same size as the trap arm, which is typically 38 mm or 50 mm. A 50 mm vent is generally considered safe practice for commercial lavatories to prevent potential trap siphonage or back-pressure, although 38 mm is technically permissible for a single lavatory's individual vent if the trap arm is 38mm. Given that the fixture itself typically requires a 38mm trap, a 50mm vent is often preferred for future-proofing and better performance, especially in a battery setup. For a lav, typical trap size if 38mm, meaning the vent must be at least 38mm. However, in Canada under NPC, the minimum trap arm for a lavatory is typically 38 mm, therefore the individual vent cannot be smaller than 38 mm. Given the options, 50mm would be the safest and commonly used practice for a commercial lavatory's individual vent.