How does plumbing venting work?

Plumbing venting maintains atmospheric pressure in the drainage pipes, essential for preventing siphoning and ensuring proper flow. According to the Quebec Plumbing Code:

Venting principles:

• Each plumbing fixture must be individually vented or via a secondary vent
• The vent pipe must have a minimum diameter of 1 ¼ in (31.75 mm)
• The maximum distance between a fixture trap and its vent depends on the drain pipe diameter (distance rule: 1.2 m for 1 ¼ in, 2.4 m for 2 in, etc.)
• The vent must terminate through the roof at least 300 mm (12 in) above the finished roof

Types of vents:

• Primary vent: Extension of the main drain above the roof
• Secondary vent (revent): Connection between a fixture drain and the main vent
• Circuit vent: For fixture batteries (complete bathroom group)

Inadequate venting causes gurgling, sewer odors, and slow drains.