What drain size is required for a shower or bathtub?

Fixture drain sizes are defined in the Quebec Plumbing Code:

Bathtub:

• Drain diameter: 1 ½ in (38 mm) minimum
• The trap must be a 1 ½ in P-trap, accessible via an access panel
• The overflow: connected to the drain downstream of the trap

Shower:

• Drain diameter: 2 in (51 mm) minimum (larger than bathtub to drain large water volume quickly)
• The shower base must have a slope of at least ¼ in per foot (2%) toward the drain
• Maximum distance from trap to vertical drain: 1.5 meters
• A shower drain must have a vent stack (revent) within 1.2 meters

Typical flow rates:

• Bathtub: 10-15 GPM when filling
• Shower: 2-5 GPM depending on the showerhead (WaterSense heads limit to 2 GPM)

A walk-in shower (zero-entry) requires a linear drain or a specifically designed base and an accessible trap.