Wet Vent

A wet vent is a drain pipe that simultaneously serves as a vent for other fixtures — it carries both wastewater and venting air in the same pipe at the same time. This is the opposite of a dry vent, which carries only air. Wet venting is a code-allowed technique (under strict conditions) that reduces the total number of pipes needed in a bathroom group, making it popular in space-constrained new construction and remodels.

How wet venting works

The drainpipe from an upper fixture (typically a lavatory or bathtub) is oversized so that wastewater flows along the bottom while air flows along the top. This air "above the water" vents the lower fixtures connected downstream — the air prevents siphoning of trap seals in those lower fixtures. The IPC (International Plumbing Code) allows a wet vent only within a single bathroom group and sets strict pipe-sizing rules.

Code requirements (IPC)

• Wet vent pipe must be at least one pipe size larger than the minimum drain size for the highest fixture it drains
• The wet-vented section must remain horizontal or pitch slightly toward the drain
• Maximum 6 fixture units can be wet-vented on a single 2-inch line
• Only certain fixture combinations qualify: lav + tub/shower and lav + toilet are common configurations

Why it matters for remodels

In a bathroom addition or basement bathroom, getting a vent stack up through finished floors to the roof can be the most expensive and disruptive part of the project. A properly designed wet vent can eliminate one or two vent runs, saving hundreds to thousands in labor and patching costs. Ask your plumber about wet vent options whenever adding a bathroom.

When wet venting is NOT allowed

A wet vent cannot serve a kitchen sink (grease), a laundry drain (lint and surge flow), or a floor drain. It also cannot serve fixtures on different floors. Any configuration outside the specific IPC rules requires a conventional dry vent.