Short definition
A wet vent is a drain pipe that simultaneously serves as the vent for an upstream fixture. It’s the most economical way to vent a compact bathroom group — the lavatory drain wet-vents the toilet, the shower drain wet-vents the lavatory, and so on. UPC 908 permits wet venting for up to four fixtures of a bathroom group on the same floor; Washington uses UPC.
What it is
In dry venting, every fixture’s vent is a separate dry pipe running up to the roof. Wet venting collapses some of that pipe by letting a drain do double duty: water flows through the bottom of the pipe while air for the upstream fixture’s trap flows around it.
UPC 908.2 sets the rules for residential wet venting:
- The wet vent serves only fixtures within a single bathroom group.
- The dry vent connects at the most upstream fixture.
- The drain pipe is sized one size larger than required by drainage fixture units (DFU) when serving as a wet vent.
IPC 909.1 has slightly more permissive rules (up to eight fixtures), but Washington follows the UPC.
Why it matters to a homeowner
Wet venting is what makes a stacked or compact bathroom feasible without a forest of vents in the wall. When a remodel quote includes “wet-vent the bath group,” the plumber is choosing the cheaper, code-compliant option. The risk is install error, not concept error:
- Capacity overload during simultaneous use — shower + toilet + lavatory at once can overload a wet vent and cause induced siphonage.
- Undersized drain pipe — the one-size-up rule is easy to miss.
- Wet vent extended beyond a single bathroom group — UPC violation.
- Adding a fifth fixture later without redesign — silently overloads the original wet-vent design.
Common variants / not the same as
- Wet vent vs. dry vent. Wet doubles as drain. Dry is air only.
- Wet vent vs. common vent. Wet protects multiple fixtures sequentially in a bathroom group. Common protects two paired fixtures at one point (back-to-back lavatories).
- Wet vent vs. circuit vent. Circuit is for a battery of similar fixtures, often non-residential. Wet vent is the residential bathroom group code path.