Short definition
A common vent is a single vent that serves two fixtures placed back-to-back or side-by-side on the same horizontal branch at the same level — most often a pair of lavatories sharing one wall. It uses the same protection logic as an individual back-vent but only requires one vertical run of vent pipe instead of two, which makes it the standard economical choice for paired bathroom sinks.
What it is
Two fixtures with their trap arms entering a single fitting, with one vent rising from that fitting to serve both, is the common vent arrangement. UPC 908 permits it for two fixtures on the same horizontal branch at the same elevation, as long as the fixtures have matching drainage fixture units (DFU) — or the vent is upsized to handle the difference.
Two specifics matter for the install:
- The fixture trap arms must enter the fitting at or near the same level.
- For different elevations, the smaller-DFU fixture’s drain must enter at or above the larger-DFU fixture’s trap weir, so its discharge doesn’t dump on top of the other trap.
Why it matters to a homeowner
If your remodel quote says “common-vent the lavatories,” the plumber is choosing the cheaper, code-compliant option — fewer holes through the top plate, less vent pipe, faster install. That’s a fine choice for paired vanities. The thing to watch is what happens later: if a future remodel relocates one of the two fixtures, the remaining “common vent” becomes an uncommon vent, and the surviving fixture loses proper venting. A future plumber will need to re-vent it.
When you’ll encounter this term
- Remodel quote for a paired vanity with two sinks: “common-vent the lavatories.”
- Permit drawing shows a tee fitting joining two trap arms before a single vertical vent — that’s the common vent.
- Pre-purchase inspection notes a relocated fixture left an old common vent serving only one trap.
Common variants / not the same as
- Common vent vs. wet vent. Common is a dry vent shared between two fixtures. Wet is a drain that doubles as a vent for upstream fixtures.
- Common vent vs. individual back vent. Individual is one vent per trap. Common is one vent for two traps.
- Common vent vs. circuit / loop vent. A circuit serves a battery of multiple fixtures, often on a branch with toilets. Common is for back-to-back pairs only.