UPC Section 909.0 covers special venting for island fixtures like a kitchen island sink that has no wall behind it. The vent rises to drainboard height, loops over, and drops to a foot vent below the floor. A cleanout must be provided so the loop can be rodded clear.
The Uniform Plumbing Code is published and copyrighted by IAPMO. This page explains the section in our own words with a short excerpt only. Read the full official text at the source.
Most sinks vent up through the wall behind them. A kitchen island sink has no wall behind it. The drain still needs air, or it will gurgle, drain slowly, and siphon its own trap dry. UPC Section 909.0 gives island fixtures a special fix called an island vent, or loop vent. It routes the vent up and back down under the countertop instead of into a wall.
What this section says
Section 909.0 is titled Special Venting for Island Fixtures. It applies to a sink or similar fixture set in an island, with no next-door wall for a normal vent. The vent pipe rises off the fixture drain:
vent as high as possible, but not less than the drainboard height and then returning it downward
From that high loop, the vent drops back down and ties into the horizontal sink drain just past the fixture drain. That upside-down U shape is the loop. Below the floor, a second vent called the foot vent branches off with a wye fitting. The foot vent carries the air connection over to a wall or up through the roof. The drain still has to slope about 1/4 inch per foot back toward the main line.
Because the loop dips down under the counter, it can trap water and debris. So the code requires an accessible cleanout in the vertical part of the foot vent. That lets a plumber run a rod through and clear the vent if it plugs. Island venting is allowed for sinks. A kitchen sink that also drains a dishwasher or a food waste disposer can use this method too.
When this comes into play
This is the go-to method any time a sink sits out in the open. Picture a kitchen remodel that adds an island with a prep sink. There is no wall to vent into, so the plumber builds a loop vent under the cabinet. The vent rises to just below the countertop, loops over, and drops to a foot vent that runs to the nearest wall. Inspectors look closely at island vents, because a loop built too low or one with no cleanout is a common defect.
What this means for you
If an island sink gurgles or drains slowly, its loop vent may be built wrong or partly blocked. A vent problem is not something you can plunge away. See what a plumbing vent is for the basics, and plumbing vent termination rules for where vents finally exit the roof. The loop still has to reach the home's vent stack and terminate like any other vent, which ties back to UPC 906. A related fix for the same open-fixture problem is UPC 910 combination waste and vent. Island venting is fussy work, so it is worth having a licensed plumber lay it out.
Full text and source
UPC Section 909.0 is part of the Uniform Plumbing Code. IAPMO publishes and holds the copyright on it. The short excerpt above reflects the rule as adopted; Phoenix enforces the 2024 UPC with local amendments. Read the current section on UpCodes, or confirm local amendments through the City of Phoenix Planning & Development Department: phoenix.gov/pdd.
Keep Reading
- UPC 1003: Which Fixture Traps Are Allowed (and Banned)
- UPC Section 1101: Storm Drainage and Roof Runoff
- UPC Section 603: Cross-Connection Control and Backflow Protection
- UPC 705: Joints and Connections in Drainage Piping
- What is a plumbing vent and why do vent pipes matter?
- How high does a plumbing vent have to be above the roof?