When a trap connects to a vertical drain, install a tee fitting. A “sanitary tee” gives the waste arm a slight downward slope, enough for good flow, but not enough to create a siphon.
Never Connect a Trap to a Vertical Drain Like This
Updated Dec. 18, 2023
Connecting a trap to a vertical drain with a wye fitting and a 45-degree elbow seems like a good idea. But it’s not. As water drains down the steep slope at the wye, it can create a siphon effect such that water will be sucked out of the trap. And the empty trap will allow sewer gas to flow into your home.
Note: Joining ABS (black) pipe to PVC (white) may look like a blunder, but it’s actually allowed by many local codes—as long as you use “universal” cement to glue them together.
The Right Way
When a trap connects to a vertical drain, install a tee fitting. A “sanitary tee” gives the waste arm a slight downward slope, enough for good flow, but not enough to create a siphon.
