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.

Photo: Courtesy of American Society of Home Inspectors

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.

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