Short definition
A sanitary tee is a drainage-tee fitting with a curved sweep at the side branch entry, used to bring a horizontal branch drain into a vertical stack. The single rule that matters: it can only be used to take a horizontal flow into a vertical drain — never horizontal-to-horizontal, and never vertical-to-horizontal. Used wrong, it’s one of the most common DWV code violations.
What it is
In drainage fittings, the angle of the side-branch sweep determines whether the fitting can carry flow without splashback or solids deposit. A wye fitting has a 45-degree sweep; a combination wye-and-1/8 bend (“combo”) brings that to 90 degrees with two gentle 45-degree bends in series. A sanitary tee has a 90-degree side branch with a tighter sweep — gentle enough to carry waste into a stack from a horizontal line, but too tight to redirect downward flow without trouble.
By Uniform Plumbing Code 706.4 (and IPC 706.3), vertical drainage lines connecting to horizontal drainage lines must enter through a 45-degree wye, a combination wye-and-1/8 bend, or another approved sweep — not a sanitary tee.
Why it matters to a homeowner
This is one of the small details that turns a DIY remodel into a failed inspection or a future clog. A sanitary tee installed “on its back” — laid horizontally with the side opening pointing upward, used as a vertical-to-horizontal transition — sends waste straight into the bottom of the run, splashes upstream, and deposits solids at the elbow. The clog returns within weeks of being snaked.
When a plumber’s quote says “replace san-tee with combo,” they’re fixing exactly this mistake. When an inspector flags it on a permit job, the section gets cut out and the right combination fitting goes in. It’s a cheap fix during rough-in and an expensive one after the wall goes back up.
When you’ll encounter this term
- An inspector flags a sanitary tee installed on its back during a remodel rough-in.
- A sewer-scope or DIY review finds a sanitary tee at a vertical-to-horizontal transition; the plumber’s repair line says “replace san-tee with combo.”
- Permit drawings specify “wye + 1/8 bend at base of stack” — the standard alternative.
- Reading about DWV layout for a basement bathroom and trying to figure out which fitting goes where.
Common variants / not the same as
- Sanitary tee vs. combo (combination wye-and-1/8 bend). Combo has a 45 + 45 sweep; san-tee has a 90 + sweep. The combo is what’s required where the san-tee fails.
- Sanitary tee vs. wye. A wye is a 45-degree branch; a sanitary tee is a 90-degree branch with a sweep.
- Sanitary tee vs. vent tee. Vent tees look similar but are labeled DWV; geometry varies by manufacturer. Read the fitting markings — DWV san-tees and vent fittings aren’t always interchangeable.