A sanitary tee is a drain fitting with a gentle curved sweep. It is used to turn drainage flow from a horizontal pipe into a vertical pipe. The sweep guides waste downward so it does not clog. It is different from a wye, which is used for other turns.
A sanitary tee is a drain fitting used in DWV (drain, waste, and vent) systems. It has a gentle curved sweep where the side branch meets the run. That sweep is what makes it a drainage fitting and not a plain plumbing tee. Plumbers often call it a san tee.
How it works
A sanitary tee is built for one main job. It turns drainage flow from a horizontal pipe into a vertical pipe, such as a soil or waste stack. The curved sweep guides the waste as it makes the turn, so solids keep moving and do not hang up in the fitting. It can also connect a fixture drain to a vent stack, part of the home's plumbing vent system. The sweep always points in the direction of flow.
Sanitary tee versus wye
A wye is a different fitting with a branch that meets the run at a 45-degree angle. Because of that gentle angle, a wye is used for horizontal-to-horizontal turns, often with an eighth bend added. A sanitary tee is not allowed for horizontal-to-horizontal changes. Under UPC section 706, a sanitary tee is approved to go from horizontal to vertical, while wyes handle the flat runs.
The most common mistake
The biggest DIY error is laying a sanitary tee on its back and using it flat, as a horizontal-to-horizontal fitting. In that position the short sweep chokes the flow and can trap waste. It also breaks code. On a flat run, use a wye and an eighth bend instead. Match the fitting to the direction of flow and the drain will run clean.