Sanitary tees and wyes

Entry in Fittings and valves · Updated October 29, 2026 · Written by the Know Your Pipes editors from the sources listed under References

A sanitary tee and a wye are both drain fittings that join a branch to a main line; the sanitary tee vs wye difference is the angle of entry. A sanitary tee turns the branch into the run with a short curved sweep, while a wye brings it in at a shallow angle.

The choice matters because drains work by gravity. Fittings that steer waste smoothly in the direction of flow help keep solids moving, keep air moving through the vent system and let a drain cleaning cable pass^[1].

1 Shape and naming

Seen from the side, a sanitary tee looks like a T whose branch curves down into the main run. A wye looks like the letter Y: the branch joins the run at a slant. A combination wye and eighth bend (often shortened to "combo") pairs a wye with an eighth bend, the 45-degree bend, so the branch turns a full right angle on a long, gradual radius^[1].

Plastic DWV makers list these shapes by name in their fitting lines, alongside quarter bends, eighth bends and couplings^[3]. They are made in PVC DWV, ABS and cast iron, and a wider view of shapes is in fitting shapes.

2 Allowed directions

Model codes say which pattern may be used for each change of direction. The 2018 International Plumbing Code (IPC), Table 706.3, as reproduced in the International Code Council's code essentials guide, allows the following^[1]:

The same guide's drawings show a sanitary tee standing upright in a vertical stack with the branch coming in from the side, and a wye or combination wye where a drain turns from vertical to horizontal or runs flat^[1]. It also notes that a sanitary tee may be turned the other way on a vent line, because vents are graded back toward the drain^[1].

Local plumbing programs state the rule in similar terms. King County, Washington, describes the sanitary tee as a drainage fitting for the vertical position only, most often used where a trap arm meets a vertical drop, and the combination tee-wye as usable vertically and horizontally but not on trap arms^[2].

3 Why a sanitary tee can't lie flat

Laid on its back in a horizontal drain, a sanitary tee's short sweep sends incoming waste across the main flow rather than along it. The wye's gentle angle merges the two streams instead, which is why codes keep the wye family for horizontal runs and for the turn at the base of a stack^[1].

The rules above are from one code edition. Later IPC editions, the Uniform Plumbing Code and state amendments have their own tables and footnotes, so the plumbing inspector's adopted code decides what is accepted; see plumbing codes.