Yes, with an approved drainage fitting and a properly vented trap arm. A sanitary tee commonly connects a horizontal arm to a vertical drain. Avoid a steep drop before the vent, which can siphon the trap seal. The applicable code sets trap-arm slope and vent distance; wyes can be suitable elsewhere in the system.
What goes wrong with a steep connection
The trap holds a plug of water that blocks sewer gas. The code's own summary of its vent chapter says a drainage system depends on an air path throughout the system, to keep waste and odor from blowing back into fixtures and to keep the trap seal from being siphoned out. Section 901.2 sets the limit: the vent system must be arranged so that the seal of any fixture trap "shall not be subjected to a pressure differential of more than 1 inch of water column."
A trap arm that plunges steeply into a vertical pipe is well outside the slopes the vent-distance table below allows for. If the seal is lost, nothing stops sewer gas from reaching the room.
The fitting the code allows
Table 706.3 lists which fittings can change direction in a drain. A sanitary tee is allowed for a horizontal-to-vertical change; it is not listed for horizontal-to-horizontal or vertical-to-horizontal drainage changes. That is exactly the job of a trap arm entering a vertical pipe.
Wyes, combination wye and eighth bends and long sweeps are listed for all three kinds of direction change. The slope is the catch. Section 909.1 requires the slope and length of the fixture drain, from the trap weir to the vent fitting, to fall within Table 909.1. The table pairs each trap size with a slope of 1/4 inch per foot (1/8 inch per foot for 3- and 4-inch traps). A 45-degree arm drops about 12 inches per foot, roughly 48 times steeper than 1/4 inch per foot. A steep drop before the vent connection does not meet that trap-arm arrangement. Wyes and combination fittings can still be appropriate elsewhere after proper venting; their mere presence is not a diagnosis.
How far the trap can sit from its vent
Table 909.1 sets the maximum distance from the trap to the vent:
| Trap size | Slope | Maximum distance |
|---|---|---|
| 1 1/4 inch | 1/4 inch per foot | 5 feet |
| 1 1/2 inch | 1/4 inch per foot | 6 feet |
| 2 inch | 1/4 inch per foot | 8 feet |
| 3 inch | 1/8 inch per foot | 12 feet |
| 4 inch | 1/8 inch per foot | 16 feet |
Self-siphoning fixtures such as water closets have no limit.
Section 1002.1 also limits how far the trap can be from the fixture. The vertical distance from the fixture outlet to the trap weir can't exceed 24 inches, and the horizontal distance can't exceed 30 inches.
Two codes in Phoenix
Phoenix's amendments to the 2024 IPC don't touch these sections. But Phoenix also lets designers follow the Uniform Plumbing Code, and the UPC uses its own trap-arm table. Our page on how far a trap can be from its vent gives the UPC numbers. Ask your city which code applies before you rough in.
If you already have this connection
Look under the sink or behind the access panel. A steep arm into a vertical pipe, gurgling when other fixtures drain, or a sewer smell that comes and goes can all point to a trap that's losing its seal. See why a bathroom drain smells like sewer for the other causes. If the connection needs to change, HQ gives free estimates and upfront pricing, and we're licensed and insured. For the basics, start with what is a P-trap.