UPC Section 1003 describes which fixture traps are approved and which are banned. The standard P-trap is allowed. S-traps, bell traps, drum traps, and traps with moving or hidden parts are prohibited. Each fixture gets one approved trap, and double-trapping is not allowed.
The Uniform Plumbing Code is published and copyrighted by IAPMO. This page explains the section in our own words with a short excerpt only. Read the full official text at the source.
A trap holds a small pool of water that blocks sewer gas from rising into your home. But not every trap shape does that job well over time. UPC Section 1003 describes which fixture traps are approved and which are banned. The rule is not just about having a trap. It is about having the right kind of trap. Some old designs are prohibited because they lose their water seal or hide problems inside.
What this section covers
Section 1003 sits in Chapter 10 of the UPC, which covers traps and interceptors. Its heading reads:
1003.0 Traps - Described
This section sets what a legal trap looks like and lists the trap types that are not allowed. It works with the rest of Chapter 10, which requires a trap on each fixture and limits how far the trap can sit from its vent.
The approved trap
The standard fixture trap is the P-trap. It is a simple U-shaped bend that holds water by gravity and refills every time you use the fixture. It has no moving parts and nothing hidden inside, so an inspector or plumber can see the whole thing. For a plain-language walk-through, see what a P-trap is. The water it holds is the trap seal, and that seal is what stops sewer gas.
Traps the code bans
The code prohibits several older trap designs. Common banned types include:
- S-traps. An S-shaped trap can siphon itself dry as water rushes past, which leaves the seal empty and lets gas in.
- Bell traps. These rely on a loose cover and lose their seal easily.
- Drum traps. These older cylinder-shaped traps trap debris and are hard to clean. See what a drum trap is.
- Traps with moving parts or hidden interior partitions. If part of the trap can wear out, stick, or hide a clog where no one can see it, it is not allowed.
The reason is the same across the list. A good trap keeps a steady water seal, cleans out, and shows its condition in plain view. Designs that fail those tests are out.
One trap per fixture
The code also limits double-trapping. A fixture gets one approved trap. Putting two traps in series on the same drain is prohibited, because air can get stuck between the two water seals and choke the flow. So the fix for a slow or smelly drain is almost never a second trap. It is a correct single trap that is properly vented.
Why this matters in Phoenix
Older Phoenix homes often have S-traps or drum traps left from past decades. They may pass unnoticed until a remodel, a fixture swap, or an inspection. When that happens, the old trap has to be replaced with an approved P-trap. If a drain keeps smelling like sewer even after cleaning, a bad or dried-out trap is a common cause. See why a bathroom drain smells like sewer. How far the trap can sit from the drain opening is a separate rule, covered in the trap arm length limit. Keeping the seal from drying out is covered in UPC 1007 on trap seal protection.
Full text and source
UPC Section 1003 is part of the Uniform Plumbing Code. IAPMO publishes it and holds the copyright. The UPC viewer shows the section heading, not the full rule text, so only the heading is quoted here and the banned-trap list is paraphrased from the code's described prohibitions. The exact wording is described, not quoted, because the body text is not shown on the viewer. Phoenix enforces the 2024 UPC with local amendments. Read the section on the UPC viewer at UpCodes, review the official code at IAPMO, or confirm local amendments with the City of Phoenix Planning & Development Department at phoenix.gov/pdd.