Are S-traps legal in Arizona, and why are they banned?

Not for new installations in Phoenix. Valley cities adopt their own code editions, so check the one your city uses; our plumbing code library lists them. In Phoenix, both of the adopted codes that cover homes put S traps on the prohibited list:

• The plumbing code. Phoenix's adopted 2024 International Plumbing Code, Section 1002.3, lists the prohibited traps. The list includes traps that depend on moving parts to maintain the seal, bell traps, crown-vented traps, drum traps, and "S" traps.
• The residential code. Phoenix's adopted 2024 International Residential Code, Section P3201.5, says "The following types of traps are prohibited" and includes "S" traps, along with bell traps, drum traps and trap designs with moving parts.

ASPE, the society for plumbing engineers, sums up the idea behind the list: these trap types are prohibited "due to their tendency to lose the seal or create maintenance or safety problems."

### What code wants instead

A trap that holds its water and a vent that protects it. Phoenix's IRC says trap seals "shall be protected from siphonage, aspiration or back pressure by an approved system of venting." The plumbing code sets the size of that seal: each fixture trap "shall have a liquid seal of not less than 2 inches (51 mm) and not more than 4 inches (102 mm)," with deeper seals allowed for special designs relating to accessible fixtures. In practice, that means a P-trap connected to a vented drain. See what a P-trap is and why it matters for how that seal works.

Because they siphon themselves dry. The water sitting in a trap is the only thing stopping sewer gas from coming up the drain. If that water gets pulled out, the drain is open to the sewer.

An unvented downward path can create self-siphonage as water drains, pulling water out of the trap. The adopted code requires an approved venting arrangement to protect the seal.

The shape is the problem. After the water leaves the trap, it drops straight down. A full sink draining through that drop pulls the trap water along behind it. With no vent to let air in, nothing stops the seal from following the rest of the water down the pipe.

### Why the smell matters

A lost seal leaves a path for sewer gases into the room. Correct the trap and venting condition rather than masking the odor.

### Signs an S trap is losing its seal

• A sewer smell from that fixture, especially after heavy use.
• A trap that seems to have little or no water in it right after draining.

A sewer smell has other causes too. See why a bathroom drain smells like sewer for the full list.

### What else is on the banned list?

S traps aren't the only old design the code rules out. Phoenix's adopted codes also prohibit bell traps, drum traps and traps with moving parts, and the plumbing code adds crown-vented traps. The plumbing code does allow drum traps in one narrow case: "Drum traps used as solids interceptors and drum traps serving chemical waste systems shall not be prohibited."

Two more trap rules catch people during remodels. Phoenix's residential code says "Building traps shall be prohibited," which means no extra trap on the main building drain. And it says "Fixtures shall not be double trapped," so a sink can't drain through two traps in a row. Each fixture gets one trap, and the code limits how far it can sit from the fixture outlet: no more than 24 inches vertically and 30 inches horizontally.

Not automatically, as long as it works and isn't causing a hazard. Phoenix's adopted plumbing code allows existing systems to stay in use under certain conditions. It says plumbing systems "lawfully in existence at the time of the adoption of this code shall be permitted to have their use and maintenance continued" as long as the use, maintenance or repair follows the original design and no hazard to life, health or property is created.

That protection has limits:

• If it's causing a problem, such as a lost seal and sewer gas, it's no longer the kind of safe, existing system the code protects.
• If you remodel or repair, new work has to meet the current code. Phoenix's code says additions, alterations, renovations or repairs "shall conform to that required for a new plumbing system." Minor repairs can be done the old way only when the work "is not hazardous and is approved."

If the project alters the drain or venting, ask the local authority whether the existing installation must be brought to current requirements.

### What does converting it involve?

It depends on what's behind the wall. Swapping the trap itself is simple. The real question is venting. A P-trap needs its trap arm to run into the wall and connect to a drain that's properly vented. If the drain comes straight up from the floor with no vent nearby, that part of the drain may need to be rerouted or a vent added.

This is also where DIY fixes go wrong. Rebuilding the trap with extra bends, or dropping the trap arm steeply toward the drain, can recreate the same siphoning problem. The applicable trap-arm slope and vent-distance rules still have to be met.

### How far can the trap be from the vent?

There are limits on that too. The trap arm, the pipe between the trap and the vent, can only run so far for a given pipe size. Our page on how long a trap arm can be covers the distance rules that decide where a vent has to be. When the vent is too far away, the fix is usually in the wall, not under the sink.

If you've got an S trap and a smell you can't shake, we can look at the drain and the venting behind it. HQ is licensed and insured, and every estimate is free with upfront pricing.