Yes — P-traps are required by plumbing code on virtually every drain fixture in a home, including sinks, showers, bathtubs, floor drains, and laundry tubs. The requirement exists for a straightforward safety reason: P-traps hold a small amount of water that physically blocks sewer gases — including hydrogen sulfide and methane — from flowing back up through the drain and into your living space. Without that water seal, your home would regularly smell like a sewer, and in severe cases, combustible or toxic gas could accumulate.

How a P-Trap Works

A P-trap is the curved section of drain pipe — shaped like a sideways “P” — installed directly below a drain opening. When water flows down the drain, it passes through the curve, and a small amount remains trapped in the bottom of the curve by gravity. That standing water creates a liquid seal. Sewer gases can’t travel past a water-filled trap; they’re blocked and vented out through the home’s vent stack to the exterior instead.

The trap also catches small objects — rings, screws, earrings — before they travel deeper into the drain system where they’d be much harder to retrieve. This is why the cleanout cap on a P-trap under the sink is a useful first stop when something gets dropped down the drain.

What Plumbing Code Says

Both the International Plumbing Code (IPC) and the Uniform Plumbing Code (UPC) — the two model codes most commonly adopted across the U.S., including Colorado — require a trap on every fixture drain. Colorado jurisdictions follow the IPC. The trap must be installed as close to the fixture as practical, the trap arm (the horizontal pipe after the trap) must slope properly toward the drain stack, and the trap must be within a specified distance of the vent. All of these requirements work together to keep the trap sealed and functioning correctly.

Common P-Trap Problems

  • Evaporation: In rarely used drains — a guest bathroom sink, a basement floor drain — the water in the trap can evaporate over weeks or months, breaking the seal. The fix is simple: run water down the drain periodically, or add a small amount of water to refill the trap.
  • Siphoning: If a drain isn’t properly vented, negative pressure can suck the water out of the trap. This is why venting requirements exist alongside trap requirements.
  • Clogs: Hair, grease, and soap scum accumulate in the trap curve. A slow or stopped drain is often a clogged P-trap that can be cleaned without calling a plumber.
  • Leaks: The slip-joint connections on plastic P-traps can loosen over time, especially if the trap is bumped or stressed. Tightening the nuts or replacing the trap washers usually resolves minor leaks.
  • Incorrect installation: S-traps (an older configuration that drains straight down) are no longer code-compliant because they siphon easily. If you have an S-trap, it should be replaced.

Toilets Are Different

Toilets don’t have a separate P-trap because the trap is built into the porcelain base of the toilet itself — you can see the curved channel at the bottom of the bowl. This is why toilets don’t require an additional trap fitting under the floor.

What About Kitchen Sink Garbage Disposals?

Yes, kitchen sinks with garbage disposals still require a P-trap. The trap is installed downstream of the disposal outlet. Double-basin kitchen sinks use a fitting called a continuous waste or a center outlet fitting to connect both basins to a single P-trap, though local codes vary on acceptable configurations. If you’re remodeling a kitchen or installing a new disposal and want to make sure the drain configuration is code-compliant, it’s worth having a licensed plumber make the connection.

When to Call a Plumber

Replacing a P-trap under a bathroom sink is a reasonable DIY task for most homeowners. But if you’re noticing sewer smells and the trap doesn’t appear to be dry or leaking, the issue may be a venting problem, a broken trap deeper in the wall, or a sewer gas entry point elsewhere in the drain system — all of which need professional diagnosis. Done’s plumbers can identify the source quickly and make sure your drains are properly configured and code-compliant.

Learn more about our kitchen and bath plumbing services, or visit our drain cleaning page if a clogged trap is the issue. For persistent sewer smells or drain venting concerns, our plumbing team is ready to help.