Why Are There U-Shaped Pipes Under Sinks?

What looks like a U-shaped pipe is actually more of a sideways “P”. That part of the pipe is known as a P-trap, or a bend in the drain and waste pipe that traps debris and odors. Do you ever wonder how the smells of the sewers never seep into your home’s breathing air? Or how dropping something down your drain doesn’t clog the entire system? The P-trap is the reason. They are usually installed under kitchen sinks, bathroom sinks and bathtubs to prevent sewage odors from penetrating into living spaces.

How Does the P-Trap Work?

When water flows through the P-trap, some of the water gets trapped in the bend, which creates a two-inch seal of water that traps debris and sewer gases. This water seal prevents foul substances from rising back up the drain and into your home, clogging your sink and polluting your indoor air quality.

What Are Some Other Types of P-Traps?

While the traditional U-shaped pipe is the most common type of P-trap, it isn’t the only one. Others include:

• Adjustable P-Trap: An adjustable P-trap functions the same as the traditional model but with more flexibility. It allows plumbers and homeowners to adjust the width and height of the mechanism to fit your plumbing, which can be beneficial if the space in which the P-trap needs to fit is irregular and can’t accommodate standardized sizes. The potential downside is the adjustable tubing can lead to increased turbulence as water passes through.

• S-Trap: An S-trap is – as one could infer – similar to the P-trap but shaped like an S. It’s an older version of the P-trap and is less common today due to physics and the tendency for the water that should stay in the trap to be siphoned and dried out in specific scenarios (most commonly when a full sink basin is drained). This effect breaks the seal and allows the sewer smells to pass through, defeating the purpose of the trap.

• Q-Trap: A Q-trap is a modified version of the P-trap that, unlike the S-trap, has an additional vertical outlet to prevent siphoning. It requires a smaller water seal than a P-trap, but it can be harder to find than a P-trap and can be harder to clean and maintain due to its complex design.

• Bottle Trap: A bottle trap is a compact version of a P-trap that resembles a bottle or flask. They are typically used in smaller areas where space is limited or by homeowners who want more under-sink storage or prefer the aesthetic.



• Drum Trap: A much rarer version of a P-trap, a drum trap is a larger, barrel-shaped trap that contains a water seal. It has a removable cover that’s intended for easy access and cleaning, but its removability also makes it easy to install incorrectly. This old style of trap was often installed directly under floors in crawlspaces or basements. In some cases, the trap’s proximity to the floor above it makes it virtually impossible to open and access for cleaning.

Do All Homes Have P-Traps?

Building codes require traps of some kind, although there could be rare instances where an old home was constructed or plumbing fixtures were installed prior to the adoption of the International Plumbing Code (IPC) or Uniform Plumbing Code (UPC).

These homes may also have one of the previously listed traps instead of P-traps on some or all plumbing fixtures. Although they can provide adequate support, older or ineffective traps, like S-traps or drum traps, may make these homes more prone to sewer gas or clogs and backups, which is why plumbers generally recommend replacing them promptly with an efficient P-trap or one of the more effective modern trap variations.

The sink repair experts at Tom’s Mechanical Inc. have decades of experience installing, replacing and maintaining Arlington residents’ plumbing. Our plumbers will use their expertise and resources to help you find the best type of trap for your home.

Get Help From Reliable Arlington Plumbing Experts

At Tom’s Mechanical Inc., we’ve been maintaining pipes for Arlington homeowners since the 1960s. We’ll provide you with fast and hassle-free sink repair so you can use your sinks freely without worrying about hazardous sewer smells permeating your living spaces.

Call (817) 277-4493 to schedule service or learn more about our plumbing services today.