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S-Traps vs. P-Traps

S-Traps vs. P-Traps

Drain traps have been around for a long time — probably longer than you realize. In fact, a Scottish watchmaker filed the first patent for an S-trap in 1775 after he designed a way to stop losing small watch parts down the drain. The curve in the S-trap collected the watch pieces. It also held just enough water to form a seal in the drain that could stop sewer gases and odors from entering the room through the drainpipes, leading to significant advances in bathroom design.

The S-trap was such a successful design that it remained common in houses through the early 20th century. It’s still possible to find some older homes with S-traps under the sinks or extended from bathtubs — there’s no need to fix something that does its job. However, homeowners who understand the difference between S-trap and P-traps should consider upgrading the plumbing.

What Is the Difference?

The most noticeable difference between an S-trap and a P-trap is the shape of the pipe. An S-trap is shaped like the letter S, and a P-trap is shaped like the letter P. This is more than cosmetic, though. Changing the shape of the curve fixed a serious flaw in the S-trap design: dry trap. By design, the curve in the pipe holds water, and when this water is gone — through gravity or evaporation — the seal is gone.

A P-trap has a piece of pipe that runs horizontally, and this gives water and air a chance to move about and leave the seal in place in the curved portion. It’s important to note that P-traps can still go dry, but that usually happens if the sink or tub has not been used for an extended period of time. The water evaporates and leaves the trap dry. Compared to the S-trap, a P-trap is much less likely to get a dry trap when it’s being used.

Pros and Cons of S-Traps and P-Traps

A dry trap is actually dangerous, and not because items can go down the drain and be lost forever. It’s what can come back up the drain that’s a problem. Without water to seal the pipe, gases and odors from the sewer system can creep back into the home. It also gives vermin easy access to the home through the pipes. For this reason, S-traps was officially banned under the Uniform Plumbing Code, and P-traps became the new standard.

The shape of the P-trap is just one feature that helps stop this from happening. In addition to the different shape, a P-trap also has a vented pipe that rises from the fixture to and through the roof. This helps maintain air pressure in the drain, which in turn prevents gravity from pulling all the water through the pipe and leaving a dry pipe behind.

Installing a P-Trap

Homeowners converting an S-trap design to a P-trap may want to seek the help of a professional for the job. It’s not that replacing the pipes is difficult; all you need are a few basic supplies and some time. The tricky part can be installing the vent pipe correctly, according to the most current code. However, if the room already has a P-trap, replacing it is a fairly simple DIY job that can transform the look of a bathroom.

An exposed P-trap is more than functional. It can also add to the room’s overall design. After all, who wants to look at white PVC pipe jutting out from the wall and into the sink? Homeowners who love the look of an open vanity or prefer a minimalistic approach to design no longer need to hide their plumbing behind a bulky cabinet. Best of all, it helps tie in all the fixtures in the bathroom.

Matching Accessories

In some cases, replacing the P-trap is just the beginning of a bathroom makeover. In others, it’s the finishing touch. Along the way, you may find that you need additional accessories to complete the project. Perhaps you change out the sink with a replacement that’s in a slightly different position compared to the original fixture. In that case, you may need to expand the pipe with an extension tube. You can find extension tubes in multiple finishes, which makes it easy to match with the P-tube for a seamless fit.

If replacing the P-trap is part of a bigger room refresh, you may want to replace more than the pipe. A plumbing trim kit is a convenient way to replace an aging P-trap when you like a uniform look in the room. In addition to the P-trap, the kit includes angle stops, flanges, and supply lines, all in coordinating finishes. Kits come i