What is a P-Trap?

Dee Saale
Dee Saale

Almost any household equipment that drains water has a p-trap. Sinks, bathtubs, washing machines, all have p-traps to drain the waste water. A p-trap is comprised of a tailpiece, the curved trap piece, and a drain elbow. The drain elbow for a p-trap fits into the drain pipe which goes directly into the wall.

Various p-traps.
Various p-traps.

Codes require a p-trap (or an s-trap) any place there is an open drain line that flows into the drain-waste-vent system. As the name implies, the drain-waste-vent system removes solid, liquid or gas waste from the home through drains and vents. For example, as the water empties from a sink, it goes through the p-trap, on to the drain line, and ends up in the sewage system.

P-trap under a bathroom sink.
P-trap under a bathroom sink.

The important thing about the p-trap is that it is has a water seal along the curve of the trap. The seal prevents noxious air or gases to backflow from the sewer line, but the original waste can still exit into the sewage system. If the gases were allowed back into the home, not only would they smell, but they could cause illnesses and have even been known to explode.

A p-trap can be made of metal or of plastic. Although metal is usually considered to be more durable, the reality is that the plastic will last longer. Although the metal traps may look nicer, they quickly corrode. One squeeze at the bend, even with fingers, and if it gives in – even slightly – it needs to be replaced. There is one exception to this rule: heavy-gauge brass traps will last a very long time.

Depending on where the trap is located, it may vary in size. Bathroom traps are 1.25 inches (3.2 cm) in diameter. Kitchen traps are 1.5 inches (3.8 cm) in diameter.

Every time the drain is used, water is flushed through the trap and fresh water replaces the old water. Solids will begin to stick to the trap over the course of time; consequently, clogs will occur. In those cases, it is probably time to replace the trap. A beginner can replace the p-trap in about an hour; however, an expert can do the same job in about twenty minutes. In some cases, a trap that has a cleanout plug can be cleaned out without removing and installing a new trap – simply rinse the water through the plug and remove the debris with an auger or other tool.

It is important to know the specific plumbing codes for a particular country before removing or installing a new p-trap. For example, in the United States, white plastic PVC, polyvinyl-chloride, pipe is commonly used for drains. But, in Canada, black ABS, acrylonitrile butadiene styrene, is used. Every country is different, so the proper research is necessary before delving into any plumbing project.

Dee Saale
Dee Saale

Dee is a freelance writer based in Colorado. She has a B.A. in English Literature, as well as a law degree. Dee is especially interested in topics relating to medicine, legal issues, and home improvement, which are her specialty when contributing to wiseGEEK.

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Discussion Comments


Not mentioned here is that S-traps are now generally forbidden by most plumbing codes in U.S.


What does a p trap look like in comparison to an s trap? Sorry, I don't know about plumbing.


My husband works for a municipal wastewater department, so professional advice coming through! A P-trap is a section of plumbing located underneath most drains in your home. These U-shaped pipes remain full of water at all times to prevent horrible odors from escaping into your home from the main sewer line. In addition to preventing those pesky roaches from becoming houseguests, it also serves as an emergency jewelry catcher.

Very important! Remember to maintain your trap with a plumber 'snake' to clean the grime that can coat your plumbing. That grime can also cause stinky situations and water to build up into your sink.

Pixiedust, I hope your P-traps stay grime and stinky free!


I stayed at a hotel once where every time I flushed the toilet a horrible smell, similar to the smell of an outhouse, would fill the room. Turns out the ptrap wasn't working properly. That's when I learned that it's the pee trap we have to thank for nice- or at least neutral-smelling bathrooms.

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