What is a Plumbing Trap?

R. Anacan

A plumbing trap is a curved pipe that is located underneath a plumbing fixture. A plumbing trap is an essential part of a building’s plumbing for two main reasons. The first is that a trap prevents the entry of odors into a building. Secondly, a trap also catches items that may cause major clogs or blockages if allowed to travel further into the drainage system. In most municipalities, the installation of plumbing traps on all plumbing fixtures is required.

P-traps prevent gases from returning up a drain and into the house.
P-traps prevent gases from returning up a drain and into the house.

The plumbing system of a home or building generally drains into a municipal sewer system or into a septic tank system. In a sewer or septic system, the products in the sewage and wastewater generate fumes and gases that most would consider unpleasant at best and downright unbearable at worst. Without the plumbing trap, the gases and fumes would enter a building through the drainage system.

Traps are located underneath a plumbing fixture.
Traps are located underneath a plumbing fixture.

Plumbing traps work by utilizing a curved shape in the pipe. The earliest traps were S-shaped, but most plumbing traps today are made in a U-shape. Plumbing traps are also often referred to as P-traps as they resemble the shape of a P when facing a certain way.

A plumbing trap is simple, yet extremely effective. The design allows water to drain through it, but the curve of the pipe keeps a small amount of water in it at all times. The water that remains in the curve of the pipe forms a barrier that prevents unpleasant fumes produced by a septic or sewer system from entering a building. The curve design is also what allows the trap to catch items dropped into a drain, such as jewelry, before it is completely drained away.

When there is a clog or blockage that is preventing the effective drainage of a sink or a bathtub, the clog is often located in the curve of the trap. A plumber’s snake is a recommended tool to clear a blocked pipe. A plumber’s snake is a cable that is inserted into the drain. The end of the cable has an auger head that enables the snake to dislodge any debris that is creating the blockage.

Other suggestions for clearing a blocked drain include using a plunger or chemical drain opener. Many plumbing traps are now manufactured with a clean-out valve at the bottom of the curve in the pipe, to make it easier to access the plumbing trap for cleaning and draining.

A plumbing trap can help keep foreign objects from becoming lodged in a septic tank.
A plumbing trap can help keep foreign objects from becoming lodged in a septic tank.

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Discussion Comments


Is there any system that prevents rat traps from forming in the traps?


I've used a variety of methods to unclog the drain. I tried the long thin end of a rat tail comb to get a bunch of twisted hair released and down the drain - this didn't work too well. I've used a commercial drain cleaner. This worked, but I decided that I didn't like putting chemicals down the drain.

Using a snake seemed to work well for other people, but not too well for me.

Then my daughter suggested I try a regular "plumber's friend" (plunger). This worked really well - just have plenty of water in the sink and give it a few good thrusts and it's unclogged!


After all these years of living in houses or buildings, I never knew that one reason to have plumbing traps was to prevent odors from coming up into the house, nor did I know that they were mandated in most all districts. I did know that traps were used to catch stuff that could cause a big clog down the line and to keep little things from going too far.

Over the years, we've had to retrieve a couple of rings, a contact lens, and a variety of other small objects from the plumbing trap. Panic was rampant until the lost item was retrieved.


@MrsWinslow - Here’s what it boils down to: If you don’t already know how to get your ring out of the trap, call a plumber. Seriously. Way too much can go wrong when you start messing with the plumbing. Trying to save eighty-five bucks on a plumber can cost you way more than that!

I once cost myself over two hundred. All I wanted to do was cut off the water to my sink, but the valves underneath were stuck. I loosened them with WD-40, but didn’t realize I had loosened them for real! There was leak under there that I didn’t notice for a couple of days. But the time I realized it, the floor of the under-sink cabinet had been ruined and I had to have it cut out and replaced. Should have realized I was in over my head and called a pro.

So get Roto-Rooter on the phone. Obviously, don’t use the sink in the meantime! I hope you get your wedding ring back.


I think I lost my wedding ring down the sink! I'm hoping it's in the trap. How do I get it out? Mine is new enough that it is U-shaped, but old enough that it does not have the clean-out valve.

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