The next time you use your bathroom or kitchen sink, take a quick peek beneath at the pipes that drain the fixture. These curvy pipes, known as traps or interceptors, serve an important purpose and are essential to the health and safety of household occupants. Below is more information about these essential plumbing components:


A common misconception about plumbing traps is they are installed to capture dropped items, such as jewelry or coins. While they do occasionally prevent valuables from being swept away, plumbing traps are designed for capturing something much more mundane, yet also much more hazardous: sewer gas.
Sewer gas is composed of methane and hydrogen sulfide, two substances known to cause a variety of problems. Methane is flammable and sufficient accumulations can even be explosive; hydrogen sulfide carries a terrible, sulfuric odor and is implicated in health problems such as sinusitis.


To prevent sewer gas from entering your home, traps use water to form a barrier between the "open" sewer line and your fixture drains. As long as the water is in place, gases cannot pass the trap, and your home is kept safe. However, not all traps are created equal, and problems can occur if the design is flawed. Here are a few trap styles that are obsolete:

  • S-Traps. Plumbing traps are identified by the letters of the alphabet that most closely resemble the shapes of the pipes. An S-trap resembles a sideways letter "S" and appears to be up to the job of blocking gases. Unfortunately, the "S" trap is prone to strong suction created when large quantities of water are emptied from nearby fixtures, such as toilets or bathtubs. This suction can evacuate the remaining water from the S-trap, leaving it high and dry.
  • Bell Traps. A bell trap utilizes a basin with a bell-shaped component that fits over the vertical drain pipe. Combined, these components are designed to maintain a water "plug" that prevents gases from passing around the trap. However, bell traps fail due to their vulnerability to drying out and potential for accumulating decaying, organic debris.
  • Drum Traps. Another trap design found in some homes, drum traps use a relatively large canister of water to form a gas barrier. Their use was popular due to their ability to capture lost objects and the unlikelihood of losing water due to their high capacity. Drum traps, much like bell traps, are prone to becoming clogged with debris and can create plumbing maintenance headaches as a result.

Other trap designs have been introduced, including some unorthodox or ad hoc configurations prone to failure or those that failed to work at all.


In current plumbing design, the prevailing trap in use is the P-trap. Shaped like a letter "P" lying on its side, the P-trap consists of a lower curved section attached to a nearly horizontal drain pipe. It maintains a small volume of wastewater, which is flushed out on a continual basis as the fixture is drained. The nearly horizontal drain section prevents suction from nearby fixtures by permitting air to vent. The design is simple and highly effective in most circumstances.
As good as P-traps are at working to prevent gas leakage, they do need to be installed correctly to provide their benefits. The horizontal drain section should not be perfectly level, nor should it be angled too steeply. If it isn't aligned properly, the drain section will either permit siphoning or fail to block gases.
To ensure your home's drains are flowing properly and the traps are functioning as they should, a professional plumber can perform a detailed inspection and let you know what needs to be repaired or updated. He or she can remove obsolete traps and replace them with newer P-traps as well as clean out clogged traps.