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What is the Purpose of a P-Trap?

Updated on October 1, 2016


Wastewater traps are used in residential plumbing applications to prevent harmful and unwanted substances from entering the home through the plumbing pipes. Wastewater traps are used in sinks, showers, and a variety of other plumbing applications. There are several types of wastewater traps, but the “P” trap is the most commonly used trap.

What is a P-Trap?

A P-trap is a plumbing pipe normally used in kitchen and bathroom under-sink plumbing; however, there are also washing machine P-traps. Most commonly made of Polyvinyl Chloride (PVC), P-traps are also available in chrome, brass, copper, cast iron and Acrylonitrile Butadiene Styrene (ABS). As its name implies, the pipe is in the shape of a “P,” as opposed to S-traps that are shaped like an “S.”

Sewer Gas

A P-trap captures sewer gas and prevents it from entering the house through the plumbing drain. Sewer gas contains ammonia, carbon dioxide and sulfur dioxide - in addition to bacteria and other viruses. Depending on your municipality or sewer treatment system, the sewer gas may also contain chlorine bleach, solvents and even gasoline.

Sewer Gas Effects

Exposure to sewer gas can cause several severe side effects including hydrogen sulfide poisoning. Exposure to small amounts of sewer gas - which has the scent of rotten eggs - can result in eye and respiratory tract irritation, as well as nausea, dizziness, headaches and drowsiness. High levels of exposure may result in unconsciousness and be fatal. In addition, high levels of sewer gas can also decrease oxygen in the air, resulting in suffocation. Also, sewer gas is both flammable and explosive.

Other Considerations

Besides stopping sewer gas, a P-trap also stops rodents and other small creatures from entering the house through the plumbing drain. In addition to catching substances from entering from the outside, the P-trap also catches hair, grease and other gunk that results from using household toiletries. However, these items can clog the P-trap and cause a slow drain. In this case, the P-trap should be removed and either cleaned or replaced.


The Permaflow has been introduced on the market as an alternative to the P-trap. The Permaflow is a clear P-shaped pipe that has an external wheel, which can be manually turned to push out debris and force it down the drain. Other substitutes include mechanical P-traps. However, the website, Ask the Builder, advises against mechanical devices since they contain moving parts which are susceptible to breaking or not closing tightly enough to stop sewer gas and rodents.


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