A curved plumbing fitting that creates a water seal to prevent sewer gases from entering buildings while allowing wastewater to flow through.

A P-trap is a curved plumbing fitting shaped like the letter “P” that creates a water seal in drainage systems to prevent sewer gases, odors, and vermin from entering buildings through drain lines while allowing wastewater to flow freely to the sewer or septic system. The trap works by maintaining a standing water barrier in its curved section that blocks the passage of gases while permitting liquid waste to pass through when fixtures are used. P-traps are essential components in virtually all plumbing fixtures including sinks, lavatories, floor drains, and other drainage applications, and they are required by plumbing codes to protect building occupants from harmful and unpleasant sewer gases.

P-traps are installed in various plumbing applications including bathroom sinks and lavatories where they connect the fixture drain to the waste line, kitchen sinks including single and double bowl configurations that may require special trap arrangements, floor drains in basements, garages, and commercial facilities, utility sinks and laundry tubs, bar sinks and prep sinks in commercial kitchens, and specialty applications such as condensate drains for HVAC equipment. Each application may require different trap sizes, materials, and configurations based on the fixture type, drainage load, and local code requirements.

The construction and materials of P-traps include various options such as PVC (polyvinyl chloride) traps that are lightweight, corrosion-resistant, and commonly used in residential applications, ABS (acrylonitrile butadiene styrene) traps that offer similar properties to PVC with different chemical resistance characteristics, brass and bronze traps that provide durability and are often used in commercial applications or where appearance is important, stainless steel traps for corrosive environments or food service applications, and cast iron traps for heavy-duty commercial and industrial applications. Trap selection depends on the application, local codes, chemical compatibility, and aesthetic requirements.

Fun Fact: The P-trap was invented in 1775 by Scottish mechanic Alexander Cumming as part of the first flush toilet design, and the basic principle remains unchanged after nearly 250 years of use in plumbing systems worldwide.

• House Trap