The illegality of S-traps in modern plumbing stems from their inherent design flaws that can lead to system failures. An S-trap is a curved pipe, resembling the letter “S,” installed under a sink or other fixture to prevent sewer gases from entering a building. Its function is to hold a water seal that acts as a barrier. However, the extended vertical drop in the “S” configuration makes it prone to self-siphonage. This occurs when the flow of water evacuates the trap entirely, leaving no water seal.
The vulnerabilities of this design became apparent over time. Self-siphonage and evaporation are major concerns. Evaporation, particularly in infrequently used drains, can break the water seal. More significantly, the momentum of water flowing through the drain can create a vacuum effect, pulling all the water out of the trap. This leaves the drainpipe open to sewer gases, resulting in unpleasant odors and potential health hazards. Additionally, S-traps are more susceptible to clogging because their shape can trap debris more easily than designs with smoother flow paths.
