Which trap design is prohibited according to the IRC?

The prohibition of certain trap designs in the International Residential Code (IRC) is based on their tendency to trap water or wastewater in a way that can lead to backflow, blockages, or odors in the plumbing system, which diminishes the overall effectiveness of sanitation.

S traps are particularly problematic because they can easily lose their water seal due to siphoning effects, allowing sewer gases to enter living spaces. Drum traps, which hold water in a cylindrical design, can collect debris, leading to potential blockages and failure to retain the necessary water seal. Bell traps, similar to S traps, can also experience issues with maintaining a water seal and are thus not suited for modern plumbing systems.

The IRC emphasizes trap designs that ensure effectiveness in preventing backflow and facilitating proper drainage, thus ensuring safe and sanitary conditions within residential plumbing systems. Consequently, all of these trap designs—S traps, drum traps, and bell traps—are prohibited under the IRC due to their inherent flaws that could lead to health and safety hazards.