Why Are S-Traps Illegal?

Understanding the Difference Between S-Traps and P-Traps

When it comes to plumbing, ensuring the safety and functionality of your drainage system is critical. One essential component of any plumbing system is the trap, a U-shaped pipe designed to hold water and block sewer gases from entering your home. Among the various types of plumbing traps, the P-Trap is code-approved, while the S-Trap is strictly prohibited by U.S. plumbing codes. This blog explains what S-Traps are, why are S-Traps illegal, and how they compare to the more reliable P-Trap.

What Is an S-Trap?

An S-Trap is a type of plumbing trap shaped like the letter “S.” Its primary purpose, like all plumbing traps, is to retain water in its curved section, creating a “trap seal.” This seal is crucial as it prevents sewer gases, foul odors, and insects from escaping the drainage system and entering your home.

However, the unique design of an S-Trap makes it prone to failure, especially in maintaining the integrity of the trap seal. Without a reliable seal, sewer gas has a direct pathway into your living space, posing health risks and discomfort.

S-Traps vs. P-Traps: Key Differences

At first glance, the main difference between S-Traps and P-Traps is their shape. The S-Trap forms an “S,” while the P-Trap resembles a “P.” However, the most significant difference lies in their functionality and code compliance:

1. Code Compliance:
   • S-Traps are prohibited under major U.S. plumbing codes including the International Residential Code (IRC P3102.5).
   • P-Traps are the only code-approved plumbing trap due to their superior design and functionality.
2. Ventilation:
   • S-Traps lack a proper vent connection, making them susceptible to siphonage.
   • P-Traps are installed with a vent, ensuring pressure stabilization and maintaining the trap seal.

Why Are S-Traps Illegal?

S-Traps are banned because they are prone to siphonage, a condition where negative pressure in the drainage system draws water out of the trap. Without water in the trap to create the seal, sewer gases can flow freely into your home.

The lack of a vent is the root cause of this issue. Proper ventilation is essential in any drainage system, as it minimizes pressure fluctuations that could compromise the trap seal. P-Traps, on the other hand, are vented by design, which ensures reliable performance and compliance with plumbing codes.

Prohibited Plumbing Traps

According to U.S. plumbing codes, the following traps are not allowed:

• S-Traps
• Drum Traps
• Bell Traps
• Traps with Moving Parts
• Traps with Interior Partitions
• Crown Vented Traps

P-Traps remain the standard for code compliance due to their proven effectiveness and reliability.

Conclusion: Avoid S-Traps for a Safe and Code-Compliant Home

Understanding the differences between S-Traps and P-Traps is essential for maintaining a safe and efficient plumbing system. While S-Traps might appear functional, their design flaws make them unreliable and non-compliant with modern plumbing codes. Always opt for P-Traps, as they provide a secure, well-ventilated trap seal that keeps harmful sewer gases out of your home.

If you suspect your home has S-Traps or other outdated plumbing components, consult a licensed plumber to evaluate and update your system. Staying informed about plumbing code requirements and best practices ensures a safe and healthy living environment for you and your family.