Why S-Traps Are Banned: A Complete Plumbing Guide - 33rd Square (2024)

S-traps were once common in plumbing systems, but have been illegal for decades due to serious health hazards. This comprehensive guide explains everything you need to know about outdated S-traps, why they were prohibited, and safer alternatives.

What is an S-Trap?

An S-trap uses a curved, S-shaped pipe to connect sinks, tubs, and other fixtures to wastewater drain lines. The curved trap arm allows wastewater to flow down and away from the drain opening.

[Diagram of S-Trap Design]

S-traps were widely installed in the late 1800s into the early 1900s before modern plumbing codes existed. The S-shape provided a simple way to connect fixtures, especially when the bathroom or floor was lower than the sewer line:

• Outlet Curve – Allows wastewater to flow downward with gravity.
• Inlet Curve – Connects to the fixture‘s drain line.
• Trap Arm – Bottom curved section of the S-shaped pipe.

Though convenient at the time, the S-trap posed deadly hazards…

Dangers of the S-Trap Design

While S-traps easily connected plumbing fixtures, the design allowed poisonous sewer gases to enter homes, causing sickness and even death.

Raw sewage produces dangerous gases like:

• Hydrogen sulfide – flammable and toxic; causes headaches, nausea, and irritation at low concentrations. In high doses, can cause rapid unconsciousness and death.
• Methane – flammable and an asphyxiant; causes fatigue, nausea, vomiting, and death through asphyxiation. Also increases risk of explosions and fires.
• Ammonia – irritating to eyes and respiratory system.
• Radon – radioactive; long-term exposure causes lung cancer.

Before the 1920s, hundreds died annually in the US from "sewer gas poisoning" caused by fixtures like S-traps leaking gases into homes.

Why Are S-Traps Illegal Today?

Due to the lethal hazards they posed, health authorities prohibited the use of S-traps in the early 20th century. Let‘s examine why S-traps are no longer up to code:

Allow Toxic Sewer Gases to Enter

The S-trap‘s curved design provides very little resistance or barrier to gases passing from the sewer lines into the home. Poisonous and explosive methane and hydrogen sulfide can leak through and accumulate.

Easily Siphon Dry

The S-trap‘s curves enable water to siphon completely out of the trap, leaving it dry. When dry, gases can freely pass through into the dwelling.

In contrast, P-traps maintain a reliable water seal barrier even if water flow stops.

Drain Poorly

The indirect route and tight bends restrict wastewater flow, leading to backups and gurgling drains as waste gets caught. P-traps provide a straighter path for smoother drainage.

Prone to Clogging

Debris and buildup collects easily at the S-trap‘s tight corners, leading to more frequent clogs. P-traps‘ wider curves are less likely to clog.

Difficult to Clean

Snaking out clogs in an S-trap is extremely tricky due to the indirect, curved route. P-traps can be snaked fairly easily.

Can‘t Contain Pressure

Strong pressures from drain line blockages can blow water out of the S-trap. This eliminates the water seal and allows gases through.

Against Plumbing Codes

S-traps cannot meet modern safety standards. For example, the International Plumbing Code prohibits S-traps as they pose ventilation hazards.

When Were S-Traps Banned?

In the US, the Uniform Plumbing Code officially prohibited the use of S-traps in the 1920s due to health dangers.

By the 1930s, over 80% of US homes still had S-traps. But after World War 2, widespread renovations led to most old S-traps being updated to P-traps:

Today, S-traps remain in some older homes but any found must be replaced with approved traps.

P-Trap Replaced the S-Trap

The S-Trap was phased out in favor of the P-trap – the current standard for safe, reliable plumbing:

[P-Trap Illustration]

Benefits of the P-Trap

• U-shape maintains water seal even if water flow stops
• Smooth curves drain wastewater quickly
• Harder to clog than S-traps
• Easy to snake if clogged
• Meets plumbing codes for safety

Along with P-traps, vent stacks are also installed to release any lingering sewer gases outside rather than inside the home.

The P-trap and vent system prevents dangerous gas infiltration far better than outlawed S-traps.

Converting S-Traps to P-Traps

Can you convert an illegal S-trap to a code-approved P-trap? Unfortunately, no.

S-traps and P-traps use completely different shaped pipes to function. The S-shape cannot be bent into a U-shape.

If you have an S-trap, your only option is to remove it and install a new P-trap assembly according to building codes. This will ensure your plumbing is safe and up-to-date.

Identifying S-Traps vs P-Traps

Visually inspecting the pipes under your sinks and fixtures can reveal if you have an outdated S-trap:

S-Trap Signs

• S-shaped curve without a U-bend
• Pipe connections form a "Z" shape
• Found in older homes

P-Trap Signs

• U-shaped bend or U-pipe section
• Smooth, wide curves
• Connects with a 90-degree elbow joint

Calling a professional plumber can also identify any lingering S-traps that need replacement.

Key Takeaways

• S-traps easily allow toxic sewer gases like hydrogen sulfide and methane to enter homes, causing serious health hazards.

• S-traps were banned in the 1920s after hundreds died annually from gas exposure.

• The P-trap‘s reliable water seal and smooth drainage meet modern plumbing codes.

• S-traps cannot be converted to P-traps – they must be replaced.

• Inspect under sinks to check for S-trap or P-trap installation.

I hope this guide has helped explain the dangers of prohibited S-traps and why P-traps are now standard. Please let me know if you need any other plumbing or home improvement advice!

Related