s trap vs p trap toilet

As an Australian homeowner, you may have heard the terms “S trap,” “P trap,” and “p trap toilet” used when discussing plumbing.

But what exactly are these traps, and why is it important to understand the difference? In this article, we’ll dive into the world of plumbing traps and explore the pros and cons of each type. 

What is an S Trap?

An s trap toilet is an older style of plumbing trap that gets its name from its shape, which resembles the letter “S.” These traps were commonly used in Australian homes built before the 1970s and are particularly noted for their application in s trap toilet installations. 

Here’s how they work:

  • Water flows down from the fixture (e.g., sink or basin) and into the curved portion of the trap.
  • The water creates a seal that prevents sewer gases and odours from entering the home.
  • As more water flows through, the trapped water is pushed out and replaced.

The design of the s trap toilet bowl and s-trap toilet bowls is specifically shaped to ensure this water seal, but they can face issues such as drying out and allowing bacterial buildup, which is common drawbacks.

While S traps were once popular, especially for ground floor installations due to their specific design and water flow characteristics, they have some significant drawbacks:

  • They can be prone to siphoning, which means the water seal can be lost, allowing sewer gases to enter the home.
  • They can be difficult to clean and maintain due to their shape.
  • They don’t meet current Australian plumbing standards and regulations.

What is a P Trap in a Toilet Bowl?

A P trap, on the other hand, is the modern standard for plumbing traps, particularly notable in p trap toilet bowl designs. 

These traps get their name from their shape, which resembles the letter “P,” and are celebrated for their efficiency and suitable setup in modern bathrooms. Here’s how they work:

  • Water flows down from the fixture and into the curved portion of the trap, just like an S trap.
  • The water creates a seal that prevents sewer gases and odours from entering the home.
  • The trap has a vent pipe that connects to the main vent stack, which allows air to enter the system and maintain the water seal.

P trap toilet bowls feature U-shaped or P-shaped pipes that are crucial for preventing sewer gases from entering through the drain by holding water in the bottom. They offer a significant design advantage over S traps.

P traps have several advantages over S traps:

  • They are less likely to lose their water seal due to siphoning.
  • They are easier to clean and maintain thanks to their more accessible shape.
  • They meet current Australian plumbing standards and regulations.

S Trap vs P Trap: Key Differences

Now that we’ve covered the basics of each trap type, let’s compare them side by side:






Shaped like the letter “S”

Shaped like the letter “P” with a horizontal waste arm extension


No vent, closed system

Has a vent stack that releases gases outside 

Siphoning Risk

Higher risk of siphoning out the water seal, allowing sewer gases to enter 

Lower risk of siphoning due to vent and waste arm extension 

Drying Out

More prone to drying out, losing the water seal 

Less prone to drying out due to design 

Plumbing Code

Prohibited in many areas for new construction due to siphoning risk 

Code-compliant and considered safer 

Space Required

Smaller footprint but may require more vertical space 

Requires more horizontal space due to waste arm 

Typical Use

Suitable for low-flow fixtures, less common in new construction 

Used for most modern plumbing fixtures


Problems with S Traps

If your home has S traps, you may experience some common issues:

  • Foul odours: If the water seal is lost due to siphoning, sewer gases can enter your home, causing unpleasant smells. Plus, a lost water seal in S traps can lead to sewer gasses entering the home, emphasising the importance of maintaining trap seals to prevent toxic gasses and vermin from compromising indoor air quality.
  • Gurgling sounds: If the trap is not functioning properly, you may hear gurgling noises coming from your drains.
  • Slow drainage: S traps can be more prone to clogs and blockages, leading to slower drainage.

If you notice any of these signs, it may be time to consider upgrading to a P trap.

Upgrading from an S Trap to a P Trap

Replacing an S trap with a P trap is usually a straightforward DIY project. Here’s a step-by-step guide:

  1. Gather your tools and materials: You’ll need a new P trap assembly, adjustable wrench, bucket, and towels.
  2. Turn off the water supply to the fixture and place a bucket under the trap to catch any water.
  3. Unscrew the slip nuts connecting the trap to the fixture and the wall pipe.
  4. Remove the old S trap and clean the area thoroughly.
  5. Install the new P trap, making sure all connections are tight and secure.
  6. Turn the water supply back on and test for leaks.

Key Takeaways

  • S traps, an older style of plumbing trap that can be prone to siphoning and are not compliant with current Australian plumbing standards, lack the efficient toilet trap design seen in modern systems.
  • P traps, the modern standard, are more effective at preventing sewer gases and odours from entering the home, highlighting the importance of toilet traps in maintaining a healthy living environment.
  • Upgrading from an S trap to a P trap is usually a straightforward DIY project, but it’s important to know when to call a professional plumber. Proper selection and maintenance of the waste outlet and trap can significantly impact the system’s efficiency.
  • Common signs that an S trap needs to be replaced include foul odors, gurgling sounds, and slow drainage.
  • Regular maintenance and cleaning of your plumbing traps can help prevent issues and keep your home’s plumbing system functioning properly.

If you need help with your toilet installation, make sure you contact our plumbers to get a tailored quote and guidance on the best solution for your needs.