If you’ve ever peered behind a toilet or sink, you may have noticed curved pipes shaped like the letters “P” or “S.” These are plumbing traps, and their job is to prevent harmful sewer gases from entering your home. Choosing between a P-trap and an S-trap is an important decision in modern plumbing, as it affects performance, compliance with building codes, and long-term convenience. In this guide, we’ll break down the differences between P Trap vs S Trap plumbing, explore why P-traps are the modern standard, and provide helpful tips for upgrading outdated plumbing. Ready to modernize your plumbing setup? Keep reading to learn more!
What Is a Plumbing Trap and Why It Matters?
A plumbing trap is a curved section of pipe that holds a small pocket of water at all times. The water forms a seal that prevents sewer gases from flowing back into your home, keeping out both unpleasant odors and dangerous methane. Plumbing traps also provide other important benefits that make them a key part of any safe, reliable plumbing system. Here’s why they’re so important:
- Prevents sewer gases: Creates a water barrier that keeps foul odors out.
- Stops pests: Prevents insects and small rodents from entering through drains.
- Catches debris: Traps hair, dirt, or small objects before they clog main drain lines.
- Ensures safety: Protects indoor air quality and maintains hygiene.
Every sink, toilet, and floor drain in a home requires a properly installed trap to work effectively. In most houses, this trap is either a P-trap or an S-trap. The choice between P vs S trap is more than just shape; it affects compliance with building codes, ease of maintenance, and overall safety, which we’ll explore in the next sections.
What Is a P-Trap?
A P-trap is a curved pipe shaped like the letter “P” when viewed from the side. It connects the sink or toilet outlet to the wall drain, and it’s the standard choice in modern plumbing. The water held in the bend creates a reliable seal, while the horizontal outlet design makes it easier to clean and maintain. P-traps are widely used because they comply with building codes and offer long-term convenience.
Key features of a P-trap:
- Positioned with the outlet pipe leading into the wall
- Holds water to block sewer gases and odors
- Easy to clean and replace
- Preferred in modern construction and plumbing codes
- Compatible with most sinks, basins, and toilets
What Is an S-Trap?
An S-trap is shaped like the letter “S” and directs waste straight into the floor drain. While once common in older homes, this design is now considered outdated and often non-compliant with current building standards. S-traps are more prone to siphoning, which can break the water seal and allow odors or gases to escape into the home.
Key features of an S-trap:
- Outlet pipe connects directly to the floor
- Holds water but is more likely to lose the seal due to siphoning
- Less hygienic and less reliable compared to P-traps
- Commonly found in older bathrooms
- Often requires conversion to a P-trap for code compliance
S Trap vs P Trap: What's the Difference?
To make the choice clearer, here’s a side-by-side comparison:
| Feature | P-Trap | S-Trap |
|---|---|---|
| Shape | U-shape with a horizontal arm | S-shape with a vertical outlet |
| Outlet Direction | Connects to the wall | Connects to the floor |
| Ventilation | Designed to work with vent pipes, preventing siphonage | Poor ventilation, more prone to siphoning and seal loss |
| Code Compliance | Meets modern building codes | Often non-compliant in modern standards |
| Reliability | Maintains water seal consistently | Water seal can break easily |
| Maintenance | Easy to clean and replace | Harder to access and maintain |
| Common Use | Standard in modern bathrooms and sinks | Found in older homes, now outdated |
| Odor Control | Strong, reliable barrier against sewer gases | Less effective, risk of odors entering home |
The table makes it clear: while S-traps served a purpose in older systems, P-traps dominate modern plumbing for safety and compliance reasons.
Why P-Traps Are Preferred in Modern Plumbing?
When looking at P Trap versus S Trap plumbing, the differences are easy to spot. Modern plumbing favors P-traps for their reliability, safety, and compliance. They outperform older S-traps by offering stronger odor control, easier maintenance, and better compatibility with today’s building standards.
Strong Odor and Gas Protection
The curved shape of a P-trap holds water securely, creating a barrier that blocks unpleasant odors and harmful sewer gases from entering the home. Unlike S-traps, the seal is far less likely to break, which keeps your bathroom smelling fresh and safe.
Works Seamlessly with Venting Systems
Modern plumbing relies on vent pipes to balance air pressure and keep water seals intact. P-traps are designed to work with these systems, reducing the risk of siphoning and ensuring the trap always functions as intended.
Easy to Clean and Maintain
P-traps are built with accessibility in mind. They can be removed, cleaned, or replaced without much hassle, making them a practical choice for both homeowners and plumbers. Regular upkeep is straightforward, which helps prevent bigger issues later on.
Code-Compliant and Future-Proof
In most regions, building codes specifically require P-traps. Choosing one ensures your plumbing system is not only compliant today but also ready for any future inspections or renovations.
Tips for Upgrading from an S-Trap to a Code-Compliant P-Trap
If your home still has S-trap fixtures, especially in toilets, it may be time to consider upgrading. While replacing an S-trap requires professional work, there are steps you can take to prepare.
- Confirm Local Codes: Check the plumbing rules in your area to be sure the new setup is approved.
- Take Accurate Measurements: Measure the distance between the fixture outlet and the wall drain so you can choose the right P-trap and fittings.
- Choose Quality Materials: Pick a P-trap made from durable materials like PVC, ABS, or chrome-plated brass for longer life and easier maintenance.
- Plan the Pipe Layout: Switching to a P-trap usually means connecting the outlet into the wall. Sketch or map out how the pipes will align before starting work.
- Call a Plumber if Needed: Realigning pipes can be tricky. If you’re not comfortable with the work, a licensed plumber can ensure everything is installed safely and correctly.
- Test the Setup Thoroughly: Once the trap is installed, run several flushes or sink drains to check for leaks. Make sure the trap holds water and seals properly.
Conclusion
When comparing plumbing S trap vs P Trap, the advantages of the P-trap are clear. It offers better odor control, easier maintenance, and full compliance with modern building codes. By contrast, S-traps are outdated, less reliable, and often the source of sewer smells in older homes. If your bathroom still uses an S-trap, upgrading to a P-trap is a smart move for both comfort and hygiene. The change not only brings your plumbing up to code but also prepares your bathroom for future upgrades like smart toilets, giving you a system that’s efficient, reliable, and built to last.
FAQs
Is an S Trap Legal in Australia?
S-traps were once common in Australia, but today they are generally non-compliant in new builds. Existing S-trap toilets may remain in older homes, but most building codes encourage upgrading to P-traps for safety and efficiency. Always check your local council regulations before replacing fixtures.
When to Use an S Trap vs P-Trap?
An S-trap connects directly into the floor, making it suitable only for older bathrooms where the drain line is set into the slab. They’re mostly found in houses built before stricter plumbing codes came into effect. Even then, retrofitting to a P-trap is strongly encouraged. A P-trap, on the other hand, connects into the wall and is now the standard for modern toilets, sinks, and showers. It’s compatible with venting systems, holds the water seal more reliably, and is usually the only legal choice in new builds.
Can an S-Trap Cause Sewer Smell?
Yes. Because of their shape, S-traps can lose their water seal through siphoning after a flush. Once the water barrier is gone, sewer gases can escape into the bathroom, creating foul odors and even potential health risks. P-traps address this problem more effectively. Their horizontal outlet works with vent pipes to prevent siphoning and keep the water seal intact. This makes them far more reliable at controlling odors and maintaining healthy indoor air quality.
