Even the smallest components in a plumbing system play an important role in ensuring a smooth and safe operation. Among these essential components are traps, specifically P-traps and S-traps. While they may seem similar at first glance, these two plumbing traps have critical differences that can affect functionality and compliance with modern plumbing codes.
In this blog, we’ll detail the differences between P-traps and S-traps, show how they work, and explain why one is now preferred over the other in modern plumbing systems.
What is a P-Trap, and What is it Used For?
A P-trap is a U-shaped pipe commonly installed under sinks, bathtubs, and shower drains. Its primary purpose is to create a water seal by trapping a small amount of water inside the curve. This water barrier prevents harmful sewer gases from escaping into your home through the drain.
Key Features:
- Ventilation: P-traps incorporate a vented pipe that prevents suction, ensuring the water seal remains intact.
- Debris collection: P-traps can catch small objects, such as rings or other items accidentally dropped down the drain.
- Code compliance: P-traps are approved for all modern plumbing systems and satisfy building code requirements.
P-traps are ideal for drains that require a reliable and effective water seal. They are a staple in residential and commercial plumbing systems, ensuring safety and efficiency.
Example: Dearborn® Plastic 1-½” Tubular P-Traps with Threaded Adapter are manufactured of polypropylene plastic with a PVC or ABS adapter. This product features blue TPE washers that prevent drying or cracking while providing a long-lasting seal.
Looking to install a P-trap? Check out our Project Guide for step-by-step instructions.
What is an S-Trap, and What is it Used For?
An S-trap, shaped like an “S” on its side, serves a similar purpose as the P-trap by creating a water seal. However, the design does not include a vent pipe, which is a major limitation.
Key Features:
- Vertical drain pipe orientation: An S-trap drains water directly down into the floor, unlike the P-trap, which connects horizontally to drainage piping in the wall.
- Water siphoning issues: S-traps are infamous for causing siphoning. This occurs when water flows down the trap, pulling water out and leaving the trap dry, thus breaking the water seal.
- Not code-approved: S-traps are no longer allowed in modern plumbing codes due to their inability to maintain a consistent water seal.
S-traps were commonly used in older homes before plumbing codes evolved. While they can still be found in some properties, they are often replaced during remodeling or professional inspections.
What are the Differences Between P-Traps and S-Traps?
Understanding the difference between P-traps and S-traps is crucial for those working with plumbing fixtures. This chart provides a quick breakdown of their main contrasts:
| Feature | P-Trap | S-Trap |
|---|---|---|
| Shape | U-shape with a horizontal arm | S-shape with a vertical outlet |
| Ventilation | Includes a vented pipe | No dedicated venting |
| Water Siphoning | Rarely occurs due to vents | Common due to vertical drainage |
| Code Compliance | Approved and recommended | Not allowed in modern systems |
| Typical Placement | Wall-connected sinks, tubs, showers | Floor-connected in older homes |
Why Are P-Traps Preferred Over S-Traps?
There are several reasons why P-traps are now the standard in modern plumbing:
1. Sealing the Joint
- P-traps maintain water seal: The design of a P-trap includes a consistent water barrier that prevents the backflow of sewer gases into the living spaces. This water seal is more reliable due to the trap’s shape, which retains enough water after each use to block the gases effectively.
- Prevention of sewer gas entry: Sewer gases not only carry an unpleasant odor but can also be hazardous to health. The efficient water seal in P-traps is a critical barrier, enhancing indoor air quality and safety.
2. Ventilation Efficiency
- Facilitates vent system integration: P-traps are designed to work seamlessly with venting systems. This design choice ensures that air pressure within the drainage system is balanced, preventing the risk of the trap’s water seal being siphoned away.
- Eliminates siphoning risks: S-traps are more susceptible to having their water seals siphoned away by the flow of draining water. This can lead to the aforementioned sewer gases entering the home. P-traps, with their effective ventilation integration, mitigate this risk substantially.
3. Code Compliance
- Adheres to modern plumbing standards: The design flaws of S-traps have led to their prohibition in most modern plumbing codes. P-traps, however, are designed to comply with these regulations, making them standard in new construction and renovations.
- Meets safety and efficiency criteria: By conforming to plumbing codes, P-traps ensure a safer and more efficient drainage system. This compliance is crucial for both residential and commercial properties.
4. Conversion Friendly
- Easy replacement of S-traps: Converting to a P-trap system is relatively straightforward for older structures still equipped with S-traps. This conversion significantly enhances the plumbing system's efficiency and safety.
- Use of air admittance valves (AAV): When replacing an S-trap with a P-trap, the addition of an AAV or connection to an existing vent stack can provide the necessary ventilation. This step is essential in maintaining the drainage system's function and compliance.
When comparing the P-trap vs. S-trap, it’s clear that P-traps are the superior choice for modern plumbing. Their design minimizes the risk of sewer gas leaks and ensures compliance with building codes, making them a must-have for safe and functional plumbing systems.
If you’re working on a DIY plumbing project or simply want to better understand your home’s drainage system, knowing the differences between these traps can save you time, effort, and money.