S-Traps vs. P-Traps: What’s the Difference and When to Use Each in 2025

Plumbing traps may seem like small components, but they are critical in keeping your home safe and odor-free. Two of the most common types—S-traps and P-traps—serve the same basic purpose: to create a water seal that blocks sewer gases from entering your living space. However, they differ in design, code compliance, and function. In this guide, we’ll compare S-traps and P-traps in depth to help you understand which is best for your project.

Key Differences Overview

Now, let’s take a closer look at each of these differences to see how they affect performance, code compliance, and everyday use.

Key Differences Between S-Traps and P-Traps – Detailed Explanation

Before discussing each trap type in detail, it’s essential to understand the core differences that distinguish them. While both S-traps and P-traps serve the same essential function—blocking sewer gases from entering your home—they differ in installation method, venting capability, code compliance, and long-term reliability. These differences can directly impact plumbing safety, performance, and legality in your region. Below, we summarize the six most important factors that distinguish the two trap styles.

1. Shape and Orientation

S-traps have a vertical design that resembles the letter “S” when viewed from the side, while P-traps have a horizontal design that looks like a sideways “P.” The shape directly affects how the trap connects to the drainage system. S-traps drop into the floor, whereas P-traps bend toward a wall drain.

2. Drain Connection Point

S-traps connect to floor drains, making them suitable for fixtures without nearby wall plumbing. P-traps connect horizontally to a wall drain pipe, which allows for easier venting and better long-term performance. This distinction is crucial for understanding where and how each trap can be installed.

3. Venting Capability

P-traps are typically vented either through a wall vent pipe or with an air admittance valve (AAV), which helps prevent the siphoning of water from the trap. On the other hand, S-traps often lack proper venting, which can result in suction and the loss of the water seal. This makes S-traps more prone to allowing sewer gases into the home.

4. Code Compliance

P-traps are fully compliant with modern plumbing codes across North America and many other regions. S-traps are often considered outdated or even banned in new construction due to their venting issues. If your home is undergoing renovation, converting S-traps to P-traps is often necessary to pass inspection.

5. Long-Term Reliability

Because P-traps are vented and maintain a consistent water seal, they’re more reliable in the long run. S-traps can dry out more easily, especially in fixtures that aren’t used often, creating health and odor issues. P-traps require less frequent maintenance to remain effective.

6. Installation Scenarios

S-traps may still be found in older homes, rural buildings, or temporary setups where wall access or venting isn’t feasible. P-traps are standard in virtually all new builds and major remodels. If you can access wall plumbing and proper ventilation, a P-trap is the preferred solution.

Now that we’ve reviewed the main differences, let’s take a closer look at the S-trap—what it is, how it functions, and when (if ever) it makes sense to use one.

Introduction to S-Traps

S-traps were once a standard in residential plumbing but have become less common in modern installations. While they are still found in older homes or makeshift setups, they’re typically flagged during inspections due to poor venting. However, understanding how S-traps function can help you troubleshoot plumbing problems or plan for a future upgrade.

Understanding what an S-trap is begins with its core features. Each of the characteristics below helps explain why S-traps were once common—and why they’ve fallen out of favor.

Key Characteristics of an S-Trap

Each characteristic of an S-trap contributes to how it functions and where its limitations lie.

• Connects to Floor Drains
  S-traps discharge waste vertically into the floor, making them suitable for rooms without wall plumbing. This is often the case in older homes, detached structures, or temporary sinks. However, the vertical drop increases the risk of water siphoning from the trap.
  
• Lacks Built-In Venting
  Unlike P-traps, S-traps don’t include a vent pipe to equalize pressure. This causes the water in the bend to be pulled out by suction during drainage, breaking the water seal. Without this seal, sewer gases can rise back into the living space.
  
• Visible “S” Shape
  The most recognizable aspect of an S-trap is its double curve, which mimics the letter “S.” This shape is a dead giveaway that the plumbing might be outdated. Although visually distinct, it’s a sign the system may need to be brought up to code.
  
• Outdated in Modern Plumbing
  Most building codes have phased out S-traps in favor of safer, more reliable P-traps. Converting S-traps may be necessary if you plan to remodel or sell your home. Some municipalities may issue violations if S-traps are found in new installations.
  
• Common in Older or Remote Setups
  You’ll often find S-traps in older properties that haven’t been updated or in rural areas with minimal infrastructure. They may also appear in workshops, barns, or outdoor sinks where proper venting and wall drains weren’t feasible. In these cases, they serve as a functional but suboptimal solution.
  

Despite their drawbacks, S-traps can still be found in specific situations. Let’s look at when and where using an S-trap might still make sense.

What Is an S-Trap Typically Used For?

While S-traps are no longer recommended for new plumbing installations, there are still scenarios where they might be used temporarily or out of necessity. Here are typical examples where an S-trap may still appear.

• Standalone Utility Sinks in Garages or Workshops
  These setups often lack wall drainage and are installed quickly using available floor drains. An S-trap may provide a short-term solution in these cases.
  
• Older Homes Without Modern Venting
  S-traps may be found throughout the plumbing system in homes built before venting codes were standardized. They remain until major renovations are done.
  
• Outdoor or Garden Sinks
  For convenience, these sinks are often plumbed directly into the ground. S-traps may be used where venting isn’t feasible due to exposure or distance from the main building.
  
• Temporary Plumbing Installations
  Events, pop-up structures, or construction sites sometimes use S-traps for ease and speed. However, they’re almost always intended as a short-term fix.
  
• Rural Properties With Limited Infrastructure
  S-traps may be used in cabins, off-grid homes, or remote farm buildings due to a lack of wall access and formal plumbing systems

Now that we’ve explored where S-traps are used, it’s time to weigh their benefits and drawbacks more directly. Below are the top pros and cons to consider.

S-Trap Pros and Cons

✅ Pro: Simple Installation

S-traps are easy to install, especially when only a floor drain is available. Because there’s no need to connect to a wall or vent system, the installation can be done with fewer materials and less labor. This simplicity appeals to DIYers or those in rural settings. However, this ease comes at the cost of long-term reliability.

✅ Pro: Works in Tight or Low-Clearance Spaces

The vertical configuration makes S-traps useful in cramped conditions where a horizontal P-trap won’t fit. This is particularly handy in small utility rooms or under outdoor sinks. Their compact design allows for creative routing in unconventional setups. However, lack of venting still makes them less efficient.

✅ Pro: Low Cost for Materials

S-traps are inexpensive and widely available at hardware stores, especially in PVC form. This makes them a budget-friendly choice for quick fixes or non-permanent installations. The upfront savings can be attractive for those looking to stretch renovation dollars. Just remember that this low cost may lead to expensive issues down the line.

✅ Pro: Compatible with Floor-Only Drains

In situations with no wall drain or vent nearby, the S-trap can be a workaround. It connects directly to the floor plumbing without rerouting infrastructure. This is often the only solution when plumbing needs to be retrofitted in a non-standard location.

✅ Pro: Can Serve as a Temporary Fix

If you’re remodeling or waiting for professional installation, an S-trap can serve as a stopgap. It can keep a fixture functional while planning a proper P-trap replacement. For short-term usage, it’s better than leaving an open drain. Just don’t treat it as a permanent solution.

❌ Con: Not Code-Compliant in Most Areas

Modern plumbing codes prohibit the use of S-traps due to their siphoning risk. If your system is inspected or permitted, the S-trap will likely need to be replaced. Using one in new construction or major renovations could result in delays or fines. To avoid these issues, it’s best to upgrade to a vented P-trap.

❌ Con: Prone to Siphoning and Dry Traps

One of the S-trap’s biggest flaws is its tendency to siphon water out after each use. When this happens, the barrier between your home and sewer gases is gone, leading to unpleasant odors and potential health risks. Frequent use or additional venting may reduce, but not eliminate, this issue.

❌ Con: Difficult to Retrofit Properly

Upgrading an S-trap to a P-trap often requires rerouting the drain pipe from the floor to the wall, adding a vent, and sometimes opening up walls or floors. This can be labor-intensive and costly. While worth it for long-term value and compliance, it’s a significant investment upfront.

❌ Con: Not Ideal for Infrequently Used Fixtures

S-traps are more likely to dry out when the connected fixture isn’t used regularly. This makes them a poor choice for guest bathrooms or seasonal outdoor sinks. Once the water seal evaporates, sewer gases can leak into the home.

❌ Con: Limited Lifespan in Permanent Installations

Even with proper use, S-traps aren’t built for modern plumbing standards. Over time, the lack of venting causes performance issues, and replacing them becomes inevitable. Investing in a compliant setup from the start saves time and money.

Still have questions about S-traps? These FAQs cover common concerns about legality, safety, and whether upgrading is worth it.

S-Trap FAQ (Expanded)

Q: Are S-traps illegal?
A: While not federally banned, S-traps are prohibited under most modern plumbing codes in the U.S., Canada, and other countries due to their tendency to lose their water seal. This can lead to sewer gases entering living spaces, which is both a health and safety risk. If found in a home inspection, they are often flagged for replacement. Some jurisdictions may allow them with added venting or air admittance valves, but they are rarely approved for new construction.

Q: Can I convert an S-trap to a P-trap?
A: Yes, but it usually requires more than just swapping out parts. You’ll need to reroute the drain line from the floor to the wall and install a vent pipe or an air admittance valve (AAV). While this can be a DIY project for an experienced homeowner, hiring a licensed plumber is highly recommended to ensure code compliance and proper performance.

Q: Is there any situation where an S-trap is acceptable?
A: In rare cases, S-traps may still be allowed for temporary or outdoor fixtures where no wall access is available and venting is impractical. Even then, many local codes require additional measures like an air admittance valve to reduce siphoning. Always check your local plumbing regulations before installing one.

While S-traps have their place in older or temporary setups, modern plumbing relies on a more reliable and code-compliant solution: the P-trap. Let’s explore what makes the P-trap the preferred standard in today’s residential and commercial plumbing systems

Introduction to P-Traps

P-traps are the modern standard in residential and commercial plumbing. Designed with a horizontal orientation and built-in venting capability, they are both highly effective and fully code-compliant. If you’re doing any type of new construction, renovation, or fixture replacement, a P-trap is the solution you’re most likely to encounter—and for good reason.

Key Characteristics of a P-Trap

In most scenarios, each of the following traits makes the P-trap a superior and more reliable plumbing option.

• Connects to Wall Drains
  P-traps are designed to connect horizontally to wall drainage lines, which allows for proper venting. This configuration supports long-term water seal stability and prevents siphoning. The wall connection also makes for a cleaner, more concealed installation in bathrooms and kitchens.
  
• Includes Built-In Venting Capability
  P-traps are usually connected to a vent stack or paired with an air admittance valve (AAV), allowing air to enter the drain system and equalize pressure. This venting prevents the suction effect that can dry out the trap. It’s one of the most important reasons P-traps are so reliable.
  
• Recognizable “P” Shape
  P-traps resemble the letter “P” when viewed from the side, thanks to their horizontal bend toward the wall. This design retains water after each use to form a barrier between your home and the sewer line. It’s a simple but effective shape used in nearly all modern fixtures.
  
• Required by Modern Plumbing Codes
  Nearly all plumbing codes in North America and abroad require P-traps for new installations. Their performance, venting capability, and durability make them the safest and most compliant option. Inspectors will look specifically for this configuration during permitting and resale assessments.
  
• Versatile and Widely Available
  P-traps come in various materials such as PVC, ABS, brass, and chrome, making them suitable for everything from utility sinks to luxury vanities. Their widespread availability means you can easily find compatible parts for repair or upgrade. They’re also available in different sizes for kitchen and bathroom drains.

These features help explain why P-traps are used in so many applications. Here are the most common places you’ll find them in action.

What Is a P-Trap Typically Used For?

P-traps are the go-to choice for almost any permanent or code-compliant installation. They’re used across residential, commercial, and industrial plumbing systems and are adaptable to various fixtures.

• Bathroom Sinks and Vanities
  P-traps are the standard under-sink configuration, providing clean drainage and consistent odor prevention. Most come pre-installed or pre-configured with modern vanities.
  
• Kitchen Sinks
  Because kitchen drains deal with food waste and grease, a stable water seal is essential to avoid foul odors. P-traps are durable and easy to clean, making them ideal in this high-use area.
  
• Bathtubs and Showers
  In-floor drains are usually routed through a P-trap hidden beneath the surface, providing safe and silent protection from sewer gases.
  
• Laundry Rooms and Utility Sinks
  P-traps handle heavy wastewater loads and are vented to prevent disruption during fast-draining cycles. They’re also easy to service in the event of a clog.
  
• Commercial Restrooms and Public Buildings
  P-traps are the required standard across nearly all public-facing plumbing systems, from schools to office buildings. Their code compliance and reliability are unmatched.

Now that you know where P-traps are used, let’s discuss their biggest advantages and a few limitations.

P-Trap Pros and Cons

✅ Pro: Fully Code-Compliant

P-traps meet all major plumbing code requirements across the U.S., Canada, and other developed countries. Whether it’s a renovation, a brand-new build, or a commercial upgrade, you’ll never run into compliance issues with a P-trap. This makes it the safest long-term choice. It also streamlines permitting and inspections.

✅ Pro: Maintains a Consistent Water Seal

The horizontal orientation and built-in venting help the P-trap retain its water barrier with every use. This water seal is essential for preventing sewer gases from entering your living space. You won’t have to worry about the trap drying out under normal conditions. It’s ideal for both high-traffic and occasional-use fixtures.

✅ Pro: Works with Venting and AAVs

Whether connected to a traditional vent stack or an air admittance valve (AAV), P-traps function optimally in properly vented systems. This makes them compatible with both conventional and modern plumbing setups. It also enhances performance and reduces the likelihood of gurgling or slow drainage.

✅ Pro: Easy to Maintain and Repair

Most P-traps have slip joints that allow for quick disassembly and cleaning. This is especially useful in kitchens or laundry rooms, where clogs are more common. Replacement parts are inexpensive and widely available. Even novice DIYers can service or replace a P-trap without hiring a plumber.

✅ Pro: Durable and Versatile Design

P-traps are available in multiple materials and sizes, making them adaptable to a wide range of fixture types and installation needs. Whether you’re working in a tight cabinet or an exposed area, there’s a P-trap that will work. They’re also more aesthetically pleasing when using designer finishes like chrome or brass.

❌ Con: Requires Wall Access for Proper Installation

Because P-traps connect to wall drains, they’re not a good fit for floors-only plumbing systems unless modifications are made. This can be a challenge in older homes or garages without wall access. Retrofitting for wall plumbing adds time and cost to the project. It’s not ideal for quick or temporary fixes.

❌ Con: Not Suitable for Floor-Only Drains Without Modification

If your drain exits through the floor—like in many older homes or outdoor buildings—you can’t simply install a P-trap without rerouting the pipe. You’ll need to convert the system or use an AAV with special venting considerations. This limits the flexibility of the P-trap in legacy plumbing environments.

❌ Con: Slightly More Complex Installation

Compared to the plug-and-play simplicity of an S-trap, installing a P-trap requires more precise alignment and potentially venting connections. If the wall pipe isn’t perfectly positioned, you might need extension kits or angled fittings. It’s not overly difficult, but it’s a step up from basic installs.

❌ Con: Prone to Clogs Without Maintenance

Although P-traps are easy to clean, they can still collect hair, grease, or soap scum over time. If left unchecked, this can lead to slow drainage or backups. Regular cleaning or installing a drain screen can prevent buildup. Fortunately, access for cleaning is very straightforward.

❌ Con: May Require Professional Help in Retrofits

If you’re switching from an S-trap to a P-trap in a space without existing wall plumbing, you’ll likely need a plumber. Converting the drain line and adding a vent pipe isn’t always DIY-friendly. This can add to the cost of otherwise simple remodels. However, it’s worth the investment for long-term compliance.

Do you have more questions about installing, maintaining, or identifying a P-trap? The FAQ section below covers everything you need to know before proceeding.

P-Trap FAQ (Expanded)

Q: Is a P-trap required by building code?
A: Yes. P-traps are the industry standard and are required by virtually all local and national plumbing codes. They offer safe, effective, and vented drainage solutions and are preferred by both inspectors and contractors. If you’re planning a remodel, installing a P-trap will ensure your project passes inspection.

Q: How do I know if my trap is a P-trap?
A: Look for a U-shaped bend that curves horizontally toward the wall. The connection should lead to a vent or air admittance valve. P-traps are typically located under sinks, showers, or bathtubs. If your drain connects into the wall rather than the floor, you likely have a P-trap.

Choosing between an S-trap and a P-trap comes down to safety, code compliance, and long-term performance. While S-traps may serve temporary or legacy purposes, P-traps are the modern, reliable solution for most residential and commercial plumbing needs.

Eric Jensen

Eric is the founder and president of Badeloft USA. He has been the president of Badeloft’s US division for over ten years and oversees all marketing and branding aspects of Badeloftusa.com.
His expertise lies in small business development, sales, and home and bathroom industry trends and information.

Contact us with any business related inquiries.