P-Trap vs. S-Trap: What’s the Difference and Why Are S-Traps Banned?
Plumbing traps are small but extremely important parts of a home’s drainage system. If you look underneath a sink, you will usually see a curved section of pipe designed to hold water. This is called a trap, and its primary job is to prevent sewer gases and unpleasant odors from entering the building through the drain.
Two types you may encounter are the P-trap and the S-trap. Although they perform a similar basic function, their designs are different—and that difference is important.
Modern plumbing codes generally prohibit S-traps because they can self-siphon and lose their water seal. The P-trap, when properly vented and installed according to the applicable plumbing code, is the standard arrangement used in modern plumbing. The International Plumbing Code (IPC), for example, specifically lists S-traps among prohibited trap designs.
Important: Plumbing requirements can vary by state, city, and adopted code. Always check the plumbing code applicable to your location before modifying an existing drainage system.
What Is a Plumbing Trap?
A plumbing trap is a curved section of drainpipe that intentionally holds a small amount of water.
That water is called the trap seal.
The trap seal creates a barrier between the drainage system and the room. It allows wastewater to flow down the drain while helping prevent sewer gases from traveling back through the plumbing.
A typical trap can be found underneath:
- Bathroom sinks
- Kitchen sinks
- Laundry sinks
- Bathtubs
- Showers
- Some floor drains and other plumbing fixtures
The International Plumbing Code requires plumbing systems to protect trap seals from pressure conditions that could cause them to be lost.
What Is a P-Trap?
A P-trap is a plumbing trap shaped roughly like the letter “P” when viewed from the side.
It consists of a curved section that holds water and a horizontal outlet, commonly called the trap arm, that connects to the drainage system.
A simplified arrangement looks like this:
Fixture → P-trap → horizontal trap arm → drain/vented system
The P-trap became the standard modern solution because its configuration allows the trap to be properly vented.
The vent allows air into the drainage system and helps prevent the water in the trap from being sucked out when water flows down the drain.
How a P-Trap Works
Imagine filling a sink with several gallons of water and then pulling the stopper.
The water rushes through the P-trap and into the drain.
As the water flows away, the plumbing vent provides a path for air to enter the drainage system. This helps maintain appropriate air pressure and protects the trap seal from siphoning.
After the sink empties, some water remains in the curved portion of the P-trap.
That water continues to act as a barrier against sewer gases.
What Is an S-Trap?
An S-trap also has a curved section that holds water, but its outlet configuration is different.
Instead of leaving the trap horizontally through a trap arm, the pipe continues downward.
A simplified arrangement looks like:
Fixture → S-shaped trap → downward drain
The resulting configuration resembles the letter “S.”
S-traps were commonly used in older plumbing installations, particularly where fixtures drained directly through the floor.
You can still find them in older houses and buildings.
The problem is not that an S-trap cannot hold water initially. The problem is what can happen when a large volume of water drains through it.
P-Trap vs. S-Trap: What’s the Difference?
The easiest way to understand the difference is to look at the direction of the outlet.
| Feature | P-Trap | S-Trap |
|---|---|---|
| General shape | P-shaped | S-shaped |
| Trap seal | Holds water | Holds water initially |
| Drain outlet | Usually horizontal | Continues downward |
| Venting | Designed to work with a vented trap arm | Prone to self-siphoning |
| Modern code status | Generally permitted when properly installed | Prohibited by major U.S. plumbing codes |
| Main concern | Must be properly vented | Can siphon its trap seal |
| Common location | Modern sinks and fixtures | Older plumbing installations |
The critical difference is therefore not simply the shape of the pipe. It is the relationship between the trap, the drainage pipe and the venting system.
Why Were S-Traps Banned From Modern Plumbing?
The main reason S-traps are prohibited is self-siphoning.
When water rapidly flows down an S-trap and continues downward into the drainage system, the flowing column of water can create a siphoning effect.
That siphoning action can pull water out of the trap.
Once the trap loses its water seal, there is no longer an effective water barrier between the room and the drainage system.
This can allow sewer gases to enter the building.
The 2024 International Plumbing Code lists “S” traps among prohibited trap designs.
The Uniform Plumbing Code likewise specifically prohibits S-traps.
What Is Trap Siphonage?
Trap siphonage occurs when the movement of water through the drainage system causes water to be pulled out of the trap.
Consider an old sink with an S-trap.
You fill the sink with water and then release the stopper.
A large amount of water begins moving through the trap and down the drain.
Because the outlet continues downward, the moving water can create a siphoning action. Instead of simply draining the sink, the flow can continue pulling water from the trap.
The trap can therefore lose some or all of its protective water seal.
This is the fundamental problem with the traditional S-trap design.
Why Is Losing the Trap Seal a Problem?
The water inside a plumbing trap isn’t there simply to slow down the water.
It serves an important protective function.
The trap seal separates the interior of the building from the drainage system.
Without that water barrier, gases from the drainage system can potentially enter the building.
These gases can contain unpleasant odors and potentially hazardous compounds.
This is why modern plumbing codes place significant emphasis on protecting trap seals from siphonage, backpressure and other pressure-related problems.
Why Does a P-Trap Not Have the Same Problem?
A properly installed P-trap is connected to a vented trap arm.
The vent provides a path for air to enter the drainage system.
When water flows through the drain, the vent helps prevent the pressure conditions that could pull the water out of the trap.
In simple terms:
P-trap + proper venting = protected trap seal
The vent isn’t merely an optional pipe added for convenience. It is an important part of the drainage system.
The 2024 IPC states that plumbing venting must allow air to enter or leave the system so that the trap seal isn’t subjected to excessive pressure differences.
Why Can’t You Just Add a Vent to an S-Trap?
This is an important point.
Simply attaching a vent somewhere to an existing S-trap doesn’t necessarily make the installation compliant or correct.
The entire relationship between the fixture, trap, trap arm, vent and drainage system matters.
In modern plumbing, the typical solution is to reconfigure the drainage arrangement as a properly vented P-trap, subject to the local plumbing code.
Depending on the building and local regulations, an approved air admittance valve (AAV) may sometimes be used as part of a venting solution. However, AAVs aren’t universally permitted in every application or jurisdiction, so local requirements must be checked.
Can You Still Find S-Traps in Older Homes?
Yes.
If you live in an older house, you may discover an S-trap underneath a sink during a renovation or plumbing repair.
The presence of an S-trap doesn’t necessarily mean the entire plumbing system was installed recently or that the house is automatically unsafe.
Many older homes were built under plumbing requirements that differed from today’s requirements.
However, when plumbing is remodeled or replaced, the work may need to comply with the currently adopted plumbing code.
A licensed plumber can determine whether an existing configuration needs to be replaced and what venting arrangement is appropriate.
Are S-Traps Illegal Everywhere?
It’s better to say that S-traps are prohibited by many modern plumbing codes, rather than saying they are universally illegal everywhere.
For example, the International Plumbing Code specifically prohibits S-traps, and the Uniform Plumbing Code does as well.
Local jurisdictions can adopt different editions of plumbing codes and may have amendments or alternative requirements.
This is especially important in the United States because plumbing regulations can vary between states and municipalities.
For example, New York City has its own plumbing code provisions governing fixture traps and venting distances.
Therefore, always check the code adopted by your local authority before replacing or modifying a trap.
What Is the Proper Trap Seal Depth?
Trap seals need to contain enough water to remain effective while avoiding unnecessarily deep traps that can create drainage problems.
The IPC’s trap requirements generally specify a liquid seal of at least 2 inches and not more than 4 inches, with specific provisions and exceptions depending on the installation.
The purpose is to maintain an effective water barrier while allowing the trap to function properly.
What Happens If a P-Trap Dries Out?
Even a properly installed P-trap can eventually lose its water seal.
For example, consider a sink in a bathroom that hasn’t been used for several months.
The water sitting inside the trap can slowly evaporate.
Once the water disappears, sewer gases can potentially travel through the drain.
This is different from the self-siphoning problem associated with an S-trap.
P-trap water loss can happen because of:
- Evaporation
- Siphonage from an improperly vented system
- Pressure fluctuations
- Plumbing defects
- Leaks
- Improper installation
For rarely used drains, maintaining the trap seal is particularly important.
P-Trap vs. S-Trap: Why the Shape Matters
At first glance, the difference may seem insignificant.
Both traps contain water.
Both are designed to prevent sewer gases from entering a building.
So why does one design work better?
The answer is how water and air move through the drainage system.
An S-trap’s downward outlet can allow the flowing wastewater to create a siphoning action.
A properly configured P-trap uses a horizontal trap arm and appropriate venting to help maintain the trap seal.
That’s why changing the shape and venting arrangement made such a significant difference in plumbing design.
Why Plumbing Systems Need Vents
A common misconception is that a plumbing vent exists primarily to let sewer odors escape through the roof.
While vents are connected to the drainage system and ultimately provide a pathway for air, one of their most important functions is protecting trap seals.
When wastewater flows through a drainage system, air needs to move appropriately through the pipes.
Without proper venting, pressure changes can affect traps.
The IPC specifically states that the drainage system depends on maintaining an air path and that venting protects fixture trap seals from siphoning and other pressure effects.
Signs You May Have an Old S-Trap
You may have an older S-trap if you see:
- A curved trap underneath a sink
- The pipe drops vertically into the floor immediately after the trap
- No visible horizontal trap arm
- A plumbing configuration that resembles a large letter “S”
- A sink installation that appears to drain directly into the floor
However, don’t rely solely on appearance.
Some plumbing configurations can look unusual while still using an approved venting arrangement.
If you’re unsure, a qualified plumber can inspect the system.
Should You Replace an Old S-Trap?
If you discover an S-trap during a renovation, it’s worth having the installation evaluated.
In many situations, the solution involves converting the old arrangement to a properly vented P-trap.
The exact method depends on:
- The fixture location
- Existing drain piping
- Vent location
- Pipe size
- Building construction
- Local plumbing code
- Whether an AAV is permitted
- Accessibility for future maintenance
Don’t simply cut out the S-trap and install a P-trap without considering the venting arrangement.
A P-trap that isn’t properly vented can have drainage and trap-seal problems of its own.
Can an S-Trap Cause Sewer Smells?
It can.
If an S-trap siphons enough water from its trap seal, the remaining water may no longer provide an effective barrier.
That can allow drainage-system gases and odors to travel back through the fixture drain.
However, sewer odors can have many other causes, including:
- A dry P-trap
- Leaking drain connections
- Improper venting
- Damaged plumbing
- Missing or defective traps
- Cracked drainpipes
Therefore, an odor underneath a sink doesn’t automatically prove that an S-trap is the cause.
P-Trap vs. S-Trap: The Simple Explanation
If you want to remember the difference, think of it this way:
P-trap:
The trap exits horizontally and works with a vented drainage arrangement.
S-trap:
The trap continues downward and can siphon itself empty.
Why is the S-trap prohibited?
Because self-siphoning can remove the trap’s protective water seal.
Why is the P-trap used today?
Because its configuration, when properly vented, is designed to protect the trap seal from siphonage.
Frequently Asked Questions
Is a P-trap better than an S-trap?
Modern plumbing codes generally require or favor properly vented trap configurations and specifically prohibit S-traps. A properly installed P-trap provides the standard modern arrangement for many plumbing fixtures.
Why were S-traps banned?
S-traps were prohibited because their configuration can allow wastewater to siphon the trap seal out of the trap. Losing the trap seal can allow sewer gases to enter the building.
Are S-traps still used?
You may find them in older homes and buildings, but they are generally prohibited for new installations under major U.S. plumbing codes.
Can I replace an S-trap with a P-trap?
Often, yes, but simply swapping the trap isn’t necessarily enough. The drainage and venting arrangement must be properly configured according to the applicable local code.
Does every P-trap need a vent?
A fixture trap generally needs approved protection against siphonage and other pressure effects. The exact venting method depends on the plumbing code and installation.
What is the main purpose of a P-trap?
Its primary purpose is to maintain a water seal that helps prevent sewer gases from entering the building through the fixture drain.
Can an S-trap make a sink smell?
Yes, if the S-trap loses its water seal through siphonage, sewer odors can potentially enter through the drain. However, other plumbing problems can also cause drain odors.
Final Thoughts
The difference between a P-trap and an S-trap may look simple, but it is an important part of plumbing design.
Both use water to create a barrier between the fixture and the drainage system. The major problem with the traditional S-trap is that its downward configuration can allow self-siphonage, potentially removing the water seal.
The P-trap uses a different outlet configuration and is installed as part of a properly vented drainage system. This helps protect the trap seal while allowing wastewater to drain normally.
That’s why modern plumbing codes such as the International Plumbing Code and Uniform Plumbing Code prohibit S-traps and require plumbing systems to protect fixture traps from siphonage and pressure-related problems.
If you find an S-trap in an older home, don’t assume you can simply add a piece of pipe or swap fittings. The correct solution depends on the existing plumbing layout and the code adopted in your area.
In short: the P-trap became the standard because protecting the trap seal is essential—and the traditional S-trap can work against that goal.
AI-Assisted Content Notice:
Some articles, images, or other materials on this website may be created with the assistance of artificial intelligence (AI) tools. While AI helps generate ideas and draft content, all material is reviewed, edited, and published by humans to maintain accuracy, clarity, and quality. Readers are encouraged to verify information when necessary.
