HomeHome OrganizationComparing tow Plumbing Essentials S Trap Vs P Trap

Comparing tow Plumbing Essentials S Trap Vs P Trap

When we talk about ‘traps’ in bathroom plumbing, we mean a special way of setting up pipes to prevent bad smells from entering your bathroom.

At first, it might seem odd to need a certain pipe shape. You might think that water can flow out in a straight line. But the shape of the pipe is important because it helps keep bad smells away. 

The pipe is made to hold a little bit of water, which seals the pipe and stops stinky gases from the sewer from coming back into your bathroom.

We use ‘traps’ to make a water seal that catches these gases. The two common types of traps are the T trap’ and the ‘P trap.’ The main difference between an S trap and a P trap is their appearance. They’re named after the letters S and P because their shapes are similar to those letters.

Both trap types do the same job, but their shapes make them hold water differently. Because of this, the P-trap is better at keeping away the bad gases than the S-trap.

This article will explain the differences between these two types of traps so you can understand why the P trap is better.

Differences Between S Traps vs P Traps: A Quick Overview

Shape Shaped like an “S” or snake shapeShaped like a “P” with a waste arm extension
VentilationLacks proper ventilation, leading to the risk of harmful gases backing upHas a vent that lets fresh air in, balancing pressure and preventing gas backup
Water UsageRequires a larger volume of water for flushing due to siphoning effect and shapeUses less water due to the vent and design; no excess water is pulled down the drain.
Plumbing CodeIt is prohibited in many countries and is being phased out in modern buildings.Commonly used in new buildings, follows plumbing codes for safety
Water RetentionLess water kept in the sinkEffluent held in the trap
SizeRequires larger piping to accommodate water conservation needsUses smaller pipes since it doesn’t need to conserve water, takes up less space
MaintenanceOften requires frequent maintenanceTypically it does not require regular maintenance
Modern PreferenceRarely preferred nowadaysPreferred in modern setups
Water ConservationNeeded in the sinkThere is no need to conserve water in the sink
Draining System OutletPositioned on the floorMounted to wall

What Is the Function of a Plumbing Trap?

A trap is a helpful part of pipes in sinks, tubs, and toilets, even if it sounds complicated.

A trap system has curvy pipes that stop smelly gases from sewage systems from coming into your building.

Some common gases are:

  • Carbon Monoxide
  • Methane
  • Nitrogen
  • Hydrogen Sulfide

The best thing about plumbing traps is that they keep your family safe and healthy.

Those harmful gases could come into your bathroom or kitchen without a trap.

When you flush your toilet or use your tub or sink, the water stays in the pipes, usually the curves. This trapped water prevents bad gases from entering your kitchen or bathroom.

Understanding the S-Trap in Plumbing

The S Trap: How It Worked and Why It Wasn’t Enough

The S trap was made before the P trap. It was a smart shape that could hold water to prevent bad gases from entering your bathroom. These gases, like methane, hydrogen sulphade, nitrogen, and carbon monoxide, are dangerous. You don’t want them in your bathroom.

Simple things like washing your hands or flushing the toilet were okay. But when a lot of water goes down the drain, like filling a sink or pouring a bucket of water into the toilet, it creates a strong vacuum. This vacuum was strong enough to pull water out of the trap.

So, sometimes, there was too little water in the trap. We say the trap was ‘dry.’ This trap lets the bad gases get back into the bathroom. This was a big problem and was not allowed in the United States and many other countries. Older homes with these traps need to change to a better P-trap design.

Understanding the P-Trap: What Is It

After the S-trap design didn’t work well, changes were made to fix its problems. This change led to the creation of the P trap.

Like the S trap, the P trap is named after its shape, which resembles the letter P.

The goal of the P trap is the same as the S trap: to hold a bit of water that stops harmful gases from coming into our home through the pipes.

Two significant modifications to the S trap were made to create the P trap:

  1. A ‘vent’ was added to the pipes. This vent brings in air from outside as water goes down the sink. This vent helps balance the pressure in the pipes and makes it much less likely for the water to be pulled out, leaving the trap empty.
  2. A more extended part was added to the trap (this makes the tail of the P shape). This change reduces the pulling effect that the tighter S shape caused. This longer part is called a ‘waste arm extension.

The P trap could be better, but it’s better than the S trap. Sometimes, the water in the trap can still go away, like if the sink isn’t used for a long time and the water evaporates. But this happens much less often with a P trap than an S trap.

If you have an old S trap, you can change it to a P trap. You would add an AAV (Air Admittance Valve) to balance the pressure and the important waste arm extension. Compared to replacing all the pipes, this could be simpler. Even kits to help you switch from a S trap to a P trap are available for purchase.

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Converting an S-Trap into a P-Trap: Enhancing Plumbing Efficiency

Many places stopped allowing S-traps in new plumbing setups a long time ago. S-traps can let gases flow through because they don’t keep enough water, creating an air gap in the trap. 

Vent and Drain Line:

P-traps replaced S-traps, which have a similar shape but have an extra part called a vent and drain line. 

How do you change an S-trap into a P-trap?

To change an S-trap into a P-trap, you must put in a vent pipe with an air admittance valve (AAV) and add a longer pipe called a waste arm extension. There are two main steps for this change: 

  • Adding the vent pipe with the AAV 
  • Installing the waste arm extension.

The P-trap must connect to a vent pipe so it doesn’t siphon water away. If you don’t have a vent pipe nearby, you can use an AAV. This valve lets air in but keeps sewer gas out. Place the AAV on a short vent pipe near the sink, and ensure it’s higher than the drain.

Next, you add the waste arm extension to the drain line. Its length should be two to 2 ½ times the diameter of the pipe to stop siphoning. For example, if the pipe is 1 ½ inches wide, the waste arm extension should be at least three ¾ inches long.

After putting in the waste arm and vent pipe, connect the P-trap to the sink’s drain line and then to the waste arm. The waste arm is connected to the vent and drain line via a unique wye connector.

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Why Are S-Traps Not Permitted

The main reason why S-traps are not allowed is because they are hard to take care of and can be risky. An S-trap is not allowed because it can be slippery and cause accidents. 

The S-trap has an open part sticking up, which can make liquids fall into it and overflow when it’s full. 

The rules called the UPC Code say that you can’t use any “S” trap that might let stinky gases from sewers come out of a sink or appliance.

The Uniform Plumbing Code (UPC) says all plumbing must follow the rules. For instance, the UPC says you can’t use an “S” trap under a sink if it might let sewer gas enter the room. If there’s no way for the gas to escape, it can stay in the room and be bad for people’s health.

Outdated Plumbing: The Reason S-Traps Are No Longer in Use

The S-trap has been around for a very long time, almost 200 years. By using gravity, it’s a way to make water flow out of a building. But this design has stayed the same since the 1800s, and it’s time to improve it.

The S-trap is a pipe that helps water leave a building, especially from the lowest point. However, you don’t see it much in new buildings because it could be better than the newer U-trap.

Your s-traps in older buildings are cheaper to put in than U-traps. Water goes into this pipe through a curvy bend and flows down a straight pipe. There’s usually a little dip or a vent to stop stinky sewer gas from coming in. 

But there needs to be a solution. If the pipe starts to get rusty, it can spring leaks around that curvy bend and cause more problems.

The big issue with the S-trap is that the water at the bottom of the curvy part might freeze during really cold weather. Then, when it thaws, the water can back up and flood your basement.


If your old toilet is giving you trouble, like leaking, and you want to put in a new one, there are many different toilet options to pick from. When choosing a toilet, you not only have to think about the brand but also decide between two types: S-trap or P-trap toilets. The differences between them are explained earlier.

Remember, no matter which type of toilet you go with, it’s a good idea to have a professional plumber install it. They know much about this and have the experience to do a better job.

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Gabriel, a skilled individual with a wealth of knowledge and expertise, he specializes in diagnosing and resolving various problems that arise with Smart TVs. Additionally, he offers valuable advice on how to maintain and prolong the optimal functioning of these devices.


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