S trap vs P trap: 5 Differences You Need to Know Now

When we talk about ‘traps’ in bathroom plumbing, we are referring to a particular piping arrangement designed to protect your bathroom from bad sewer odor.

At first glance, it might seem strange to need a specific piping shape. Indeed the water just needs to be drained away by a straight line.

The shape of the pipe matters a lot because that drain is designed to ‘pool’ a bit of the water, which creates an air-tight seal that prevents dangerous and harmful sewer gasses from making their way back up the system and into your bathroom.

So, we use ‘traps’ to create a water seal that traps these gasses. Two of the most common trap types we use are called the ‘S trap’ and the ‘P trap.’

The main difference between an S trap and a P strap is their shape. They are named S and P because their piping shapes loosely resemble the letter they are called after.

While both trap types are fundamentally there to serve the same purpose, their unique shapes make them pool water differently, and as such, the P-trap is much more effective at stopping these harmful gasses than the S-trap.

In this article, we’ll take an in-depth look at the differences between these two plumbing trap types so you can fully understand why the P trap is preferred over the S trap.

What is an S trap?

The S trap came before the P trap. It was a logical shape that could create that pool of water that would make a seal and hold back unwanted gasses from backing up into your bathroom.

This includes gasses like methane, hydrogen sulfide, nitrogen, and carbon monoxide, which are dangerous. You do NOT want these making their way into your bathroom.

The S shape is compact and convenient and has the added benefit of catching any objects that fell down the drain, such as watches or wedding rings, so that they could be easily fished back out.

It seemed ideal, but there was a fundamental flaw in its design. 

While simple tasks like washing your hands or a toilet flush were not a problem. 

Anytime a large amount of water was flushed down the drain (such as a full sink of water or if a bucket of water was dumped in the toilet), it would create a siphoning vacuum effect that was strong enough to suck excess water from the pool we needed to have sitting in the trap.

This meant that sometimes there was so little water in the trap (we call this leaving the trap ‘dry’) that there was a direct air gap allowing those gasses to seep back into the bathroom.

This was quickly deemed unacceptable and is now considered prohibited throughout the entirety of the United States and in many other countries worldwide. 

Older homes found to be using these traps will be required to replace them with the updated P trap design.

What is a P trap?

After the overall failure of the S trap design, some modifications were made to it to address its issues, and we were left with what we now call the P trap. 

Much like the S trap, it’s named this way as the piping shape resembles that of a P.

The goal is precisely the same as the S trap, to create a small pool of water that ‘traps’ those harmful gasses down in the plumbing and prevents them from entering our home.

Two significant adjustments were made to the S trap for us to arrive at the P trap.

1: Firstly, a ‘vent’ was added to the piping system. This means as the water is drained down the sink, it also pulls in air from a vent that leads outside. 

This air allows the pressure inside the drain to balance itself and dramatically reduces the possibility of water being siphoned out and leaving the trap ‘dry.’

2: A long extension was added (this makes the tail of our ‘P’ shape), reducing the ‘pulling’ aspect that the tighter S shape caused. This is called a ‘waste arm extension.’

While the P trap is not 100% perfect, there are still certain circumstances in which the water pool can be compromised (for example, through evaporation if you end up not using the sink for a prolonged period). The chances of experiencing this on a P trap are considerably lower than on an S trap.

It’s also possible to ‘convert’ an older S strap to a P trap by adding an AAV (Air Admittance Valve), the air vent that will help equalize the pressure and that all-important waste arm extension which requires a little extra piping to be added.

This can sometimes be easier than just re-doing the whole plumbing system. You can even buy S trap to P trap conversion kits to make this process easier.

What are the differences between an S trap and a P trap?

Hopefully, you now know better why the S trap is no longer used and the risks involved with having one installed.

Let’s take a deeper look at these differences so you can better understand how they differ.

1. The shape

Their shape is the most notable difference between the two plumbing trap types.

Both traps take after their respective letter, so the S trap is shaped like an S or snake shape, while the P trap consists of a single curved seal which then extends out into a waste arm extension, just like the letter P.

This arm extension has the added benefit of allowing the water to drain while air is taken in through the vent equalizing the pressure. This P shape, in turn, directly counteracts the siphoning issue that the S trap runs into.

The S shape by itself does fundamentally work under many use-case scenarios. It is more prone to having its water siphoned off when large amounts of water have to be drained away.

In conclusion, the P shape is much more effective than the S shape.

2. The use of a vent

When S traps were first invented in 1775, they were used for over 100 years until Thomas Crapper invented the U-bend (another term for the P crack) in 1880.

At this time, less thought was put into health and safety, and the consequences of those harmful gasses were not fully understood.

A big factor that makes the P trap superior to the S trap is the use of a vent.

While the P trap introduced the waste arm extension to help reduce the siphoning effect and increase the reliability of the trap. That alone was not enough.

So we also introduced a vent allowing outside air to be pulled into the drain as the water flows away. This reduced (or equalized) the pressure within the system and dramatically reduced the chance of excess pressure sucking that all-important trap water away.

Nowadays, a vent is a mandatory part of the plumbing code, and almost all modern homes will have one.

3. Volume of water used

Another aspect to consider is the volume of water used when flushing or draining.

Generally speaking, an S trap requires a large volume of water to drain the waste and refill the trap. This is because of the siphoning effect and the shape. 

The P trap, on the other hand, thanks to the addition of the vent, equalizes the pressure as soon as the waste is removed, meaning no excess water is siphoned down the drain, which therefore means less water is needed to keep the trap filled.

4. Plumbing code standards

Because of its inherent design flaws and risk of going dry, resulting in harmful gasses backing into your bathroom, S traps have slowly been phased out of new builds. They are illegal to install in many countries.

Almost all new constructions will use the P trap as it’s up to code and far safer than the S trap.

S traps can still be purchased and aren’t illegal to buy, but by installing one, you are breaking the plumbing code, and if you are caught, you will either need to replace it with a P trap or use a mod kid to turn it into a P trap.

5. Size

At first glance, it might seem like the S trap has a smaller footprint in your bathroom due to its tight snake-like shape and the fact a P trap has that long waste arm coming out from it.

But in fact, it’s the opposite way around. Because the S trap needs to conserve water in the sink to function, the piping is made larger to accommodate this.

Whereas the P trap doesn’t need to do this and can get away with holding far less water and, as such, uses thinner piping, leaving a smaller footprint than that of the S trap.

S trap vs. P trap: Are they the same?

Regardless of what shape piping you use, traps are all designed to do the same thing, prevent harmful gasses and foul odors from backing up into our bathroom. We delineate the two in their effectiveness at doing this job.

The S trap is inherently flawed as a suctioning effect can pull the water out, leaving it ‘dry.’ This means there is no complete seal and an air gap allows those gasses back up to the bathroom.

These traps are now illegal in most places and should be replaced with a P trap which does a far better job trapping those harmful gasses.

  • As the names might suggest, the S trap is shaped like an S, whereas the P trap is shaped like a P.
  • The P trap uses a vent that helps equalize the air pressure, thus reducing the chance of water being pulled out of the trap.
  • The S trap requires a larger volume of water to operate, whereas the P trap is more efficient and better at conserving water.
  • The S trap does not meet modern plumbing code standards and is illegal in many countries. The P is the current most common choice for most installations.
  • The P trap is smaller in size than the S trap.
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