S Trap vs. P Trap Drain Pipe Differences: The Ultimate Guide

Plumbing traps are essential parts of your home’s draining system, preventing sewer gases from entering your house through the drain. Choosing the right trap can be challenging, especially if you don’t know the difference between S and P traps. This guide can help you pick the right one.

The main difference between an S trap vs. a P trap is the shape of the drain pipe. S traps are shaped like the letter S and connect the sink tailpiece to the floor drain. P traps are shaped like the letter P and connect the tailpiece to the wall drain. 

The table below shows a quick overview of the differences between a P trap and an S trap.

FeaturesS TrapP Trap
Installation Easy, unless converting to a P trapEasy, unless converting from an S trap
Suitable setup Water closet Bathroom sink, kitchen sink, shower, bathtub, water closet
Siphoning riskHighLow
Drain connectionFloor drainWall drain
Efficiency LowHigh
BackflowReverses water in the trapLittle to no backflow issues
Maintenance FrequentRare
OdorFoul odor associated with the trapNo odor
Pest invasion riskHigh Low 
Plumbing Code approvedNoYes 

S Trap vs. P Trap: Differences & Comparison

While S traps and P traps have the same role, there are important differences between the two. Let’s have a look at them and see why you should upgrade to P traps if you have S traps in your home.


All plumbing traps are relatively easy to install if you’re not converting from one type of trap to another. 

S traps connect to the main drain pipe through the floor, whereas the P traps connect to a wall drain pipe. Despite this difference, both traps require you to follow the same steps to replace them (disconnect the trap from the fixture and main drain line, then install a new trap in place).

Converting from an S trap to a P trap is possible – and recommended – but the procedure is more complicated. You should hire a licensed plumber for this purpose.

Suitable Setup

S-traps are the oldest type of plumbing trap, and they initially equipped all fixtures. However, a variety of problems associated with this trap style led to the invention of the P trap. P traps fix a lot of issues, including the dry trap syndrome.

Because of this, P traps have started to replace S traps in most setups, and S traps have been banned for use in new constructions

Nowadays, the only fixture that still uses S traps without problems is the toilet bowl (although P-trap toilets also exist). S traps work in toilets because a slow stream of water keeps flowing into the bowl as the toilet tank refills, filling the bottom of the bowl with water. Thus, there is no risk of siphoning. 

The only acceptable plumbing trap is the P trap for all other fixtures, including kitchen and bathroom sinks, tubs, and showers. P traps are also recommended for appliances connected to a drainpipe, such as a dishwasher or washing machine.


Siphonage refers to the emptying of a plumbing trap due to the downward rush of water and air in the plumbing system. The pressure built up in the systems sucks all the water in the trap, and unless slower flowing water is refilling it (as it happens with the toilet), the trap will fail to seal sewer gases out of your house.

Siphonage is a frequent issue in S traps and the reason why they were replaced with P traps in the first place. This phenomenon doesn’t affect P traps.

Drain connection

Generally, P traps connect to a wall drain and the S traps to a floor drain. However, while you can adapt a P trap to use with a floor drain, it is nearly impossible to adapt an S trap for a wall drain connection.


Perhaps the most important difference between S traps and P traps is their efficiency. S traps do their work properly as long as the water pressure is low. A higher pressure leads to siphonage.

The siphoning effect sucks all the water from the trap, and without the water seal, the sewer gases will get right into your home. 

P traps aren’t subject to siphonage due to the horizontal section of pipe linking the U-bend to the wall drain. Thus, these traps are perfect for modern plumbing systems with higher water pressure. 


Another essential difference between S and P traps is the required maintenance. S traps are notorious for getting clogged due to the lower water pressure and flow rate. In fact, S traps are mostly found in older plumbing systems that don’t support a high flow rate.

The slower flow fails in washing down the drain all the grime and debris, which will accumulate inside the trap. In addition to plugged fixtures, the clogs are also responsible for the foul smells often associated with S traps. 

P traps don’t have these issues and don’t need as much maintenance as the S traps – even though they also get clogged from time to time.

What is an S-Trap?

An S-trap is a plumbing trap shaped like the letter S. It connects to a floor drain and is the first type of plumbing trap ever invented. While S traps solved the unwanted sewer gas flow back into dwellings, they are not effective on modern systems due to the siphoning effect. For this reason, they have been outlawed.

Why Are S-Traps Illegal?

Are you wondering when were the S traps banned and why? S traps have been banned since 2006 due to the risk of siphonage. An empty trap is ineffective, allowing sewer gases to leak into your house. 

Sewer gases are toxic and potentially life-threatening if they accumulate in high quantities. For this reason, the International Plumbing Code bans them from use in new buildings. You may also have to replace the S traps in your house with P traps when remodeling your home.

What is a P-Trap?

A P trap is a type of fixture trap that doesn’t connect vertically to a floor drain but has a horizontal pipe section that connects to a wall drain. This difference solves the siphonage issues. Currently, P traps are the standard fixtures traps used in the USA.

Benefits of P-Traps

Proper Venting

One of the most important things to know about S traps vs. P traps is that the former is almost always used in older plumbing systems that lack venting. A lack of venting causes drainage and siphoning issues, including gurgling sounds, foul smells, and gray water backflow.

P traps are not subject to these issues because they are suitable to install on modern, vented plumbing systems. Plumbing vents allow the water to flow smoother and faster while also solving the siphoning problem

Fewer Clogs 

P traps can stop debris from flowing down the drain, thanks to the trap design. Thus, the risk of deep clogs inside the main drainpipe is lower compared to S traps. 

Valuables Retrieval 

In addition to debris, P traps can also prevent valuables from being washed down the drain – as long as the items are heavy enough to resist water pressure. 

If you accidentally drop a ring or bracelet down the drain, all you have to do is stop the water and remove the P trap to retrieve the item.


How do I know if my toilet is S-trap or P-trap?

Fixture S traps are bad, but most toilets still have S traps. This generally isn’t an issue because the toilet bowl fills slowly as the toilet tank refills after flushing. Thus, you don’t have to worry about siphonage.

However, if you want to replace your toilet, figuring out what trap your toilet has is easy. Look behind it and see if the drain outlets through the wall. If it does, the toilet has a P trap. If it doesn’t, the drain outlets through the floor, and your toilet has an S trap

Almost all toilets on the market have an S trap designed to outlet through the floor.

Can you use an S-trap for the bathroom sink?

No, you should not use an S trap for your bathroom sink. S traps are outlawed, and all fixtures (except for the toilet) must have a P trap. You may still be able to use an S trap legally if your sink has been fitted with an S trap when they were still legal, but only if you’re replacing a broken part. If you’re doing any upgrades or remodeling, you should convert to a P trap.

Can you convert an S-trap to a P-trap? 

Yes. The easiest way to do this is with a conversion kit

How do you vent an S-trap?

You can’t vent an S trap because S traps drop immediately to the drain. This is the main reason for siphonage.


Beyond design considerations, the differences between an S trap and a P trap in plumbing concern the trap’s efficiency and the installation and maintenance ease. We hope this guide can help you understand why the S traps are bad and why you should convert to P traps if you still have old-style fixture traps in your home.

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