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S Trap vs P Trap: What Every Homeowner Should Know

S Trap vs P Trap
Written by Callum Strempel

The purpose of the trap is to hold water so sewer gases with bacteria and viruses cannot enter your home through the drain.

The S trap is siphoned dry quite easily with every use without the water seal while the “P” trap always maintains a water seal.

Let’s dig deeper and see how they compare and why the S trap was banned in many states.

What is an S Trap?

The s-trap is named due to its general appearance, similar to the letter S.

Two PVC pipes are tethered together so water flows through one into the other when flushed.

What is an S Trap

What is a P Trap?

P traps gather water in the bottom of the pipe, which prevents leaks and odor. They are usually found between the toilet and the main sewer line (sometimes referred to as “wet vents”).

What is a P Trap

S Trap vs P Trap: How Do They Differ?


S trap

In our opinion, it is much simpler to install a p-trap than a S-trap. In the case of S-traps, you can mess things up if you don’t have any professional supervision.

P trap

It is easy and inexpensive to install a P-trap. Home-based DIYers can do it on their own if they want.


The P trap is recommended for DIY installation as they are easier and cheaper.

Design advantage

S trap

S-traps are simple in design – drain pipes fall into a trap at the bottom of the sink while the drain also falls into the trap from the sink.

P trap

The P trap doesn’t have a loop over then back down again. It goes into a horizontal run and then exits downward. As a result, water moves downward when it is forced into a 90° turn by the horizontal run.


P-traps offer more advantages in terms of design than s-traps – the reason S traps have become quite obsolete in this field.


S trap

The problem with S-traps is that they tend to dry out quickly, which may cause gas leaks. They also tend to be difficult to maintain and often need to be replaced entirely, which can be expensive.

P trap

P traps maintain water traps consistently and effectively. Five-row cylinders and four-row cylinders are very similar and they both receive the same volume of water as a result.


P-traps are the most efficient and are dependable with holding water traps in place.

Water Waste

S traps

S traps drain wastewater by using a large volume of water.

P traps

This type drains wastewater away with a small amount of water.


P traps are better for saving water since they use only a small amount.

Backflow Prevention

Backflow Prevention

S traps

S traps are equipped with air filters. As a result, your home is protected from sewer gases backing up into it without getting rid of them completely.

P traps

P traps function by creating a natural seal to prevent water from leaking. The overflow pipe can carry water away from the sink, but not back into it.

There is a water seal tin the trap below that prevents water from leaking back into it. As a result, sewer gas cannot enter the trap.


For this criterion, the s-traps win since the air filter is advanced.


S trap

Both connect up to the water trap for drainage.

P trap

They have the same connection as the S trap to drain the water out.


Both have a similar connection, so there’s a tie here.


S trap

S traps are usually found in sinks, but you’ll also see them in toilets and floor drains.

P trap

They are the most common type of drainage but can be used for all types, including floors and toilets.


Both are used for the same things like toilet and floor drains. But the S trap is not recommended to be used anymore.


S trap

It is not recommended to use the S trap though some older home still have it. Some countries, including the United States, have banned it.

P trap

Highly recommended to use and common everywhere.


The S trap is banned in some places. It is ideal to use the P trap now to avoid a fine and continuous problems.


S trap

Basic S traps can be brought at the local hardware store starting at $8 but can end up costing over $100.

P trap

Much the same as the above, your cheap option starts at $10 but can go over $100


Both are the same in the price range with budget and more expensive options.

Which is better S-trap or P-trap?

As a general rule, P-traps maintain water traps more effectively and consistently than S-traps. Their design makes them less prone to drying out and losing their seal. In fact, a P-trap installed properly will never leak.

How to convert an S-trap to a P-trap?


In order to convert an S-trap into a P-trap, AAV must be fitted with a vent pipe and waste arm that is attached to the drain and vent pipe.

P-trap conversions consist of two parts: waste arm extensions and vent piping.


1. Why do they still sell S-traps?

Ans. You can still find S-traps in stores as they serve a valid purpose, but they should not be used beneath your sink. Despite the building codes outlawing the S-trap, you may currently have an S-trap installed before the new law came in.

2. Does a p-trap need a vent?

Ans. The answer is yes. As a result, sewer gases are able to vent, avoiding pressure buildup in your sewer lines. In addition, it prevents water from siphoning from the trap.

3. Why was the S-trap Banned?

Ans. Many jurisdictions prohibited the installation of S-traps several decades ago. S-traps leave an air gap by draining too much water, which allows gases to enter the system.

4. What if my S trap is causing odor?

Ans. Run the water slowly for a few minutes to fill the trap so it can maintain a water seal and the sewer gases cannot escape to your home.

About the author

Callum Strempel

Licensed Plumber

Hi, I’m Callum, a full time plumber with a plumbing license issued by my state. So far, I’ve spent nearly a decade and a half installing and fixing a wide variety of plumbing components and units. My profession inspires me to talk about the kitchen and bathroom (and the plumbing system) so confidently and knowledgeably that I offer my free consultations to help neighbors and fellow homeowners every now and then. As I enjoy installing faucets, sinks, showers, and fixtures, I love to see people enjoying services from those components for years to come. So, I write every week to inform you and others. Will you mind going through some of my articles on this website?

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