Informational Guide

S Trap Vs. P Trap (2024 Comparison)

Learn about the similarities & differences between an S trap & a P trap here to help maintain a sanitary environment at home.

by Ian Haynes

Plumbing traps prevent harmful sewer gases from entering the house by confining water or gas in one place. Different plumbing trap types are available for various uses. Two of these are S-trap and P-trap.

Both of these are essential for the health and hygiene of inhabitants. It is necessary to have the right type of plumbing traps adequately installed, to maintain a sanitary environment around the house.

Read ahead to learn more about the similarities and differences between an S trap plumbing and a P trap plumbing.


P traps and S traps are the same sizes for the same application. However, both types go up a size when the location changes to a more demanding one.

For example, plumbing traps for bathroom sinks are usually around 1.25 inches in diameter, while those for kitchen sinks are about 1.5 inches in diameter.

In a nutshell, they are the same size for the same location.


The S trap travels from under the sink or tub to the drain line below the floor. It locks a specific water volume in-place to stop harmful gases from entering the house through the shower or sink drain.

Meanwhile, toilets have built-in traps to keep the bowls filled to a certain level at all times.

However, the issue with an S trap drain is that it can easily be siphoned dry if too much water exits at the same time.

On the other hand, a P trap follows a strict installation process and maintains a strong water seal so that it doesn't siphon dry easily.


Kitchens no longer use S-traps because of the drying issue. P traps are more suitable because they are better at keeping sewer gases out of the kitchen sink. Additionally, you can retrieve heavy items if dropped into the P trap by shutting off the water.

For toilets, it is again preferred to have a P trap because of its water sealing properties. However, if you get an S-trap, then make sure that the setout (the distance between the finished floor and the middle of the waste pipe) is between 140-165 mm according to modern standards.

Regardless of the bathtub size, the appropriate size for a bathtub trap and drain is 1.5 in diameter. Since a P-trap is precisely that size, it can fit onto the pipe without any additional tools.


A P trap is a lot easier to install and doesn't cost much either. However, correct installation is necessary to maintain all connections and avoid leaks.

Therefore, it is better to call professional plumbers to do the job. They have the experience to install your plumbing in a way that provides longevity of use.

Also, if you have a chrome P trap, you may need a pipe wrench during installation to connect all the pipe pieces tightly. Hence, it is slightly harder to install these.

On the other hand, you are no longer allowed to install an S trap, as they do not adhere to most plumbing codes.

fixing sink

Trap Safety

One of the S trap's most significant issues is that it can quickly lose water after use, leaving behind a dry trap.

Due to the health hazard, S traps have been banned for use by the plumbing fraternity.

On the other hand, the curve in a p trap creates a water seal, preventing sewer gases from backing up in the drain while allowing wastewater to exit simultaneously.

New water enters each time the P trap is called into action, making it a lot safer to use.

If you want to find out ways to stop rats from climbing your drain pipes, click here.

S Trap Vs. P Trap Explained

S Trap Drain

The name S trap is derived from its S-like shape, and they come in metal or PVC plastic. It is connected to toilets situated on ground floors, or where the pipe installation goes through the floor.

The S trap aims to confine sewerage water and stop harmful sewer gases from seeping through the shower or sink drain.

It does this by trapping water, effectively creating a barrier between the sewer pipes and the inside of the home.

P Trap Drain

A P trap drain is made out of metal or PVC pipes. It gets its name from its P shape construction, as it goes down, incurves, turns straight, and then opens up.

Commonly found in modern kitchens and bathrooms, the trap's primary role is to prevent harmful sewer gases from entering the home. These traps are essential to protect all household members from the spread of viruses and bacteria.

bathtub and shower p-trap drain

image source: Kitchen Bed & Bar

What Is A Venting System & How Is It Connected To Plumbing Traps?

Every trap that is part of your property's plumbing network requires a venting system. Venting is needed because the water in sewer pipes builds a negative pressure that can slow down or halt the water flow.

Additionally, this pressure can help clear stagnant water from the P traps attached to tub drains, sinks, and showers.

Although vents allow sewer fumes and gases to disappear from pipes, their primary role is to let the drainage and sewer system move freely. They equalize the pressure in the lines by allowing air to flow inside.

A stack is the central part of the vent system generally meant for the main bathroom. It runs down to the sewer and up the roof, into a pipe above the roofline.

Every connection to the central plumbing fixture abides by strict plumbing codes. The most important of these codes is that plumbing links should only be a specific distance away from traps.

Converting An S Trap Into A P Trap

P-traps and S-traps serve the same purpose, but the difference is that of a horizontal pipe connecting the vent to the outflow. However, with this pipe's addition, you can convert an S-trap into a P-trap as long as it is at least 4 inches long and connects to a vent.

P-trap assemblies have sections connected using compression fittings that can be hand tightened. The procedure is relatively straightforward – the trap inlet is first connected to the drain tailpiece (the pipe that comes down from the sink) – then the outlet is connected to the drain. If the said drain is situated on the floor, you must swivel the trap, bring in more pipes and create bends to join the outflow tube.

In the absence of a cheater vent, aka the air admittance valve, the vent shouldn't be more than five feet away from the trap weir. It should also be vertically positioned so that it extends past the fixture's overflow level.

However, it is better to get a professional to convert your S-trap into a P-trap because of the drain's position under the sink.

It isn't easy to set up the pipe system needed to connect to the central vent. As a homeowner wanting to take this up as a DIY project, try to install an air admittance valve or cheater vent if your local codes allow so. 

Watch this detailed YouTube tutorial for a step-by-step guide on converting S trap into P trap.


What is the difference between a j trap and a p trap?

The P trap has removable nuts, while the J trap has a captive nut.

What is the difference between a bottle trap and a p trap?

A bottle trap takes less radial space under the waste outlet than the P trap.

How far below the drain should the p trap be?

According to Universal Plumbing Codes, the P trap should be 24 inches away from the drain.

Can p trap be installed higher than drain entry?

Ideally, the P trap should be below the drain entry because gravity has to force water out rather than naturally flowing downwards.

Is an s trap illegal?

Yes, the S trap is illegal throughout the US under the Uniform Plumbing Code.

Can I use an s trap for the sink?

Don't use S traps in sinks because of the health hazards attached to them.

Do all drains need a trap?

Yes, all drains need a trap because methane, nitrogen, carbon monoxide, and hydrogen sulfide can travel from the sewage pipes to inside your home in its absence.


Plumbing traps are essential for every fixture in your house. This is because the sewer system emits various harmful gases that can pose a health hazard to you and your family.

However, for this to work correctly, it is necessary to evaluate each trap's efficiency and then have it professionally installed so it can prevent these gases from entering your house.

Ian Haynes is an expert writer who has successfully deployed over 500 plumbing pages and other related content. He has an excellent understanding of home plumbing issues and translates his experiences via Plumbing Lab so readers can have a better understanding of common household problems. Outside of his work, Ian likes exploring Brooklyn with his Labrador.