Which one is better, P-trap or S-trap?

Author: Shannon Halvorson  |  Last update: Tuesday, June 3, 2025

Due to a now-standard plumbing apparatus, the P-trap, water is all that is needed to seal off unwanted smells from your sewer. A similar attachment, the S-trap, has a similar function but has been out of commission due to it being less effective.

Is S-trap or P-trap better?

P-traps are known for their reliability and efficiency in maintaining a water seal, which helps to prevent sewer gases from entering buildings. Additionally, P-traps are less prone to syphoning and drainage issues compared to S-traps, making them a preferred choice for both residential and commercial plumbing systems.

Which trap is best for a toilet?

While both P-traps and S-traps create a water seal against siphoning, the shape of P-traps creates an effective water seal that traps gases. The additional vent pipe most P-trap toilets require further lower the risk of gas siphoning. Most modern plumbing codes require P-trap toilets for residential installations.

Why are S-traps not allowed anymore?

The S-Trap's Major Flaw

The “s” design could cause a water siphon, removing the water built up in the curve, opening the seal, and allowing sewer gas to pass through. This flaw is whys-traps are no longer allowed in plumbing code and should be replaced if you have one.

Should I use S or P-trap for sink?

While S-traps may still be found in older properties, upgrading to P-traps ensures compliance with modern standards and reduces the risk of sewer gas intrusion. Regular maintenance and professional consultations are key to keeping your bathroom plumbing in optimal condition.

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Which trap is better for kitchen sink?

The classic shape of trap is the p-trap. Here, the water seal remains standing in a U-shaped bent pipe under the drain. It is suitable for standard to large wash basins thanks to its excellent “elbow”.

What is the problem with the S-trap toilet?

While S traps were once popular, especially for ground floor installations due to their specific design and water flow characteristics, they have some significant drawbacks: They can be prone to siphoning, which means the water seal can be lost, allowing sewer gases to enter the home.

Are p-traps illegal?

For this reason, S-traps was officially banned under the Uniform Plumbing Code, and P-traps became the new standard. The shape of the P-trap is just one feature that helps stop this from happening.

How to avoid an S-trap?

S traps can be avoided by venting the vertical drain that the p trap drains into and ensuring that the top of the horizontal drain does not fall below the trap weir. In retrofitting older systems, an air admittance valve can be used to relieve vacuum pressure, thus preventing siphonage of the trap seal.

What trap to use for bathroom sink?

A bottle trap is recognisable by its cylindrical shape and vertical waste attachment. Compared to other sink trap types, they're compact and take up much less space making them ideal for ensuite bathrooms and cloakrooms. If your sink has exposed pipework, plastic traps can look a little unsightly.

How to fix toilet P-trap smell?

To eliminate odors coming from a dry p-trap, pour half a gallon of water into the trap to restore the barrier. It will prevent the odors from seeping through the drain. Another helpful method is to add a cup of white vinegar bleach to get rid of larvae and slow down the evaporation.

Which trap is most commonly used with plumbing fixtures?

Every sink, bathroom fixture, and drain line in a home is equipped with a P-trap, making them a universal and essential component of the modern plumbing infrastructure.

When to use a S-trap?

The S-trap and P-trap are both fixtures in a plumbing system, but each function is different. The P-trap is designed to seal off the drain pipe when it meets the floor. On the other hand, the S-trap is used to seal off any water that might escape from a sink or other fixture downstream from the trap.

What are the disadvantages of P-traps?

Disadvantages of P-Traps

One common issue is that the water seal can be protected if properly maintained or if there are blockages in the drainpipe. This can lead to the escape of sewer gases and unpleasant odors. The trap is regularly inspected and cleaned to ensure its proper functioning.

Is my toilet an S or P-trap?

The S-trap is the oldest style, shaped like an “S.” It lets waste exit through the floor, so it's often found in ground-floor bathrooms. However, it can dry out more easily, which may let smells escape. The P-trap is shaped like a “P” and exits through the wall, not the floor.

What can I use instead of P-trap?

SWVs such as HepVO offer an effective and efficient alternative to traditional p-traps: versatile installation, no foul odors, no gurgling noises, no limescale, hygienic, space-saving, easy flow, improves venting to negative pressure, performs under back pressures, effective in hot and cold conditions, and copes with ...

Is it OK to have 2 P traps?

In all of these cases, water has to pass through two separate traps, called a double trap. But don't confuse this with two separate traps, which is fine. Of course, those corrugated things in the photo above aren't proper, but the configuration is correct. Plumbing codes expressly prohibit the use of double traps.

Where is S-trap used?

S-Trap is a common SWR fitting used in combination with Pushfit & Solfit Pipes. Toilet with s-trap prevents transmission of unfavourable odours from the washroom to the living area. Can be connected to the shower area and WC if the branch line is in a vertical position.

Why are S traps illegal?

The “S” trap is prohibited under the Uniform Plumbing Code throughout the United States. This is because the “S” trap will siphon or suck water out from the trap which will end up releasing methane (sewer) gases into the home. So, it is a code violation and if you are doing remodeling, you will have to replace it.

Do you need a P-trap under a toilet?

Because of its bend, it always contains a bit of water to prevent dangerous sewer gas from seeping into your home. It's also something of a catchall for anything you accidentally drop down your sink drain. Toilets and bathtubs have P-traps, too. It is a necessary part of your home's plumbing system.

How do I keep my toilet trap from drying out?

Use the Fixture Regularly

Regular usage can prevent P-traps from drying out. Make it a habit to run water through infrequently used drains at least once a week.

Why are P traps better than S traps?

The P-trap is now the standard since the S-trap was deemed ineffective and prohibited in new construction under the Universal Plumbing Code. The S-trap was found to be prone to siphoning and drying out, which leaves the apparatus ineffective.

What is the best sink trap?

Our favourite trap is the Bottle Trap. They are named as such because they have a small bottle shaped canister as their connection to the basin. This cannister is internally divided which is how it prevents the gas from rising.

Should kitchen sinks have P traps?

The Purpose of a Trap Under a Kitchen Sink

This otherwise minor tweak in the piping design plays a very important role in your home's plumbing system. Without it, your house would immediately be filled with the pungent odors of everything washed down the drain.

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