P-Trap vs. S-Trap Drain Pipes: What Are The Major Differences?


P-Trap vs. S-Trap

Whether you are building a home or remodeling an older home, you should understand drain configurations when deciding on plumbing. A licensed plumber is involved with the contractor or builder, but you should know what is up to code. Understanding the main differences between a P-trap and S-trap will help to ensure your home is a safe environment.

A P-trap is a U-shaped pipe in toilets and sinks to connect the drain to the sewer system. An S-trap has the same purpose as the P-trap, but it forms into an S-shape. Due to safety hazards, S-traps are no longer up to code in new builds or renovations. 

What Are Drain Traps?

The purpose of drain traps is to provide a water barrier between the inside of your home and the sewer. The drain trap retains a small amount of water each time the sink drains. The water barrier’s intention is to keep any sewer gases from entering your home.

There are several different types of drains. An ideal drain trap will be able to resist any damage and household chemicals. It will also be able to prevent foul smells, insects, and parasites from entering your home, as well as rodents. 

An ideal drain trap will also be able to self-clean. Quality drain traps have a long service life, are easy and quick to install, and economical. 

What Is a P-Trap?

P-traps are U-shaped pipes in toilets and underneath sinks. They connect the drain to a home septic tank or the sewer system. P-traps hold a small amount of water to prevent sewer gases from rising into the home.

What Is an S-Trap?

The S-trap is similar to a P-trap, but it lacks a horizontal arm. Instead, it curves down and then down again, forming into an S-shape. This trap connects the drainpipe to the floor.

Main Differences Between P-Traps and S-Traps

In the chart below, you can find the main differences between P-traps and S-traps.

P-Traps vs. S-Traps

Quality P-Trap S-Trap
Waste Reserved Location Reserves little to no water in the trap. Reserves wastewater in the trap.
Water Waste Uses a small amount of water to drain the waste. Uses a large amount of water to drain the waste.
Preference P-traps are preferred. S-traps are no longer up to code.
Size P-traps are smaller in size. S-traps are larger than P-traps.
Ability for Sink to Save Water The sink doesn’t have to save water.  The sink has to save water.
Odor There is no odor associated with the p-trap. The s-trap can release a foul odor.
Maintenance Does not require maintenance. Requires frequent regular maintenance.
Installation The draining system outlet installs in the wall. The draining system outlet installs in the floor.

Housing Codes of P-Traps and S-Traps

Housing Codes for a P-Trap

P-traps are now used instead of S-traps, especially under sinks. P-traps eliminated the siphoning issue discovered with most S-traps. Two main features in the P-trap stop siphoning. 

The first feature of the P-trap is a vented pipe. The P-trap vents inside a wall and through the roof to the outside of your home. When air pressure balances throughout the inside of the drain, siphoning becomes less likely. 

The second feature is an extension added to the drain side of the trap. The extended pipe reduces the possibility of gravity pulling water through the pipe. This is known as a waste arm extension. 

The required length of the waste arm extension is 2 to 2.5 times the diameter of the pipe. If your drainpipe is 1 – 1.5 inches, the waste arm extension should be at least 3 inches long. This ensures a proper P-trap configuration.

Housing Codes for an S-Trap

S-traps no longer meet code and are illegal in all new construction. S-traps are now illegal because siphoning can happen when a large amount of water drains. Siphoning pulls water through the pipe and leaves the bottom of the trap dry. 

When a trap is dry, gas and odors can travel through the drain pipe and into your home. These gases are methane, hydrogen sulfide, and ammonia which are found in sewers. Not only do these gasses have a bad odor, but they are also poisonous and explosive. 

Furthermore, pests can enter your home without some sort of barrier. It may seem unlikely for vermin to travel through the pipes, but without anything stopping them, they can succeed.

Risks Associated with P-Traps and S-Traps

Risks of a P-Trap

Improperly installed P-traps can become damaged and leak toxic sewer smells into your home. There are several issues associated with incorrectly installed P-traps. 

Dry P-traps can happen when the trap has lost its water seal, and sewer gasses build-up through the drain. Clogged P-traps are the product of a drain that runs too slow. If left untreated, leaky P-traps can occur and produce foul smells.

Risks of an S-Trap

As mentioned before, plumbing codes have now banned S-trap configurations in modern plumbing. The risk of water being siphoned out of the trap and releasing gas into your home has become too high.

This problem has existed for more than 100 years, which is why in 1880, authorities began to address it. They then developed and enforced plumbing and sanitation codes as well as drainage requirements. Some of these requirements and codes we still use today.

Choosing Between a P-Trap or an S-Trap: A Quick Glance

While the P-trap and S-trap are used for the same purpose, they are very different. When deciding between the two, you should keep housing codes and risks in mind.

Housing Codes for a P-Trap vs. an S-Trap

S-traps are no longer allowed in new construction or the remodeling of a home. The S-trap does not meet housing code requirements, and most inspectors will not pass the S-trap during an inspection. 

The Winner: P-Trap

Risks Associated With a P-Trap vs. an S-Trap

The P-trap is the right choice when deciding which trap you need. The S-trap can allow poisonous sewer gasses into your home due to its design. It can also allow vermin to enter your home if the trap goes dry for a long period.

The Winner: P-Trap

Related Questions

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

Sometimes an S-trap to P-trap conversion is necessary. If you decide to convert your drain trap, you need to change two aspects of the drain. The first thing you need to do is add a waste arm extension, then you need to vent the pipe. 

Installing a vent stack can be difficult and costly, but it’s worth it. If installing a vent stack isn’t currently in your budget, there are alternatives. You can use an air admittance valve (AAV) to equalize pressure inside the drain. 

An air admittance valve allows air to enter the drain pipe’s waste side but prevents sewer gasses from releasing. Make sure to check your local code before installing an air admittance valve, as some areas do not allow them.

When are you required to get a plumbing permit?

Each county or parish is different, but most require a permit if it is a big plumbing job. If you are replacing your water heater, the city considers this a large job. There are water heater safety codes that need to be observed, so this will require a permit.

Re-piping is a large project and requires you to change out all water supply pipes in the home. These jobs are usually done by a professional plumber and require a permit. Another job that requires a permit is drain line replacements. 

When replacing sewer lines, expect to get a permit. Any work involving sewer lines requires a permit because you need to meet specific safety codes. 

When moving existing plumbing, you will need a permit. This is considered a remodel, so there are building codes you have to follow.

Are there any other types of traps used in plumbing?

Yes, people use several other types of traps in plumbing, but they each have a different purpose. While P-traps and S-traps focus on sewerage, there are also floor traps, gully traps, bottle traps, and grease traps. 

Floor traps are located in the floor to collect wastewater from the bathroom, wash areas, and kitchen sink. A gully trade is used outside of a building and connected to an external sewerage line. It also collects wastewater from the kitchen and bathroom areas.

A bottle trap receives waste from the bathroom, kitchen sinks, and other appliances that don’t have a built-in trap. A grease trap installs in the waste pipe from one or more fixtures to separate grease from the liquid. It then retains the grease.

Our Final Take

Overall, when deciding between a P-trap or an S-trap, a P-trap is the best choice. While widely used in the early 20th century, the S-trap is now illegal. Builders no longer use the S-trap when installing plumbing in new homes, and contractors will not use it in remodels.

Stacy Randall

Stacy Randall is a wife, mother, and freelance writer from NOLA that has always had a love for DIY projects, home organization, and making spaces beautiful. Together with her husband, she has been spending the last several years lovingly renovating her grandparent’s former home, making it their own and learning a lot about life along the way.

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