What Is a P-Trap Toilet – Best Applications

While most homeowners seldom consider how vital their toilet is, they quickly realize it when it suddenly ends up out of order. Of the various things that can go wrong with a bathroom, dealing with a toilet’s P-trap can be particularly confusing. But what is a P-trap Toilet?

A P-trap toilet is a toilet with a P-trap incorporated into it. These devices serve an essential function with a house’s plumbing system. Namely, P-traps keep foul and harmful sewer gases from making their way into your home.

This article details more about what this type of trap is, whether a toilet needs one, and other info you may need as a DIYer or homeowner.

What Is a P-Trap?

DIY’ers and plumbers are bound to deal with P-trap problems from time to time, so it is worth knowing exactly what they are. You should also be able to differentiate between P-traps and S-traps according to their shape.

A P-trap is a pipe with a distinctive p-shape connected to a drain that leads to the sewer system or septic tank. P-traps hold a fresh water level as a barrier or water seal to keep noxious sewer gases out of your home. 

If those combustible gases got in, they could be potentially fatal. P-traps are manufactured with metal or PVC and feature a strong water seal to function correctly. 

toilet trap

A lot of people feel that metal P-traps are more aesthetically pleasing, but the fact is that plastic or PVC P-traps last longer. Either of the two will work just as well, though.

The Differences Between P-Trap and S-Trap Toilets

You might have noticed that some plumbing uses P-traps while S-traps are mentioned elsewhere. The differences between these two types of traps include their shape, how they work, and even their legality.

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Both P-traps and S-traps derive their respective names from their shape. P-traps resemble the letter P, and S-traps resemble the letter S. You will not encounter many S-pipes these days as they have been banned for years, but more on that below.

How They Function

Precisely because of its shape, a P-trap connects to a horizontal drain pipe, letting air in and allowing water and waste to go out. The S-trap connects to a vertical line that does not let air in, leading to a siphoning action that takes water from the plumbing trap.

s trap


As of 2006, S-traps are illegal in the U.S. because of their siphoning effect. The negative air pressure created by S-traps inside drain lines allows combustible methane gases into the home. 

The Uniform Plumbing Code prohibits these traps in the entire country for new and remodeled dwellings. Because of this, you should not install one and should replace any in your home to avoid legal issues and—above all—for safety.

Does a Toilet Need a P-Trap?

While older toilets with S-shaped traps go back to the 1800s, modern toilets do not need S- or P-traps. Toilet designs now incorporate a built-in trap which means they can be installed without attaching an additional P-trap. 

This makes installation and replacement much easier for plumbers and DIY’ers alike.

Despite modern toilets not requiring a P-trap, two major problems should be highlighted. One is that if you are dealing with a toilet that is not modern by any standard, you may need to attach a P-trap to it. 

Another is that some people do add a P-trap beneath modern toilets even though it is unnecessary. Note that unnecessary P-traps under toilets can often lead to trouble.

When you flush a toilet with an unnecessary P-trap, a pocket of air appears. The water from the bowl acts as a seal and the P-trap as another. This situation traps waste in the air pocket and blocks the toilet.

Where is a P-Trap Located in a Toilet?

Older toilet designs that used P-traps would have them right behind the toilet. Recently, some people have installed P-traps underneath the toilet, but this is unnecessary. Modern toilets have traps built inside and directly behind the toilet bowl.

Of course, you will find a P-trap right beneath your bathroom sink. Placing the sink P-trap in this position allows for efficiency. Like the trap built into your toilet, your sink P-trap will prevent bad-smelling and dangerous gases from entering your home.

Can I use a Kitchen Sink P-Trap for a Bathroom Sink?

P-traps for domestic use are intended for sinks in kitchens and bathrooms. This fact might make them seem interchangeable, but they are not. P-traps for kitchen sinks are of a different size than those intended for toilet sinks.

You cannot use a kitchen sink P-trap for your toilet sink or vice-versa. Domestic P-traps are explicitly made for sinks or toilets. While they both have a P-shape, they are of different sizes, which is the key factor here.

The concept behind P-traps made for a kitchen and those made for a bathroom is the same. However, a bathroom sink P-trap is usually 1-¼-inch in size, and a kitchen sink P-trap is normally 1-½-inch in size.

Should I Use a Metal or Plastic P-Trap?

When deciding between plastic and metal P-traps, you must consider price and style.

If price is an issue, plastic P-traps are usually a better choice. P-traps made of plastic are also easier to install and will not corrode.

On the other hand, if your kitchen or bathroom style calls for a metal waste pipe, this will be a better choice for you. Just keep in mind that this will drive up your budget and the P-trap’s lifespan will not be as long.


A P-trap toilet is a toilet with a P-trap that keeps dangerous gases out. However, modern toilets no longer require these traps as kitchen and bathroom sinks do. 

If you need to replace a kitchen or bathroom sink P-trap, you can choose from metal or plastic options according to budget and style.

If you need to replace a kitchen or bathroom sink P-trap, you can choose from metal or plastic options according to budget and style. You can also recover items easier if they fall into the drain. Overall, these traps are an important modern plumbing fixture.