J-Trap Vs. P-Trap: What Are The Major Differences?

Dennis Howard
by Dennis Howard
Understanding the difference between plumbing traps is necessary when you install fixtures. J-traps and P-traps are similar, but they vary mostly based on their shape. Whether it be the shape or ideal application, follow along as we compare J-traps and P-traps.

Talking about plumbing and drains, in particular, involves a host of letter designations. Figuring out what these letter designations mean and how to use them in drain and water plumbing can get confusing? Exactly what are J-Traps and P-Traps?

The portion of all drain and waste systems that prevent sewer gases from coming back into the structure are known as traps. The terms describing these traps include U-Traps, J-Trap, P-Traps, and S-Traps. There are slight differences in some of these traps. However, the goal is the same to make a safe plumbing installation.

Understanding the various traps and their uses can make discussing your plumbing and drain issues with your plumber. A little knowledge can make the process of dealing with your plumber. This knowledge can also save you money by preventing unnecessary repairs or replacements.

The History of the Trap – Thank You, Mr. Crapper

Alexander Cummings gets the credit for installing the first S-Bend trap in a plumbing system in 1775. However, these S-bend traps were notorious for clogging and needed an overflow to work properly.

In 1880, Thomas Crapper (yes, that is where the modern term for a toilet originated) introduced the U-bend trap. The U-bend trap was far less likely to clog and didn’t need an overflow like the S-bend trap.

All modern traps trace their roots back to the Crapper U-Bend trap and variations in this basic design. The trap concept is such a necessity that modern plumbing codes require a trap on every waste drain. Similar to the P-trap vs. S-trap debate, J-traps and P-traps share key similarities and differences.

U, P, or J – Which do You Have?

So, what kind of trap do you have on the drain under your sink? A quick visual check is all you need to determine which type of trap you have. In truth, U, P, and J traps are very similar. The subtle differences to look for are the keys to knowing which letter to add to your trap designator.

U-Traps – Just Like They Sound

No matter what kind of trap you have, it is a U-trap. On most drain pipe installations, there is a portion of pipe with the basic shape of a U. The U-trap holds a small amount of water that prevents sewer gases from escaping back into your home.

Typically one end of the U-Trap has a flared fitting and a captured nut and gasket. On the other end threads. Another flared fitting mates with the U-Trap on this threaded fitting. Both J-Traps and P-Traps include a U-Trap of some sort.

J-Traps and P-Traps – One in the Same

Add a ninety-degree elbow to a U-Trap and you get a P-Trap or J-Trap. Depending on your perspective, the U-trap with these extensions resembles the letter P or the Letter J. It is all in the way you view the assembly.

U-Traps may have an extension or elbow fitted to both ends. These extensions don’t change how the trap operated but allows almost any drain installation to fit under a sink. There are 2 P-trap shower alternatives worth considering

What size P-Trap or J-Trap Should I Install

When replacing or repairing a P-trap or J-Trap, the general rule is to go back with the same size trap. In general, trap sizes depend on the installation and follow this scheme.

Drain Trap Installation TypeDrain Trap Size
Kitchen Sinks1 ¼ inch P-Trap
Bathroom Sinks1 ½ inch P-Trap
Bathtubs1 ½ inch P-trap
Laundry Rooms (Washer Drain)1 ½ inch P-trap

If you are installing a new drain in new construction, be sure that you are familiar with your local building codes. Your building inspection department can advise you on the proper size and method of installing a P-trap.

Trap Materials

P-traps and J-traps manufacturing use three types of materials suitable for installation in your home. The tree materials most commonly used for P-traps and J-traps include:

  • ABS Plastic
  • PVS
  • Brass

The general rule is to use a P-trap made of the same material as the drain lines in your home. Trying to connect different plastics can cause problems because of the different types of glue needed to make the joints.

How does a P-trap or J-trap Work?

J-traps and P-traps serve the important function of preventing sewer gas from rising from the sewer system and into your home. The concept is amazingly simple.

The U-trap in the P-trap or J-trap captures a small amount of water that serves as a barrier to the sewer gas that wants to come back up the drain pipe. A secondary benefit of a trap under your sink is capturing small articles that may inadvertently go down the drain. Rings and other jewelry often find their way down a bathroom drain and can be retrieved relatively easily.

Check this out if you want to know How To Connect A P-Trap To A Wall Drain.

Clogs and Plugs – When the water Doesn’t Drain

Most P-traps and J-traps operate for years without problems. However, under some circumstances, clogs or plugs can prevent the drain from operating properly. Some of the most common causes of a plugged P-trap or J-trap include:

  • Hair – Ask any plumber, and they will tell you that hair is the most common cause of a clog or plug in a bathroom sink P-trap. When wet, hair tends to cling together and form clumps. These clumps catch on any rough surfaces inside the P-trap. Once attached to the inside of the P-trap, the hair clumps act like filters, capturing all manner of debris that builds up over time.
  • Foreign Objects – Anything that isn’t water can become a problem in a P-trap. Small toys are a common issue behind clogs or plugs in a P-trap or J-trap. Larger pieces of soap bars may find their way down the drain. Jewelry is also a common item retrieved from clogged traps.
  • Soap – Believe it or not, soap can be a problem in your P-trap. Soap particles can react with minerals in your water. The result is a chalky substance that tends to settle out and cling to the inside of your drain pipe. This material is sticky and can attract hair and other debris that can make the problem even worse.
  • Fats and Grease – Kitchen sinks are susceptible to plugs and clogs in the P-trap when oils and grease go into the sink. The liquid oils or grease tend to solidify and gather at the lowest point. Once the fats sink, they become sticky and attract other debris. Never pour grease or fat down your kitchen sink drain. 

The best practice to adopt is to only put water down your bathroom and kitchen drains. Keeping your P-traps free of objects and other debris ensures years of fast-flowing drains. Find out what kind of PVC pipe you need for a gutter drain.

Can I use Chemical Drain Cleaners to Keep my P-trap Unclogged?

Most plumbers strongly advise against using harsh chemical drain cleaners in your home. There are several reasons that you should not use these drain cleaning products.

You can Damage your Drain Pipes

No matter what form of chemical drain cleaner you use, they depend on a reaction with the material clogging the drain. In some cases, this is an acid, and in other cases, a caustic product such as lye.

In either case, what will eliminate the clog may also damage the pipes and fittings in your drain. If the chemical doesn’t clear the clog and washes away, it sits in your pipes, giving it more time to damage or destroy your drains.

A Dangerous Combination – Mixing Drain Cleaners

It may be tempting to add two different drain cleaners to your clog under the assumption that if one is good, two is better. In truth, you may be brewing a disaster down your drains.

Mixing drain cleaners can lead to unexpected violent reactions. In some cases, explosions can result. These explosions can damage pipes or send an eruption of caustic, corrosive liquid back up the drain where you are standing.

Never mix chemical drain cleaners under any circumstances.

Be Concerned for the Environment

Anything you pour down the drain eventually ends up somewhere. If you have a septic system, all of these materials can end up in your backyard. Chemical drain cleaners tend to be persistent in the environment and can lead to contamination problems in your septic system leach field.

In municipal areas, most drain waster ends up in a treatment facility. Harsh chemicals like drain cleaners are hard on these systems, which usually depend on a natural element as part of the treatment for the wastewater. Caustic or corrosive drain cleaners and seriously impair the efficiency of wastewater treatment facilities.

I Smell a Foul Odor in our Guest Bathroom

Drains need regular use to operate efficiently. A drain that doesn’t get regular use can dry out. The water block in the P-trap can evaporate. Without the water block in the P-trap sewer gas can flow uninterrupted back into your home.

We suggest that you periodically flow water down every drain in your home, especially those that don’t see regular use. Guest bathrooms often sit for days or months without water flowing into the drain. Make it a habit to regularly turn on the water in each sink, bathtub, or shower, and flush each toilet a few times a month.

P-Traps and J-Traps – Essential Parts of Your Plumbing

No matter what you call them, P-trap and J-Traps are essential to a healthy home. Our daily lives would be much different if our drains didn’t have traps. Unpleasant odors, unsafe conditions, and unsanitary facilities would be the result. We hope that the information in this article helps you understand P-traps and J-traps and the important place they hold.

Dennis Howard
Dennis Howard

Dennis is a retired firefighter with an extensive background in construction, home improvement, and remodeling. He worked in the trades part-time while serving as an active firefighter. On his retirement, he started a remodeling and home repair business, which he ran for several years.

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