Do You Need A P-trap For Washing Machine? (Why+How To Install)

Washing machine installation involves installing a systematic plumbing system for efficient function. While installing the plumbing system for your washing machine, you may get a question if a p trap is necessary for the washing machine. Let’s find out the clear answer to your question. 

A washing machine needs a p trap installation in its plumbing system. The primary purpose is to block the backflow of dangerous gases from the sewer system. A P trap also helps prevent the blockage in the drain from the accumulation of all the debris particles. 

In this article, you will learn the importance of the p trap, how to install it, and how to determine the height of the p trap. So, keep reading. 

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What is a P-Trap and How Does it Work with Washing Machines?

A p-trap is the plumbing material used while installing a drainage system for the washing machine. 

It has several functions to perform in the drainage function of the washing machine. 

It is a U-shaped pipe with a depth of 2 inches. 

The main function of the p-trap is to accumulate water in it and prevent the blow flow of sewer gases inside the home. 

Here is a brief insight into how the p trap works in the drainage system of the washing machine. 

At the end of the washing cycle, wastewater will reach the p trap, creating a seal with water to prevent the backflow of harmful gases from the drain line. 

Dirt and debris from the water are accumulated on the p trap preventing it from accumulating on the drain line. 

It is attached to the standpipe. For the effective function of the p trap, you must install the standpipe at the right height. 

In case of any issues in the drain line, you must start by checking the p trap for clogs. 

Clogs in the p trap can be easily cleared by removing the p trap and cleaning it with a brush and soapy water. 

And it is important to make sure the p trap doesn’t dry out and always has standing water in it. 

The Importance of P-Traps in Washing Machine Plumbing Systems

A few days ago, I was experiencing some foul smell coming out of the washing machine drain pipe. At first, I could not understand the issue, but when I did the inspection, I realized what the issue was. I came to know during my research that due to the lack of a p-trap in my washing machine drain pipe, the smell was coming, and then I installed it in the drainage system. This solved the problem completely. I, therefore, realized that the p-trap is an important component of the plumbing system of the washing machine. 

Here is the importance of p traps in the washing machine: 

Preventing the backflow of sewer gases

As I mentioned, the p trap creates a water seal between the sewer and your laundry room to prevent the backflow of water. 

It helps to prevent the harmful health hazards caused by the backflow of sewer gases like methane. 

If there is no p trap, sewer gas is more likely to blow flow to your house, causing health hazards. 

Preventing clogs in the drain line

A p trap will catch all the debris from the washing machine and prevent it from reaching a drain line.

Clogs affect the function of a washing machine by not allowing the smooth flow of water in the drain line. 

By catching the debris particles in it, p traps help increase the washing machine’s efficiency. 

It allows easy access to the drain line of the washing machine

A p trap facilitates easy access to the drain line of the washing machine. 

You must access the entire drainage system whenever clogs or plumbing issues arise. 

Open the p trap to clear the clogs and inspect plumbing system issues. 

Common Problems that Can Arise Without a P-Trap for a Washing Machine

With that said, it is obvious that not using a p trap in the plumbing system of the washing machine will cause issues. 

Here are the common issues that arise when installing the plumbing system of the washing machine without a p trap: 

  • If there is no p trap in the washing machine, sewer gases are more likely to blow flow, and they will be released inside your home. Sewer gases are toxic and cause severe health hazards if exposed for a long time. 
  • It is common for drainage to smell bad. If there is no p trap in the washing machine plumbing system, those unpleasant odors will spread in your laundry room. It will be hard to bear the smell, and you can permanently eliminate it. 
  • Without a p trap, wastewater from the washing machine will directly reach the drain line carrying all the debris particles. In the long run, the plumbing system of your washing machine will get clogged. It will result in slow draining and water accumulation in the plumbing system. 
  • P traps prevent the entry of pests and rodents from the drainage system. Not having a p trap will let the pests infest your home. It causes health problems and also destroys the hygiene of your laundry room. 
  • The plumbing system of the washing machine is prone to leakage issues with the p trap. The leakage of water will damage the appliance and also the floor of the laundry room. 
  • Not installing p traps to the washing machine’s plumbing system will violate local codes of plumbing systems. You will have to pay a fine or a penalty.                                                

You can avoid all these problems by installing the p trap in your washing machine plumbing system. 

Do Building Codes Require P-Traps for Washing Machines?

Most of the local plumbing code requires the installation of a p trap to washing machine plumbing systems. 

If you are not installing p traps, it is a violation of local plumbing codes. In such a case, you must pay a fine or penalties. 

The height of the p-trap is determined by the uniform plumbing code( UPC ). 

While installing a p-trap, it is important to follow the guidelines of the uniform plumbing code, as installing a p-trap against the code is illegal. 

According to UPC, the height of the p-traps must be 16-18 inches off the floor. 

Different Types of P-Traps for Washing Machines and How to Choose the Right One

Here are the different types of p-traps available for washing machine plumbing. 

  • ABS P-trap. 
  • PVC P-trap. 
  • Flexible P-trap. 
  • Deep P-trap. 
  • Chrome P-trap. 

You can choose the right P-trap based on your requirement. It is best to consult a professional to choose the right p-trap for your plumbing system. 

How to Install a P-Trap for a Washing Machine: A Step-by-Step Guide

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Follow the steps below to install a P-trap in your washing machine plumbing system: 

  • Gather the required material to install the p trap in your washing machine. You will need a screwdriver, wrench, p-trap assembly, PVC pipes, cement, primer, Teflon tape, and level. 
  • Unplug your washing machine from the electric outlet. 
  • Turn off the water supply valves to prevent water leakage. 
  • Move your refrigerator away from the wall.
  • Access the drain hose of the washing at its back side. 
  • Disconnect the drain hose from the standpipe by loosening the clip around it using a wrench. 
  • Connect one end of the p-trap with the drain hose and secure it with the clamp. Tighten the clamp using a screwdriver. 
  • Attach the other end to the standpipe and secure it with clamps. 
  • Check if the p trap is fitted tightly. 
  • Replug the washing machine from the electric outlet and turn on the water supply. 
  • Move the washing back to the original position. 
  • Check for any leakage near the p trap and so take action immediately to fix it.                                                                       

How to determine the height of the p-trap

The depth of the U bend does not determine the height of the p-trap. 

Instead, it is determined by the height from the floor to the center of the p trap. 

It is ideal for installing a p trap 6 to 18 inches off the floor. 

To determine the exact height of the p-trap, measure the height from the floor and add an extra 1/2 to the diameter of the pipe. 

The height of the p trap is important because if the pipe is too low, the water in the p trap will evaporate, resulting in the backflow of sewer gases. 

P-Trap Maintenance for Washing Machines: Tips and Tricks: 

Cleaning the p trap regularly will ensure its proper functioning. Follow the tips below for maintaining the p trap: 

  • Disconnect it from the sewer line and clean it with water. 
  • Inspect the p-trap for any rust. If there is rust in the p trap, use a brush to clean it. If the p trap is damaged, you must replace it. 
  • Check if the p trap is clogged; clean it using soapy water and stick. 

Common issues with p-traps and how to avoid them

Some common issues are associated with the p-trap, and you might face these from time to time.

Below I have listed some issues I faced all this while with the p-traps.


If the p-trap is damaged, it causes leakage. Check the p trap and tighten it with a wrench to fix the leakage. 

If you cannot fix the leakage, replace the p trap. 


Debris particles like hair cause clog in the p trap. 

Remove the p trap from the plumbing system and clear the clogs using a plumbing snake or stick to fix the clog. 

Not unclogging the p trap will cause draining issues in the plumbing system of the washing machine. 


If the pipes attached to the plumbing system of the washing machine are corroded, it causes leakage issues. 

It also damages the traps and other pipes. To fix this issue, you must replace all the corroded pipes in the plumbing system. 

Improper installation

Improper installation of the P-trap will cause leakage and clog issues. 

You can fix this issue by reinstalling the p trap with the guidance of the professional, or you can refer to the manufacturer’s instructions. 

The ideal standpipe height for the washing machine

The standpipe height is essential for the smooth working of the plumbing system of the washing machine. 

Standpipe is attached to the p trap. 

It is fixed in the vertical position and collects drain water from the washing machine at the end of the cycle. 

A uniform plumbing code states that the standpipe height must be 36 inches while measuring from the floor. 

However, standpipe height must be determined based on the height of the p trap. 

If you have installed a p trap 6 inches off the finished floor, you must install a standpipe at the maximum height of 30 inches. 

And the diameter of the standpipe must be about 2 inches. 

Alternative Drainage Options for Washing Machines without P-Traps

Installing a washing machine plumbing system without a p trap is not recommended. It will go against local laws and codes. 

However, if there is no way to install a p trap in your plumbing system or washing machine, there are two alternatives. 

One is installing the drain box. 

It provides closure to the washing machine drain. 

It prevents the water backflow from the drain. 

Although it couldn’t replace the efficiency of p traps, it is a good alternative if you couldn’t install a p trap. 

You can install the drain box to the wall and connect it to the washing machine’s plumbing system. 

Another one is placing a drain pan under the leakage area to collect water. 

It won’t prevent the backflow of sewer gas as a p trap does, but you can install a drain pan to prevent water damage. 

Still, the p-trap plays an important role in the plumbing system of the washing machine. 

Not installing a p trap will cause several issues. So, consider consulting a professional to make an important decision. 

When to call a professional? 

Suppose you are not confident about installing the p trap by yourself, or you cannot determine the correct height of the p trap. 

In that case, it is best to consult a professional to install it.

Final Thoughts

The p trap is the essential component in the plumbing system of the washing machine. Different types of p traps are available for you to choose from based on your requirements. P traps help to prevent the backflow of dangerous gases from the sewer system, and it helps to prevent clogs in the drain line. 

The uniform plumbing code mandates the installation of p traps in the washing machines, and not installing the p traps will violate local plumbing codes. 

P traps contain standing water to create a seal between the washing machine and a drain line. It helps to prevent the spreading of bad odors from the drain line. Consider hiring a professional if you are not confident about installing a p trap for your washing machine. 

How far should the P-trap be from the washing machine?

A p trap must be installed 30 inches far from the washing machine. However, it depends on the local plumbing codes.

Should the P-trap be full of water?

P trap must be full of water to prevent the backflow of harmful gases from the sewer system. 

Reference: Trap Wikipedia

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Jeniffer Smith

Hi, I am Jeniffer Smith, a housemaker and blogger, and I grew up in Shrewsbury, Massachusetts. I have been researching and fixing appliances and other things around the home and kitchen from a young age to help my single mother. Every time something went wrong, I couldn't rest until I fixed it. And ever since, I have enjoyed fixing things around my home, and I am always ready to help my friends and family with advice and hands-on help. Since we were always looking for affordable ways of fixing things, we rarely hired experts and tried to fix most things ourselves. That interest brought me to write for this blog, as I realized my knowledge could help many people trying to fix things around their homes on their own without spending hundreds of dollars.

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