What Are Washing Machine P-Traps? (& how do they work?)

The P-trap is an essential part of the wastewater disposal system connected to your home’s sinks, showers, baths, dishwashers and washing machines. In many cases, you can’t see the P-trap as it is hidden from view but it is constantly preventing foul smells and sewer gases from entering your home.

In this article I’ll explain what the P-trap is, how it works and the beneficial reasons why you need one connected to your washing machine’s drain hose.

What Is A P-Trap?

The P-trap, named after its shape, consists of a U-shaped pipe which acts as a barrier preventing foul smelling and potentially dangerous sewer gases into the home.

It is a simple design that uses a bend in the pipe to form a water barrier preventing sewer gases from passing.

How Does The P-Trap Work?

The way the P-trap works is by keeping a layer of water trapped in the U-bend part of the trap which blocks the pipe effectively stopping any gases from passing through and into the home.

Wastewater leaves the washing machine via the drain hose and flows down the drain. It then enters a vertical section of the P-trap and continues on its downward journey through the pipe system.

As it travels, some of the wastewater collects in the U-shaped trap which prevents any gases or bad smells from travelling through that part of the P-trap.

It’s an incredibly simple design but completely effective. The water collected in the trap creates a barrier that the gases cannot penetrate. This means any foul smells or potentially harmful gases remain in the sewer and can’t get into the home.

Where Should The P-Trap Be Fitted?

kitchen sink

The P-trap needs to be fitted at the point where wastewater leaves many different plumbing fixtures to create a water barrier and prevent sewer gases from entering the home.

Places like;

  • Kitchen, Bathroom & Utility Sinks
    P-traps are very common features under sinks of all kinds connected to the drain pipe to form a water barrier preventing sewer gas and bad smells from entering the home.
  • Baths & Showers
    A P-trap is fitted below the bath and/or shower to prevent any bad smells or sewer gas from entering the home through the bath or shower waste pipe.
  • Dishwashers
    P-traps are fitted to the drain line on the dishwasher to prevent any foul smells and gases from the sewer entering the kitchen.
  • Washing Machines
    Washing machine drain lines are connected to a P-trap to prevent the laundry or utility room from foul smells or potentially harmful sewer gas.

What Are The Pros & Cons Of Washing Machine P-Traps?

open washing machine door

Let’s take a look at the advantages and disadvantages of installing a P-trap to your washing machine drain line.

The Advantages Of Installing A P-Trap On A Washing Machine

The advantages of installing a P-trap on a washing machine include;

  • Prevention Of Bad Smells
    P-traps completely block the drain hose with trapped water which prevents any foul smells from the sewer from getting into the home. As long as there is water in the trap, no foul smells can travel through the pipe.
  • Prevention Of Illness
    The P-trap stops anything travelling through the water barrier including; gases, insects, bacteria and other potentially harmful microorganisms from entering the home. This prevents illness and promotes a hygienic home.
  • Maintaining A Safe Environment
    The P-trap not only blocks bad smells, it also prevents harmful sewer gases like hydrogen sulphide and methane which can be harmful if they are allowed to build up indoors. The P-trap prevents these gases from entering the home keeping all who live there safe and toxin free.
  • Easy To Maintain
    P-traps are easy to maintain, all you need to do is make sure that the water seal remains intact. You can do this by regularly adding water to underused P-traps. For instance if you only use your washing machine once every two weeks, it might be a good idea to add water to the standpipe every other week to prevent the P-trap from drying out.
  • Inexpensive To Install
    P-traps have a simple design and are easy to install at a relatively low price. They are widely fitted by plumbers in domestic and commercial settings.

The Disadvantages Of Installing A P-Trap On A Washing Machine

The disadvantages of installing a P-trap on a washing machine include;

  • They Can Dry Out
    If the sink or in this case, washing machine isn’t used for a while, the P-trap can dry out. This would allow foul smells and sewer gas to enter the home as the water barrier is no longer in place.
  • They Can Become Clogged
    If P-traps have a commonly complained about disadvantage it’s that they can become clogged. As the wastewater from the washing machine runs through the drain hose, it carries dirt, detergent residue, hairs, felt and other particles with it. Unless the P-trap is flushed through regularly, these particles can build up and clog the pipe. This can lead to slow drainage or even backflow and flooding if left unchecked.
  • Need A Lot Of Space
    Depending on where the washing machine is installed, it can be difficult to install a P-trap in the allotted space. This often leads to digging out floors to accommodate the U-bend needed for the P-trap to work.
  • Need Maintenance
    Whilst the only real maintenance needed on a P-trap is regular flushing to prevent drying out and the occasional application of drain cleaner, many people forget to do these simple tasks which leads to the problems mentioned above.
  • Can Become Syphoned
    Under certain conditions, a quick change in water pressure can lead to the water collected in the P-trap syphoning out. This would allow foul smells and harmful sewer gases to enter the home. This can be prevented by installing an additional feature like an air admittance valve or similar ventilation device.
  • Frost Damage
    If your washing machine is situated in a cold place like a garage or shed, it is possible for the P-trap to freeze in cold conditions. When the water collected in the P-trap freezes it will expand and that could cause the pipes to crack or burst. This could potentially cause water leaks and flooding or other plumbing issues.

Are There Any Alternatives To P-Traps?

There are alternatives that can be used in place of a P-trap which are;

Bottle Traps

As the name suggests, bottle traps are bottle shaped and also smaller than P-traps. Typically used on sinks or in small areas where a P-trap wouldn’t easily fit, bottle traps feature a bottle shape with a curved section below the fixture that creates the water seal.

The curved section is often made from a clear material and this transparency makes it a popular choice for bathroom designers.


S-traps have been around for many years and were once the most common type of trap installed by plumbers. However, due to certain issues connected with the design, they have been pretty much superseded by P-traps. In fact, in some countries including the US, S-traps have been banned.

As the name suggests, S-traps form the shape of a letter S and work in a similar way to P-traps by creating a water barrier that sewer gases can’t penetrate.

The main noticeable difference between S-traps and P-traps is that P-traps have a horizontal outlet that connects to the drain pipe whereas S-traps have a vertical outlet.

Why Have S-Traps Become Out Of Favour?

P-traps have replaced S-traps for a number of reasons but mainly because P-traps maintain a more reliable seal than S-traps meaning more protection from sewer gases. P-traps are preferred over S-traps for the following reasons;

P-Traps Seal Better

The design of the P-trap with its U-bend is made to collect water and create a seal to stop foul smells and sewer gas from entering the home. Plus the horizontal outlet flow is superior to the vertical outlet flow on a S-trap.

The S-trap still makes a water seal, but it’s less effective than the shape of the P-trap.

S-Traps Are More At Risk Of Trap Siphonage

A sudden change in water pressure is more likely to cause an S-trap to be syphoned of water than a P-trap. It’s all down to the way the traps are laid out. 

As I said above, the water in a S-trap flows through a vertical outlet which makes it easier to break the seal and allow sewer gases to enter. 

P-traps on the other hand, with their horizontal outlet, reduce the risk of siphonage as the water is less likely to change pressure in the same way.

S-Traps Are More Likely To Leak

S-traps are more likely to lose their water seal which can lead to sewer gases escaping into the home. The loss of the water seal can also lead to water leaks which can cause water damage to the home.

P-traps have a more secure water seal which minimises the risk of any water damage due to leaks.

P-Traps Are Better At Doing The Job They Are Designed To Do

P-traps are better at preventing blockages and keeping the pipes flowing properly. The constant water seal protects the home from harmful gases and foul smells.

As well as trapping dirt and debris protecting the pipe work from clogs and blockages.

Do Washing Machines Need A P-Trap?

person looking at washing machine

Your washing machine needs a P-trap installed if it drains into the sewer line. However, if your washing machine drains into a grey water tank, it doesn’t need a P-trap.

This is because grey water tanks don’t handle kitchen or sewer wastewater. Assuming your washing machine drains into a sewer line, it needs a P-trap to;

Prevent Sewer Gas Entering The Home

The P-trap forms a water barrier preventing harmful sewer gases from entering your home through the drain pipe. This creates two major benefits which are;

  1. Stops bad Smells From Entering Your Home
    The P-trap prevents sewer gases from entering your home which can smell foul. But more than this, it prevents any of those gases affecting your water supply. This keeps your water safe to bathe in, cook with and drink.
  2. Prevents Fires
    If there is methane gas present in the sewer, it could potentially cause a fire if it were to enter your home. The P-trap successfully stops these harmful gases from breaking through the water seal and entering your home.

Prevent Harmful Bacteria

The sewer gases can contain harmful bacteria which could be breathed in causing potential illness and disease. These harmful airborne microbes could also enter your home’s water supply leading to more serious infection and illness.

Collect Items Lost In The Washer

Often, items lost in the washing machine from the pockets of your clothes like coins, pins, screws, nails, etc, can get into the drain pipe. These items could potentially cause the drain or sewer pipe to become blocked over time.

It can be quite a large and expensive task removing blockages from sewer pipes. However, many of these items will become trapped in the P-trap. Which is far easier to clear than the main drain or sewer.

How To Install A P-Trap For A Washing Machine

Integrated Washing Machine

Unless you are technically minded and possess good DIY skills, I recommend contacting a plumber to install a P-trap for your washing machine. 

There are a few technical considerations and measurements that need to be taken into account which is why I recommend contacting a plumber.

If you decide to install it yourself, you will need;

  • P-Trap
  • Standpipe
  • Pipe Clips
  • Hacksaw


  1. Ensure The Standpipe Is The Correct Height
    The standpipe needs to be between 30 to 36 inches in height or 18 to 30 inches above the P-trap. Cut the standpipe to the correct size
  2. Fix The Standpipe To The Wall
    Using the pipe clips, secure the standpipe to the wall ensuring the standpipe is the correct height.
  3. Connect The P-Trap To The Standpipe & Waste Pipe
    You now need to join the P-trap to the standpipe at one end and the waste pipe at the other end. Ensure the connections are correctly sealed.
  4. Place The Washing Machine’s Drain Hose Into The Standpipe
    Place the drain hose from the washing machine into the top of the standpipe ensuring that the drain hose doesn’t protrude more than 6 inches into the standpipe (creating an air gap).

You need to ensure that the P-trap is fitted at least 6 to 8 inches above the surface of the ground to prevent sewer gases leaking into your home.

How to Clean A Clogged P-Trap

Once the P-trap has been successfully installed, it is likely to become clogged at some point. This will happen whether you install the P-trap yourself or if it is installed by a plumber. When it does become clogged, you basically have three options to clear it. Which are;

  1. Use A Commercial Drain Cleaner
    There are several commercial drain cleaners available on the market which all claim to clear drains and P-traps. They will remove hair, dirt, soap scum (detergent residue) and many other items that can cause a blockage.
    Simply follow the instructions on the packaging to allow the drain cleaner to do its job. 
  2. Use A Plumber’s Snake
    A plumber’s snake is made from a flexible wire core wound with more metal wire. It is gently pushed into the pipe to dislodge any blockage and clear the P-trap. If the commercial drain cleaner hasn’t worked, a plumber’s snake is often more successful in removing blockages.
  3. Remove The Trap & Clean It
    If all else fails, you may need to remove the P-trap and clean it out by hand. This is the final option because it is the most difficult to implement. You will need to dismantle the P-trap completely and remove it from the standpipe. It can then be cleaned under fast running water and any foreign bodies (like coins, pins, screws, etc) removed.

Before removing the P-trap be aware that it will most likely still contain some water. This water is likely to spill onto the floor as you attempt to remove the P-trap. Place a bowl or bucket underneath it to catch any water before loosening the slip nuts holding the trap in place. 

You should also disconnect the water supply from your washing machine before removing the trap to avoid any potential flooding issues.

Common Signs Of P-Trap Malfunction

Washing Machine

There are a few common tell-tale signs that indicate that the P-trap is malfunctioning. These include;

Foul Smells

The first and most obvious sign that there’s something wrong with your washing machine’s P-trap is bad drain smells coming from your laundry or kitchen.

These foul smells indicate that there’s something not right with the P-trap. It could simply be a case of a dry trap which can be remedied by pouring water into the standpipe.

If this doesn’t solve the issue, it could be a blockage (see above).

Gurgling Sounds

If you can hear a gurgling sound coming from the drain pipe when your washing machine is running, it is also a good sign of a malfunctioning P-trap.

You should try to clear the P-trap using one of the methods above to clear the trap and stop the gurgling sound.

Slow Water Draining From The Washer

If you notice that your washing machine takes far longer than normal to drain after washing or rinsing, it could be another sign of a malfunctioning P-trap.

You should check the lint filter on the washing machine first but if this is clear, there’s a good possibility that the problem is coming from the P-trap.

In some cases, the P-trap could have failed due to a blockage or it may be due to cracks or deterioration or degradation of the plastic trap.

If this is the case, the P-trap will need to be replaced.

How To Prevent The P-Trap From Clogging

As with most things in life, prevention is better than cure. You should regularly run a hot service wash in your machine using a commercial washing machine cleaner or 2 cups of white vinegar. As well as regularly inspecting and cleaning the lint filter on your machine.

You should also pay particular attention to the amount of detergent and fabric softener you add to your machine. Too much of these products can cause clogging or blockages in the P-trap leading to further problems.

You should also regularly inspect the P-trap for any signs of damage or leaks. Plus, if you notice the washer is draining slower than usual consider using a drain cleaner to increase the flow rate.

SEE ALSO: Why Your Washing Machine Makes Your Kitchen Sink Smell (and how to fix it)

Frequently Asked Questions

What does the P-trap do on a washing machine?

The P-trap on a washing machine creates a water seal preventing any foul smells or potentially harmful sewer gases from entering your home through the washing machine’s drain pipe.

Does a washing machine drain need a P-trap?

If the washing machine is connected to a sewer or drain system, it needs a P-trap. The only time a washing machine doesn’t need a P-trap installed is if the washing machine is connected to a grey water tank. This is because grey water tanks do not handle kitchen or sewer wastewater.

Should there always be water in the P-trap?

Yes, there should always be water in the P-trap because the water creates a barrier that foul smells and sewer gases cannot penetrate. Thus keeping them out of your home.

How do I know if my P-trap is clogged?

You will know if the P-trap is clogged because the water from your washing machine will take longer to drain away, you will hear a gurgling sound and there will possibly be a foul smell in the room.

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