Does a Shower Drain Need a Trap? Benefits and Installation

A shower drain trap, also called a shower P-trap, shower U-trap, shower S-trap, shower pee trap, or simply the shower trap, is a U-shaped section of the drain pipe that keeps sewer gases from rising into your bathroom. You may not see it since it’s buried under the floor but it’s a legal requirement for all bathrooms. If your bathroom doesn’t have it or it’s clogged, you are endangering your household. 

All shower drains are required by law to have P-traps. This requirement is for all household fixtures which connect to the main plumbing system. The P-trap is a curved pipes used in shower drains, sinks and toilets to keep out sewer gases and animals from entering the home through the drain pipes.

Does a shower drain need a trap

Does a shower drain need a trap?

A P-trap for the shower drain is a legal requirement in the United States and many other countries around the world. The shower pee trap helps keep awful gases away from the shower. Besides that, it keeps animals and other items from going back up into the home given its shape and design. 

Some sewer gases such as methane and hydrogen sulfide are highly flammable. Other gases such as carbon monoxide reduce the amount of oxygen in the home and are poisonous which easily leads to suffocation. Without a P-trap in your shower drain, these gases will fill the home and cause damage and even death. 

Each shower in the building requires its own separate P-trap to work properly. This means that even bathrooms next to each other aren’t allowed to share a P-trap. 

How does a shower drain trap work?

The shower drain P-trap works in a similar way to the P-trap of your sink or toilet which is by preventing gases and objects from coming back up while allowing the same to flow down the drain. 

The anatomy of a shower drain trap is made of the following parts:

The inlet

The inlet of your P-trap is where water enters the trap. When you take a shower, for example, the water goes into the drain hole on the floor of the bathroom which is the inlet to the P-trap. 

The downward bent

The downward bent of the P-trap assumes a ‘U’ shape or a ‘P’ shape (if the outlet is included in the shape). This part holds an amount of water which is just enough to provide an airtight seal between the sewer side and the inlet side of the P-trap. When water enters the P-trap’s inlet, it displaces the water in the trap and flows down the drain. 

When sewer gases come up from the outlet side of the P-trap, they won’t pass through the water and into your home hence the ‘trap’ part of the name. It traps gases and even animals which can’t go through the water and the bend to reach your home. 

The downward bent has a cleanout which is located in the middle of the ‘U’ section of the trap and is used to drain the water in the P-trap during cleaning or repair. 

The outlet

The outlet of the P-trap is where water exits the trap and goes into the sewer line to be transported into the main sewer. 

The shape of the shower drain trap often determines the name given to it. The P-trap has a ‘P’ shape as described above. While the U-trap is just another name for a P-trap, the S-trap has an ‘S’ shape on a vertical plane where water flows downwards rather than sideways. All drain traps, however, work the same way as described above. 

Shower drain P-trap depth

The depth of a shower drain P-trap is not standard but rather determined by the size of the pipes or the fittings used. As long as the drain trap is securely covered by the flooring material and it works fine, don’t worry about its depth. 

How far can P-trap be from shower drain?

The maximum distance between the shower drain and the P-trap is 5 feet. However, the P-trap should be as close to the shower drain inlet as possible. You should even have the P-trap directly under the shower drain for the best results. 

With P-traps, the further it is from the shower drain, the higher the chances of awful smells entering the home. Some may not even be from the sewer line but the smell of decaying dirt in the section between the drain and the P-trap. 

Shower P trap dimensions

The minimum size of a shower drain P-trap is 2 inches which is the diameter of the pipe used to make the trap. This is different from other drain traps such as the bathroom sink (1¼ inches) and the kitchen sink (1½ inches)

When you look at a shower trap diagram from the side, the size of the trap is the thickness of the pipe that curves to make the P-trap. 

How does a shower drain work?

Water from the shower enters the shower drain opening on the floor of the bathroom and either directly enters the shower P trap, or enters a horizontal pipe before the P-trap. Given that there is water in the drain P-trap, the incoming water displaces the water in the P-trap and leads it to the main sewer or septic tank. 

If you’re wondering where the shower drain leads to, then the answer is that it leads to the main sewer line. Basically, the shower drain and the toilet drain lead to the same destination which is the sewer. 

How to clean a shower P-trap

There are two methods to clean the shower drain as follows:

1. By removing the P-trap

Locate the position of the P-trap on the shower floor then remove the flooring above it. Place a bucket underneath it to avoid spills then disconnect it from the outlet pipe. If there are any nuts and screws, undo them to free the P-trap. Soak it in a mixture of white vinegar and baking soda for 15 minutes then brush it before rinsing with clean water. Reinstall it back by connecting it to the outlet then returning the floor covering and the drain cover. 

2. By not removing the P-trap

Pour half a cup of baking soda in the shower drain then add a cup of white vinegar. Cover the shower drain for 15 minutes then flush it with hot water. 

You should also clean the shower drain using a plumber’s snake or a drain cleaner. Avoid harsh drain cleaners which may damage the plumbing of your bathroom. Instead, go for enzyme-based drain cleaners which slowly eat away the dirt without harming your pipes. 

These cleaning procedures should be carried out each time you clean the shower stall to keep the bathroom in good condition at all times.

How to install a shower drain trap

If your bathroom doesn’t have a P-trap or you need to replace the current one, you will need to install one. For new bathrooms, locate the P trap in the center of the shower and as close to the shower drain as possible. The steps to installing a shower drain trap are as follows:

1. Cut the subfloor around the drain

Use an electric saw to cut the subfloor around the drain of your bathroom. Cut it such that it’s 12 by 12 inches. The section cut should be above the floor boards to allow you to nail back the subfloor when you’re done. 

Don’t cut too deeply such you damage the floor and the drain pipes. 

2. Create space around the drain

Use a prybar to open up the floor under the square cutout. If you’re replacing the P-trap, cut the drain pipe right after the old P-trap. If you’re installing it, cut the drain pipe the same distance from the drain as the size of the P-trap. Basically, measure the length of the P-trap from one end of the ‘U’ shape to the other then use the same distance between the drain and where you cut the drain pipe. 

3. Connect the P-trap to the drain pipe

Connect the P-trap to the horizontal section of the drain pipe using a connector and glue. Make sure the other end of the P-trap faces upwards and is directly under the shower drain. 

4. Reinstall the floor

Put back the square subfloor cutout into the floor then measure the distance between the subfloor and the P-trap. Cut a pipe with this length and with the same diameter as the end of the P-trap. Reattach the shower drain and the drain cover. 

After an hour (for the glue to dry up), you can use your shower as usual. 

What are vent pipes?

Vent pipes are pipes connected to the sewer line in the home where gases from the sewer escape to the air rather than entering the home. 

Vent pipes work hand in hand with P-traps to keep sewer gases away from the house. The P-traps in toilets, sinks, and shower drains prevent the gases from getting back into the house while the vent pipes allow the gases to escape to the air outdoors. This also balances the air pressure within the pipes. 

Shower trap problems and fixes

P-trap failure can occur when the trap doesn’t perform its functions. Some of the common shower drain P-trap problems include the following:


Solid items, calcium build up and others may cause clogs in the shower drain. You should unclog the drain with a plunger or by pouring in it a cup of baking soda followed by 2 cups of white vinegar. Rinse with hot water afterward. 

A plumber’s snake is also great at unclogging a shower drain P-trap. Enzyme-based drain cleaners can also be used to unclog the shower drain. 

Prevent clogs in the shower drain by installing a drain cover that keeps out larger objects and dirt from entering and clogging the drain. 

Drying up

A P-trap is only good when it has water in it. When you don’t use the bathroom for long such that the water in the P-trap evaporates, sewer gases will find their way into the house leading to an unpleasant experience. 

Make sure you pour water into the shower drain at least once every week to maintain the airtight seal. 

Another cause for a dry shower drain P-trap is a leak. For this one, replace or repair it with glue meant for pipes. 

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