Does It Matter Which Way P-Trap Goes?

P-Trap got its name from the two joint pipes at 90 degrees, which makes the shape of a “P” letter, and connects the drain to the waste sewage lines. It contains water inside the pipe which acts as a sealing to prevent the toxic sewer gases in the bathroom through drains.

Does It Matter Which Way P-Trap Goes? The correct direction and installation of the P-Trap can prevent clogging and sewer gases from releasing into the bathroom through the open drains. You can install backward P-Traps with some modifications in the connections, but it will reduce the water flow and increase the chances of clogging and toxic gas leakage.

The water flowing from the plumbing fixtures into the sewage pipes through it and reversing the direction of the P-Trap can cause different problems. Professionals do not recommend changing its direction and installing it in a proper way as it is designed.

Which way should P-Trap go?

It is designed to connect the left side with the plumbing fixture drain, and the right side goes to the sewer pipe. Its correct installation is essential for the proper functioning and sealing of the toxic gases in the sewage.

The water standing at its bottom restricts the gases from leaking in this place through the open drains in the plumbing fixtures and provides better water flow to prevent clogging in drain pipes.

You can also understand the correct way to install it, as the straighter end will connect to the sewage pipe, and the curved side attaches to the shower, sink, or bathtub drain.

There are different material types of P-Traps, such as brass and polypropylene, which work the same way but differ in durability.

The connections and the arrangement of both material traps are the same. However, the brass pipes and components can corrode over time due to moisture. 

Why does it matter which way the P Trap goes?

The correct installation and placement of the drain pipes are important because improper installation can cause plumbing problems, and you have to face worse situations.

Its bend seal creates water sealing of 1.5 to 2 inches to prevent the harmful gases from the sewage lines into the drain pipes and bathroom.

Clogging can occur no matter which direction you install the trap, but the backward P-trap causes more chances of clogging than the correct direction instructed by professionals.

The trap distance increases from 4 inches if you install the backward P trap, and it cannot function properly for the long term and cause blockage.

Proper ventilation is essential because it can cause the sewer gases to leak into the nearby pipes, and you hear sounds from the empty traps. 

Can you flip the direction of the P-Trap?

You can flip or reverse the direction of the P-Trap when you do not have an accurate slope under the drain, and traps are low. It needs some modification in the installation and plumbing connections, or you can lower the drains in the walls, but it can take more time and hard work.

You can reverse its direction using a threaded connection and install it after flipping its direction. For example, you can attach 1.5 inch ID to a 1.5 inch OD using a threaded connection, and it can cover a 2-inch gap that can occur in the trap.

You can also use a metal P-Trap, but using it will allow ½ inch grip on the other loose end of the trap. This is not a better option when reversing its direction.

Another option is to flip the P-Trap, such as by creating a new PVC or ABS P-trap to make perfect connections and ignore installing the traditional trap kits. 

What happens if you install a P-Trap backward?

Changing the direction has many disadvantages; some of those cons are listed here.

Sewer gases release

The sewer gases inside the sewage lines contain toxic fumes and are dangerous to inhale. Reversing its direction can cause the trap to work inefficiently and leads to the leakage of these harmful gases in the bathroom.

The water inside the U-shaped pipe cannot stay there, and it causes the sewer gases to release through the drain into this place, as the water inside the trap holds the gases from releasing.

The plumbing regulation requires that there should be a P trap for every open drain that draws the wastewater from the plumbing fixtures to the sewage lines to prevent the sewer gases from leakage. 


It causes clogging because the plumbing codes require a minimum trap seal of 2 inches, and a maximum trap seal can be 4 inches.

However, the backward or reverse trap creates a seal of more than 4 inches, which causes the blockage inside the pipes over time.

The hair, soap scum, and other foreign particles can get trapped inside it, and the chances of clogging increase.

It can also cause flooding in the drain pipes if you do not install it as the plumbing codes require them to install.

It is designed to install in the recommended position without reversing its direction to avoid clogging and leakage. Still, you can install a backward trap with some modifications to prevent the blockage in pipes.

Water flow reduces

The backward P-Traps can reduce the velocity of water flowing down the drains into the sewage pipes, as they are designed to self-clean the drain pipes by the water flow.

The P-Trap installed correctly will allow the water flow to accelerate when it hits the pipe bottom and flows over the P-shaped hill down the drains.

The backward installation causes an increased distance of the P trap, and the water flow decrease because of poor acceleration, which leads the debris to clog the drain pipes.

Does the P-Trap need to be lower than the drain?

Professionals recommend that it cannot be installed higher to the drain entry, and the trap distance can be measured from the top trap dip to the highest point on the inside curve.

This vertical distance will tell us about the maximum liquid the trap can hold inside it. The P trap under the sink cannot be higher than the drains.

The same goes for the exit drain in the wall because the output level needs to be higher than the input level for proper water flow and better sealing of sewer gases. 

Building codes do not require any restrict minimum distance from the drains. Therefore, the horizontal connection of the trap arm from the vent pipe to the P trap can have a slope of ¼ inch per foot towards the skin drain.

However, International Residential Code recommends that the maximum distance of the sink drain to it can be 24 inches vertically.

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