Why There is a U-Shaped Pipe Under Your Sink


The U-shaped pipe under your sink is known as a “P-trap” or “sink trap.” Its primary purpose is to prevent foul odors and harmful sewer gases from entering your home through the kitchen or bathroom drain.

The P-trap achieves this through a simple design:

  • Water Seal: The U-shape of the trap holds a small amount of water in it at all times. This standing water creates a seal that prevents gases from the sewer or septic system from rising back up through the drain and into your living space.
  • Preventing Odors: As wastewater flows down the drain and into the P-trap, the water in the trap prevents any odors or gases from escaping into your kitchen or bathroom. Instead, these gases remain trapped in the sewer or septic system.
  • Trap for Debris: The P-trap can also catch and hold small debris or objects that accidentally fall into the sink drain, preventing them from entering and potentially clogging the larger plumbing system.

Why You Need a P-trap


To understand why a P-trap is necessary, let us first look at how your home plumbing is designed.

Each of the fixtures in your house that is connected to the drainage system has its own drain line. Each of those drain lines are then connected to the main waste/soil stack which will then drain all the waste to the sewer line buried in your yard.

Your private sewer lateral will then be connected to the municipal sewer line at the street. For those on a septic system, the sewer line will drain to the septic tank.

Needless to say, there will be sewer gases inside the drainage system that will need to be removed; otherwise they will force themselves into the house regardless of whether you have a P-trap or not.

And that is where a plumbing vent comes in. A plumbing vent is a vertical pipe connected to the waste/soil stack and which runs through the roof of the house.

Plumbing vents have 2 main functions:

  • They allow sewer gases to exit the plumbing system.
  • They introduce air into the drainage system. The air helps the fixtures to drain fast and toilets to flushing better.

Sewer gases, just like many things in life will follow the path with the least resistance. Without a P-trap, there is no way sewer gases will flow up the vent stack while there is any easy way through the drain lines.

The water inside the P-traps however creates a barrier, forcing sewer gases up through the plumbing vent.

A P-trap’s shape also allows it to trap solids which have the potential to unclog the drain line farther away where unclogging would be way harder.

Whenever you notice that your sink is clogged or draining slowly, the problem is usually a blocked P-trap. Luckily, unclogging a P-trap is way easier than having a clog 10 feet away from the sink drain.

P-trap Problems

For the P-trap to work as designed there must always be water inside it. If a certain fixture has not been used for a while (like the guest bathroom sink), the water in the P-trap will evaporate leaving the trap empty.

When that happens, sewer gases will enter the bathroom without and restrictions and you will without a doubt have a sewage smell in the bathroom.

It is for this reason that you should pour some water down your drains every time you come back home from a vacation and flush all toilets. Do not forget the washing machine drain as well.

The little water will replace the evaporated water in the P-trap and thus ensuring that sewer gases cannot enter your house.

Plumbing vents can also get clogged. That is usually caused by tree leaves, dirt, dead birds or rodents, tennis balls or even snow.

With a clogged vent, sewer gases cannot exit the drainage system and air cannot be introduced in.

One of the main signs of a clogged plumbing vent is gurgling drains. If you flush the toilet and you notice the bathtub/shower drain gurgling, you most likely have a clogged vent.

What happens is that since air cannot get inside the drainage system, negative air pressure builds up inside the drain lines. As a result, the system will try to pull in air from the surrounding.

In a bid to day that, the water inside the P-traps will be siphoned in. the gurgling sound is usually as a result of the water in the P-trap being siphoned inside the drain line.

The other signs of a clogged vent are:

  • Slow draining fixtures
  • Bubbling toilet when flushed
  • Weak flushing toilet
  • Sewer smell in the house

Another problem with P-traps is leaks. If you notice your P-trap leaking (or when there is water pooling under the sink), the connections could be loose.

A P-trap has 2 connections. If it is leaking, grab a wrench and gently tighten the 2 connections until the leak has stopped. If the leak persists you will need to replace the P-trap.

How to Clean a P-trap

As I mentioned, the shape of the P-trap makes it very easy to trap solids. If there is an accumulation of solids inside the P-trap, you will end up with a slow draining fixture or even one with standing water (cannot drain).

Apart from that, a biofilm can be created inside the P-trap which results in a nasty smell. As such, you should always clean your P-trap frequently to prevent these problems.

This is how to clean a P-trap:

  • Pour 1 cup of baking soda down the drain.
  • Slowly add another cup of vinegar.
  • Wait for about 15 minutes for the solution to do what it does best.
  • Blast hot water down the drain.

The baking soda and vinegar will break down any solids inside the P-trap while the hot water will melt them further and flush them down the drain line.

If you already have a slow draining or sink with standing water, removing and cleaning the P-trap is another great idea. Here is how to do it:

  • Place a small bucket or pan under the P-trap. That will help you drain the water already inside the P-trap instead of spilling it on the floor.
  • Disconnect the P-trap. You can use your hands only but if the connections are super tight use a wrench.
  • Clean the P-trap with hot water then connect it back.
  • Check if there are leaks by filling the sink with water then draining it.

Do You Have a P-trap or S-Trap?

Closely resembling the P-trap is the S-trap. S-traps are only found in old houses since they are currently never installed and are actually illegal. Read more on why they are illegal here.

The difference between an S-trap and a P-trap is that a P-trap has only one U-bend and is connected to a horizontal drain line while an S-trap has two U-bends and is connected to a vertical drain line.

The reason S-traps are illegal is because their design does not allow air to get into the trap during draining, resulting in creation of negative air pressure inside the drain line.

Negative air pressure causes the water inside the trap to be siphoned out. With no water inside the trap, sewer gases enter the house without any restriction.

If you have an S-trap under your sink you can bring in a plumber to replace it with a P-trap. Alternatively, you can install an air-admittance valve which fits perfectly under the sink.

And basically that is all regarding the U-bend under your sink. I hope you found this post to be helpful.

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