What is a P-Trap and Why is it Necessary?

A P-trap is a bend in a plumbing line that drains to a sewer line. It is commonly found beneath sinks and showers and can be made of PVC or metal. The P-trap contains water at all times, serving as a liquid barrier that prevents sewer gases from coming up through the plumbing line and into your home. Although the bend in the P-trap is subject to clogs, it is an essential piece of plumbing. Without a P-trap, foul-smelling sewer gas would fill your home.

What is a P-trap?

Why is it Called a P-Trap?

The Name “P-trap” comes from the fact that a plumbing line with a bend in it looks like a letter ‘P’ that has been tipped onto its side. This P-shaped bend in the line “traps” water to form a vapor barrier between your sewer line and the interior of your home.

Plumbing beneath bathroom sink with letter P imposed on image to highlight the P-trap
P-traps got their name because they resemble a sideways letter ‘P’.

It may sound bad to have water standing in your plumbing line, but it isn’t a cause for alarm. The force of your faucet or showerhead will flush your waste water out and into the sewer line. The water that remains in the P-trap is the final bit of clean water from your tap. Unless there is a clog, waste does not remain in the P-trap.

What Does a P-Trap Do?

The primary function of a P-trap is to prevent sewer gas from coming up through your drain pipes and entering the air in your home. Sewer gas is extremely foul-smelling and can be dangerous to breathe. The clean water that sits in the U-shaped bend of the P-trap is a vapor barrier. Sewer gas can’t pass through the water to enter your bathroom or kitchen.

Closeup of bathroom sink plumbing with highlighting and labeling to illustrate how the P-trap works.
P-traps hold water. This water acts a barrier to stop bad-smelling sewer gas from flowing up the pipe and into your home.

An extra benefit of the P-trap is that it often stops items dropped down the drain from falling directly into the sewer line. If you drop a piece of jewelry into your sink, it’s likely to settle to the bottom of your P-trap. You can quickly disassemble the P-trap and retrieve a lost ring. P-traps are so useful that most building codes require that all kitchen and bathroom sinks and bathtubs and showers have a P-trap built into the plumbing line.

How Do You Know If Something is Wrong With Your P-Trap?

The most common sign of trouble with your P-trap is a slow-draining sink or shower. If your sink or bathtub backs up, there may be something wrong with your P-trap.

  • Your sink or shower/bathtub is slow to drain.
  • There is a bad odor coming up from your sink or shower drain, even when it appears clean.
  • Water is collecting or leaking beneath your sink.

Another common issue with P-traps is a foul odor. If your sink or shower releases a smell, then it’s worth taking a closer look to find out what is the matter. If water is pooling beneath your sink, this may also be a sign of P-trap trouble.

Common P-Trap Problems

Clogs are a frequent P-trap problem. Because heavy items, gunk, and sediment tend to collect in the bottom of P-traps, they can become clogged. Large items that enter the drain can also become lodged in the P-trap, contributing to a clog. It is also common for water to leak from the joints where the bent pipe of the P-trap joins the straight pipes it is connected to. This is often a symptom of a clog. Water that cannot drain properly seeks another exit, which leads to a leaking P-trap.

  • Drain clogs.
  • Leaks at the P-trap joint.
  • Venting sewer gas due to water evaporation.

The final common P-trap affliction is venting sewer gas. If a P-trap is left unused for a long period of time, the water within it may evaporate. This removes the water seal and allows sewer gas to flow up the drain. Running the faucet can easily fix this problem.

How Do You Unclog a P-Trap?

Man using an orange drain auger to unclog a bathroom sink.
Using a drain snake is the most reliable way to unclog a P-trap.

There’s more than one way to unclog a P-trap. The first and simplest is to try a drain clog dissolver product, such as this one. Most minor clogs can be resolved in just a few minutes with one of these products. If your drain clog is more serious, it may be because something that cannot be easily dissolved is lodged in the P-trap. In these cases, this drain snake (also called an auger) can extract the clog.

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If your P-trap has an extreme clog, it may need to be disassembled. To do this, lay towels in the area beneath the drain pipes and loosen the fittings that hold the P-trap in place. Then, remove the P-trap itself and clean it out with water and a rag. If you do not feel comfortable disassembling a plumbing fixture to clean it, contact a plumbing professional.

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What is the Difference Between an S-trap and a P-trap?

The only difference between a P-trap and an S-trap is the shape. The P-trap resembles a sideways ‘P’ while an S-trap looks like a sideways ‘S.’ P-traps are a more modern form of plumbing. Because they have fewer extreme bends, they are less likely to clog. Both traps are common in homes to this day.

  • P-traps resemble a sideways ‘P’.
  • S-traps resemble a sideways ‘S’.
  • P-traps are more modern and are less likely to clog than S-traps.
  • Both traps perform the same purpose in your plumbing.

Both the S-trap and P-trap serve the same function—they retain water in the drainpipe to serve as a water seal. This prevents sewer gas from flowing back up the drain line.

What is the P-trap Used For?

A P-trap is required to be installed in most sink, shower, and bathtub drain lines by law. A P-trap is essential because it holds clean water in the drain line. This water seal stops horrible-smelling sewer gas from coming back up the drain line and into your home. A bonus of P-traps is that they are likely to catch heavy objects (such as jewelry), which stops these valuables from flowing directly into the sewer line.

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