Does a Bathtub Need a P-Trap

Does A Bathtub Need A P-trap?

Let me quickly answer your question: “Does a bathtub need a p-trap?”. It is necessary to install a p-trap in the bathtub, along with sinks and showers.

A p-trap consists of two 90-degree joints connected by a horizontal overflow pipe. The pieces come together to look like the letter “p”, hence the name p-trap. 

It is used for water drainage fixtures in the plumbing system.  

What Does A P-trap Do?

bathtub ptrap

The downward section of the curve creates a trap where water accumulates, which is known as the trap seal. When wastewater enters the trap from the fixture, it pushes the old water and any waste out of the p-trap and replaces it with new water.

This repeated process of flushing the old water out and replacing it with new is important and serves many purposes:

Why Is A P-Trap Important?

It traps debris and helps prevent clogs

Once debris and waste go down the bathtub drain, the p-trap serves to collect this gunk (such as hair) before it clogs the plumbing system. Once you notice the drainage is slow, you know that the p-trap has accumulated debris and will need to be cleared out.

Prevents harmful gases from escaping

Many harmful gases could potentially move up through the drainpipes if there were no p-traps. Water in these traps prevents the gas fumes from entering the bathroom.

Some of these toxic fumes that could escape through the drain pipes include methane gas, hydrogen sulfide, sulfur dioxide, ammonia, and nitrogen oxide. Not only would these gases be unappealing to smell, but they’re also hazardous to one’s health after prolonged exposure.

Some of the issues that could develop include headache, fatigue, light-headedness, and memory issues to name a few.

Catch basin for articles that fall down the drain

A p-trap is also very useful in the event you accidentally drop something of value down the drain. The shape of the trap helps catch the article before it’s lost forever.

Prevents rodents and vermin from entering the bathroom

Another useful reason for a p-trap is to help prevent rodents such as mice and rats and other vermin such as insects from climbing through the drainage system and potentially entering your bathroom.

So, Does A Bathtub Need A P-trap?

Yes, a bathtub does require a p-trap. P-traps are also necessary for a kitchen sink, a bathroom sink, a shower drain etc.

Plumbing codes require p-traps on all wastewater lines. A bathtub p-trap is part of the bathtub drain and serves the same function as it would for any sink or area with a drain line.

The importance of having a p-trap for the bathtub is the same as all the important factors listed above.

With that being said, homeowners may assume that a bathtub does not have a p-trap as it is often found hidden under the foundation and out of view. With a sink, you can easily open the cabinet doors to see the p-trap placed underneath the countertop.

Since a bathtub sits on the ground, the p-trap is pre-installed by the plumber before the flooring is laid. It is normally placed in the concrete or between floor joists.

Potential Issues With A P-trap

1. Infrequent Use: One of the most common issues found with a p-trap is the evaporation of the water in the trap. This is caused by leaving the plumbing fixture unused for long periods of time.

When the fixture is left without being used, this causes the water in the trap to evaporate. When the water evaporates, this leads to unwanted sewer gases dispersing up through the sewer line into the bathroom.

2. Negative Pressure: There is the possibility that the water in the trap is siphoned from other pipes suction. This happens when there is enough negative pressure to draw the water up and out of the trap.

If this happens, the p-trap could be left without water. Again, this would lead to sewer gases coming back up into the home.  

3. Positive Pressure: Alternatively, when there is positive pressure, this gives the opportunity for wastewater to back up into the drainage system. In this circumstance, the water would build up and move past the p-trap into the fixture itself.

Even in the event that the wastewater does not come up through the fixture, air bubbles could be forced up which again is a cause for gases to come up into the bathroom.

4. Powerful Winds: Strong winds can also cause potential issues with p-traps. When there is significant wind, it can cause the water in the trap to rise and fall.

This could cause backpressure and the water to siphon, impacting the effectiveness of a p-trap.

Other Important Information To Note

  • P-traps are plumbing pipes made of pvc or steel that are generally in-expensive. They can be bought at your local home building store, such as Home Depot. Given that they are fairly easy to install, this can be a DIY job that can be tackled by most homeowners for their next home improvement project.
  • Drains without proper venting will be noticeably slower to empty. One other tell-tale sign is a “glug-glug” noise coming from the drains. This would indicate improper ventilation. In this case, it would be wise to have a plumber install new pipes throughout the home.
  • For homes that are used less frequently such as cottages or seasonal homes, it is important to use a trap seal primer. The primer is a water seal that works to produce water inside the P-trap. The purpose of this seal is so that the p-trap does not dry up when not in use and pass sewer gases into the home.
  • Over time a P-trap can wear out just like anything else in your home. Make sure to stay aware of any sign of wear and tear and replace it as needed.
  • If you have any odd smells coming from your drains, remember the function of the P-trap. If you’re not comfortable handling the situation yourself, make sure to call in a plumber or a local handyman to avoid any potential health risks.


P-traps are a very important function within your bathtub drain system and should always be incorporated. These mechanisms are not only integral for maintaining the proper functioning of the fixtures but also for preventing toxic gases from entering your home, keeping you and your loved ones healthy and safe.