What Causes Sewage Smell in the Bathroom (and How to Prevent It)

Disclaimer: This post may contain affiliate links, meaning we get a small commission if you make a purchase through our links, at no cost to you. For more information, please visit our Disclaimer Page.

In the house I grew up in, there were two bathrooms. The second bathroom was built rather late, and it had newer equipment and plumbing system and everything. But, for some reason, there was a constant sewage odor coming from it.

Still, we were sure it was something wrong with our old bathroom on the first floor since it seemed obvious and logical. It was built in the '80s; the plumbing system might have been a little outdated, and owing to gravity, all water, gases, and odors would travel down to the first floor.

Oh, how wrong we were. If only we considered other possibilities, I could have gotten rid of my pulsating headaches sooner rather than later.

Sewage Smell in Bathroom

You read that right, sewage odors in your home can prove harmful for your health, as well as a safety hazard – It is actually good that I only dealt with a headache. 

Sewage odor is a mixture of gases, but the main components (the ones responsible for the awful smell) are actually methane and sulfur. Methane in large quantities is flammable.

Furthermore, inhaling methane for a very long time can cause pneumonitis – a lung inflammation that is usually accompanied by difficulty breathing and coughing. Methane can also cause frequent loss of consciousness.

So, with just these few mentioned problems that come with smelly bathroom gases, I think it’s clear that this issue mustn’t be ignored. I will try to present you with several possible reasons for why there is a sewage smell, and how it can be resolved and prevented.

After all, nobody likes foul-smelling bathrooms, right?

Plumbing System 101

I feel like many people automatically assume that the foul sewage odor only comes from the toilet. So, they get their cleaning gear on, use strong chemical cleaners to scrub away at the toilet bowl, and after a while (i.e. after the cleaner’s fragrance evaporates), the pungent smell is still present.

Before you work yourself to exhaustion with vigorously cleaning your bathroom, it might be better to first become familiar with some basics about your home plumbing system. No need to worry, this will be quick and simple!


So, you might’ve noticed that there is a P or U shaped pipe under your bathroom sink. This pipe is called a P-trap.

One end of the trap runs from the sink to the sewer system, and the other end (called the plumbing stack pipe) leads all the way up through the roof. All the gases and smells evaporate through the plumbing stack and the fresh air is let in.

Moreover, the same system is set up beneath your shower and the tub.

This P-trap is essential in keeping your bathroom smelling fresh. It collects a small amount of (waste)water after each sink use. The wastewater goes through the inlet pipe of the P-trap and disturbs the still water in the trap.

The still water acts as an impenetrable plug that forbids any foul odors from going back to the bathroom.

See? This wasn’t so hard. Now, let’s continue with the reasons why your bathroom is stinky.

Cause of Sewer Smell in the Bathroom

There are several reasons for the sewage smell, but I will present you with the six most common culprits. They vary in seriousness – some you can resolve yourself, and to resolve the others, you will need the help of a professional.

Cause #1: the water in the P-trap has evaporated

This is the usual problem with the bathrooms that are not used often.

As I have mentioned previously, there is always water in the P-trap, and it prevents the gases from going back. Therefore, if you’re not using that particular bathroom, the water will evaporate, and there will be no trap for the bad smell.

This is the simplest problem to fix. Just pour water down the sink, tub or a shower, so the P-trap refills with water. Yes, it’s that easy.

Cause #2: the drain becomes clogged

You may think that you’re being very careful about what goes down your drain. But it may come as a surprise to you, but even soap leaves some debris behind.

So if you don’t clean your drains regularly, that soap debris will eventually clog the drain.

Besides soap debris, when hair decomposes, it will also leave a trail of debris behind. So, over time, this will cause a smelly bathroom fixture (a sink, or a shower/tub).

Oftentimes, to remove the clog a plunger will do the job. If not, then your plumber will have sufficient tools (a drain snake or a hydro-jetter) to successfully remove the clog.

Cause #3: broken toilet seal

Just like the water in the P-trap, there is always water at the bottom of the toilet seat – it has the same purpose, to act as a barrier from the odors. But, if you see that the toilet doesn’t fill up with the water after use, the problem might be with the seal.

The seal is actually a layer of caulk that’s used around the base of the toilet to ensure no leaks. Maybe the caulk has been worn down and small cracks have appeared.

The water then seeps through the cracks and doesn’t dry, and thus creates a perfect environment for bacteria growth.

You will need to add another bead of caulk around the base of the toilet. It is also recommended to caulk the bolt holes because the smell can escape through there.

There is another component to the toilet seal that keeps your toilet bowl from wobbling and prevents water seepage.

This component is called a wax ring. The wax ring can also become damaged, or worn out, which causes the bad smell.

To resolve this, you will need to replace the wax ring.

Cause #4: blocked roof vent

It can be tricky to distinguish between a blocked drain line and a blocked roof vent. Because of this, it is advised you call up your plumber before attempting anything yourself.

But, if you have determined that it is the blocked roof vent, then let’s see what the problem is.

The roof vent ensures all the foul gases evaporate into the sky, and the fresh air constantly travels through the pipes. But, due to weather, or some curious birds in need of a home, your roof vent can become clogged.


If there was a wicked snowstorm, you may need to carefully climb up to the roof (or call a professional) and clean the pipe from snow, or in case of rainstorm, remove the leaves or other debris from the pipe.

Another frequent scenario is that a bird or another small animal has made a nest on top of your pipe. It’s a rather convenient place for a nest for them, but not so much for you.

You will need to remove the nest, but please don’t be cruel and destroy it. Maybe you can place it on a tree or the ground so they can move somewhere else.

Cause #5: improperly installed, cut or cracked vent pipe

If your vent pipe (that leads to the roof vent) hasn't been properly installed or has a crack, it can cause your bathroom to smell bad.

This is a more serious problem which you won't be able to resolve yourself. This is mainly due to the fact that the vent pipes are usually located inside a wall.

It can happen that the contractor that built your house vented the pipes to the wrong place. In this case, the correction of the vent pipe installment will be needed.

If your plumber determined that the pipes are properly installed, but there’s a crack on the pipe, he will use a tool known as a smoke-machine to pin-point the crack and fix it.

Don’t worry – the smoke in the smoke-machine is non-toxic and odorless.

Cause #6: Buildup in your sink’s overflow

Normally, your bathroom sink will have an overflow mechanism. This is a mechanism responsible for preventing your sink from – you guessed it – overflowing!

In other words, this little hole close to the rim of the sink serves as a drainage system if the water level rises to it. And, like anything else that is near water, it is no stranger to grime and debris build-up.

The solution for this issue can be found in your kitchen! You can clean the mechanism with a small bottle brush, or use a turkey blaster with a white vinegar-water solution.

Mix a half cup of water with half a cup of white distilled vinegar and pour it down the mechanism to clean it. Other - slightly less safe - way is to use chlorine bleach instead of vinegar. Be very careful when using chlorine bleach. Make sure to read all of the instructions and warnings!

best drain cleaners

How to Prevent It

There are two basic plans to follow – regularly clean your bathroom and have your plumbing system inspected by professionals from time to time. It is always better to be safe than sorry.

  • If you rarely use, say, the guest bathroom in your house, make it a habit to pour some water in every bathroom fixture, every two weeks or so.
  • To prevent clogged drains, pour half a cup of baking soda down every drain, and follow it with a cup of white vinegar. This will remove any small debris and grime build-up, safely and successfully.
  • For every other problem (such as clogged and cracked vent and drain pipes) call up your plumber every couple of months for a thorough inspection.

Conclusion: Sewage Smell in Bathroom

Although some may think that the sewage smell in the bathroom will be resolved with air-fresheners. Or, you can cope with it by opening the window or turning on the air vent.

Since we have seen that the foul smell can be a safety hazard and a health issue, it is highly recommended to find the source and deal with it appropriately. 

I hope this article has helped you in determining the problem, provided you with common solutions and ways to prevent it.