Sewer Smell in the Basement? Why and What to Do

Why Does My Basement Smell Like Sewage?

A sewage smell means the presence of methane and/or hydrogen sulfide gas which also smells like rotten eggs. In homes, that is usually caused by a problem in the drain-waste-vent system

A sewer smell in the basement is a sign that the floor drain trap has dried up, the ejector pit is not sealed properly, the sewer cleanout plug is missing, the toilet wax ring is not sealing or there are improperly vented fixtures. In rare occasions, it could also mean that the sewer line is damaged.


To get rid of the sewer smell in the basement, pour about a gallon of water down the floor drain and other rarely used drains. Make sure that the sewer cleanout plug is properly installed, the ejector pit is sealed properly and that the toilet wax ring is not leaking. If all those fail, you will need to have the sewer line inspected.

If there is a sewage smell in your basement especially after it rains, it is due to the fact that the rain causes changes in atmospheric pressure causing the air to be dense. When that happens, the sewer gases do not flow out through the plumbing vent but instead forces their way out through basement floor drains.

If you have an unused bathroom that smells like a sewer, the water in the sink or toilet trap has dried up. When that happens, sewer gases flow out through the drain into the house instead of flowing out through the plumbing vent.

In electricity, we say that electric current flows through the path with the least resistance. The same also applies to gases.

Sewer gases will easily flow out of the drain line through the basement floor drain instead of moving up through the plumbing vent.

Let us now look in more details why you have a sewer smell in your basement and what you can do to get rid of the odor.

How to Get Rid of Sewage Smell in Basement

`We will start with the most common cause for sewer smells in the basement which also affects the whole house especially the bathroom.

1. Dried Basement Floor Drain Trap

Your basement being the lowest level of your house is designed with a floor drain in mind which is connected to the sewer line. The floor drain is meant to prevent the basement from flooding by channeling excess water to the sewer line.

In plumbing, every drain line connected to the sewer line needs to have a drain trap also known as a P-trap. As such, every fixture in your house has a drain trap.

A P-trap is a U-shaped piece of pipe which also resembles an inverted P and hence its name. You can clearly see it under your kitchen or bathroom sink but cannot see the one in your basement floor drain or in shower/tub drain.


The shape of a P-trap allows it to hold water at all times. This is the water you see at the bottom of a toilet bowl at all times.

The water in the P-trap acts as a barrier preventing sewer gases from coming up through the drain. Instead, sewer gases flow out through the plumbing vent which runs through the roof of the house.

If your floor drain has not been used for some time, the water in the P-trap is bound to evaporate. When that happens, the sewer gas barrier will have been broken allowing the sewer gases to flow through it and hence the sewage smell in the basement.

The solution for this problem is usually a very easy one. You will only need to pour about a gallon of water down the floor drain to replace the evaporated water in the P-trap.

If you have other fixtures in the basement like toilets, sinks and washers that you have also not used in a while, they could also be the cause of the sewer gas smell.

Flush the toilets and turn on faucets on the sinks for a few seconds. This should solve the problem for you.

2. Missing Sewer Cleanout Plug


Sewer lines have a cleanout which is basically a pipe used to access the entire sewer line when you want to have it inspected or unclogged.

In some houses, the cleanout is located outside the house but in some the cleanout is inside the house usually in the basement. The cleanout is found inside the floor drain on the side.

It is usually covered by a plug to prevent sewer gases from coming up through it. If the plug is missing, damaged or loose, sewer gases will flow from the sewer line to your basement without being restricted.

Start by removing the basement floor drain grate then check on the side to see if the plug is there. If it is missing or damaged, you can buy another one and replace it in minutes.

3. A Bad Toilet Wax Ring

Do you have a toilet in your basement? If you do and the sewage smell in there is not caused by dried drain traps or a missing cleanout plug, you could be having a toilet wax ring that is not sealing.

A toilet wax ring is a round ring of wax that is used to create a watertight seal between the bottom of the toilet and the top of the closet flange. Although a wax ring can last for a long time, a wobbly toilet or an uneven floor can cause the wax ring to stop sealing thereby allowing sewer gases to penetrate between it and the toilet.

A bad wax ring will need to be replaced.

Start by turning off water to the toilet then flush the toilet and drain all the water from the tank and the bowl. When that is done, use a wrench to loosen the bolts on each side of the toilet then rock it about and lift it off.

Scrape off old wax from the bottom of the toilet and the top of the closet flange. Set a new wax ring on top of the flange then gently place the toilet on top. Tighten the bolts and turn the water back on.

4. Ejector Pit Not Sealed Properly

Wastewater from your house is designed to flow out to the city sewer lines or septic tank via gravity. If you have a bathroom in the basement, the waste cannot move out via gravity and you will therefore have an ejector pit and pump.

Once the wastewater flows inside the pit, the pump pumps it out into the sewer line. To prevent the ejector pit from smelling, it is usually covered with a sealed lid and vented as well.

If the lid is not sitting properly on top of the pit, the seal is broken or if the vent is clogged, sewer gases will find a way to your basement and hence the sewage smell.

The first thing you need to do is make sure that your ejector pit’s lid is properly installed and that the seal is intact. The next thing you will need to do is make sure that both the vent and water discharge pipes are not cracked or clogged.

If they are, you will need to unclog them or replace if they are cracked.  You may need to hire a professional plumber for this kind of a job.

6. Damaged Sewer Line


If you have tried all of the above remedies but still the sewage smell persist in your basement, you could be dealing with a broken sewer line.

A broken sewer line needs a professional plumber to figure out and fix. A DIY is not recommended and you may even make the problem worse than it originally was.

The first thing the plumber will do is run a sewer line inspection camera to check the condition of the sewer line. If the sewer line is broken they will be able to see although it is sometimes hard to see leaks using the camera.

In modern plumbing, a damaged sewer line does not need to be dug out and replaced. Trenchless sewer replacement is a fast, effective and cheaper way of fixing broken sewer lines. The 2 main methods used are pipe bursting and structural pipe lining.

Wrap Up

And basically that is how to get rid of a sewage smell in your basement. In most of the times, you will be able to fix the problem by simply pouring some water down the basement floor drain.

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