Bathroom Smell Like Sewage? Possible Causes and Fixes

If your bathroom smells like sewage, it could be due to a few common issues.

1. Clogged Drain

The buildup of hair, soap scum or other debris in the drains can cause blockages. These clogs trap moisture, creating a breeding ground for bacteria that can emit foul, sewage-like odors.

2. Dry P-Trap

The P-trap, a U-shaped pipe beneath sinks and drains, holds water and prevent sewer gases from entering the bathroom. If it dries out (e.g., in unused sinks or shower drains), the barrier disappears, allowing sewage smells to seep through.

3. Vent Pipe Blockage

Vent pipes allow sewer gases to escape outside your house. If leaves, debris or even animal nests block the vent pipe, it can force the gases back into your bathroom.

4. Leaky or Damaged Pipes

Cracks or leaks in pipes can release sewer gases into the bathroom. If there’s a hidden leak under the floor or behind the walls, the smell may worsen over time as gases escape through the damaged areas.

5. Toilet Seal Problems

The wax seal beneath the toilet connects it to the sewer line and prevents gases from seeping into the bathroom. A worn-out or improperly installed wax ring can lead to leaks and allow unpleasant odors into the room.

6. Sewer Line Issues

In more severe cases, a broken or blocked main sewer line could cause a backup that results in sewer odors throughout the bathroom. This may also cause issues in other drains in the home.

A basic understanding of how the plumbing in your bathroom works can help you pinpoint the issue. (Don't worry, this will be quick!)

How the P-Trap Works

At some point, you've probably looked under the bathroom sink and noticed the U-shaped pipe that runs from your sink drain (B) to a larger wastewater pipe in the wall. This pipe is called the P-trap.

One end of the P-trap runs down to your sewer or septic system (E); the other end leads all the way up through the roof (D), letting fresh air in while allowing any smelly sewer gases to vent out to the sky.

And although you can't see it, the same basic drain setup is at work behind or beneath your tub and shower (C).

The U-shape of the P-trap allows it to collect a small amount of water after each sink use, which acts as a barrier against sewage odors. When everything's working as it should, the water stays in the P-trap after you turn off the sink, empty the tub or flush the toilet.

That little bit of water is enough to prevent gases from drifting out of the sewage system and into your house. Under normal conditions, those gases flow right past your bathroom and out through the vent pipe in your roof.

The Importance of the P-Trap

"The role of the P-Trap is to keep the sewer gases out of the home," says Ray Patrick, a master plumber with Roto-Rooter. "There are misconceptions that the traps are also there to catch objects that might be dropped in the drain, which is not the case. Traps are there to prevent sewer gases from escaping into the home."

Failure to properly vent those stinky gases has the potential to be more than just annoying.

"It could be dangerous because of other gases that can be present in sewer systems. Methane is the largest cause of sewer smell but there is also danger of hydrogen sulfide, carbon dioxide, ammonia, nitrogen oxides and sulfur dioxide," explains Patrick in an email interview, adding that the main culprit of such added toxins are people disposing of gasoline and other chemicals down their drains.

Signs of exposure can include dizziness, headaches, nausea and drowsiness. Clearly, it's important to figure out exactly what's causing the sewage smell and make sure it's corrected quickly and effectively.

There are a few fixes you can try before calling the plumber.

1. Unclog the Drain

A clogged drain is kind of gross but fairly easy to resolve. Often, a plunger can take care of the clog, but if that doesn't work a plumber might have to use a drain snake or hydro-jetter to remove it. Sometimes, it's necessary to completely remove the P-trap to solve the issue.

If the suggestions above don't lead you to the source of the sewage smell, you may be dealing with a more complicated issue.

2. Fill the the P-trap

A dry P-trap is by far the easiest to fix, and more likely to occur in your bathroom if you haven't used the fixtures recently. For example, you may not use the sink in a guest bathroom or the tub in a bathroom with a separate shower.

The issue in situations like these is that the water in the P-trap has simply evaporated due to lack of use, effectively removing the barrier between your bathroom and the gases in the sewer system.

If that's the case, the solution is as simple as running some water to allow the P-trap to fill up again. If plain running water isn't making a dent in those sewer odors, Patrick recommends using pink RV water-safe antifreeze.

3. Check the P-Trap for Leaks

If there's still a sewage odor after you've run plenty of water or pink antifreeze down all the drains or if you notice sewer smells in a bathroom that gets regular use, check for leaks in or around the P-trap.

If even a small amount of leaks out of the bottom of the "U," sewage gases may be able to sneak in. "If anything leaks, it would be noticeable," Patrick explains.

To fix a leaky P-trap, first, place a bucket underneath it to catch any water and turn off the water supply. Then, carefully loosen the slip nuts on both ends of the P-trap using a wrench and remove the trap. Inspect for cracks or worn-out washers; replace any damaged parts or washers. Seal any cracks or openings. Reassemble the trap tightly, ensuring all connections are secure, then run water to check if the leak has stopped.

4. Address Leaky or Damaged Pipes

Check for visible leaks under sinks and around pipes. Use a flashlight to inspect areas under the sink or behind the toilet, and listen for any dripping sounds. A plumber’s inspection is best for leaks in walls or under flooring.

Minor leaks from accessible areas can be DIY fixes with a plumber’s tape or sealant. For hidden or severe leaks, contact a plumber to prevent further damage.

5. Test if the Toilet Seal Is Intact

To determine if there's an issue with the toilet seal, you can run a few tests:

Replacing the wax ring is possible, but it requires lifting the toilet. According to Vaughan HVAC, "This form of toilet repair is not an overly complicated process, but it does involve some heavy lifting, and many people are uncomfortable with it."

If you want to take it on yourself, follow these steps:

6. Examine the Roof Vent

To check for a vent pipe blockage, head to the roof and look for debris like leaves, dirt or nests. Clear any blockages if you feel comfortable doing so. If it’s inaccessible or you’re unsure about clearing it, a plumber can safely handle this task.

7. Correct Sewer Line Issues

If you suspect it's a sewer line issue, you’ll need a professional inspection. Plumbers can use cameras to assess the main sewer line and determine if there are blockages or breaks. This is best left to a plumber because they have the tools and expertise to address complex issues, like tree root blockages or broken sewer lines.

So, there's water in all the P-traps, you have no visible leaks and the pipes are free of clogs. What next?

Check Outside the Bathroom

Occasionally, the problem is actually coming from somewhere else, even though the smell itself is inside the bathroom. For example, a roof vent blocked by a bird's nest, leaves, or snow and ice prevents fresh air from coming into the plumbing system and preventing the sewage system from venting the way it's supposed to.

It can be tricky to distinguish between a blocked drain pipe and a blocked vent, so homeowners often spend plenty of time focusing their efforts on a single pipe.

A few telltale signs (in addition to the sewage smell) that a blocked vent is actually the culprit are that all of the drains in the home are slow to drain, and that water makes gurgling noises or bubbles up when trying to drain.

A vent pipe that's been improperly installed, cut or cracked can also send offensive gases into your home. A cracked vent pipe can be even harder to track down, since the break is probably hidden somewhere within your walls.

Fortunately, a plumber can locate a vent pipe leak with a device called a smoke machine, which fills the drain system with a harmless visible smoke. When the smoke finds its way out, the source of the leak is obvious.

Hire a Professional

Although some of the fixes are done easily enough by the average homeowner, you should evaluate your comfort and skill level before tackling a project like this.

If you're handy, you could take care of cleaning drains or perhaps replacing a toilet's wax ring. But if you're not comfortable doing home repairs, or you're not sure of where the smell is coming from, it might be best to hire a pro.