You know that moment when you notice a sewer smell in the bathroom and can’t figure out where it’s coming from? Everything looks clean, there’s no mess. But the musty bathroom odor is definitely there. Really, this is more common than you think . But I guess you don’t usually hear your friends or relatives casually talk about how their bathroom smells like mildew or sewage over dinner. Anyway, I’ve seen this happen in both new renovations and old homes. 

The cause isn’t always obvious, which makes it even more frustrating. Sometimes it’s not the toilet. Not the shower drain. Not even the ventilation fan. You spray something, open a window, maybe light a candle, eliminating bathroom odors only temporarily. That’s because sprays can only hide the problem. They don’t quite fix it. And no, having a moldy odor in your bathroom doesn’t mean your space is dirty or poorly maintained. I’ve seen spotless bathrooms stink like a sewer for the weirdest reasons.

There are a few spots people never think to check. And one or two that surprise even experienced fitters like me. So, let’s walk through the 11 real reasons for that drain smell in your bathroom, and more importantly, how to fix it.

The First Things I’d Check in a Smelly Bathroom

There’s a lot you could check, but here’s a quick list that comes to mind. First, notice when the smell appears. Does it smell like mould after a shower, or does the odor linger all the time? Then, think about the ventilation. Is there a fan or a window to help air things out? Next, check what kind of shower you have. Is it a stand-up or a tub-shower combo? And how old is it? Make sure all the plumbing fixtures (sink, toilet, vanity, and shower) have proper traps. If there’s an access panel near the shower, try turning the water on and looking inside with a flashlight. Check for any wet spots, mould, or signs of leaks. 

Also, look where the tub meets the floor. If there’s a gap or missing caulk, water might be sneaking in when you step out of the shower. In older showers, they may have used drywall behind the tiles, which can trap moisture and smell bad once it gets damaged. Pay attention to the grout too. Are there holes or cracks in the lines where water could seep in? If the bathroom is upstairs, check the ceiling below for water stains. That can also point to hidden moisture. From my experience, this is a solid starting point!

Sewer Smell in Bathroom: Common Causes and How to Fix Them

Below, I’ve broken down the most common causes I’ve run into on real jobs, plus what you can do about each one. You can handle some fixes on your own. And others might need a professional. But either way, you’ll know exactly where to start when there’s a sewage smell in your bathroom.

1. A Poorly Sealed Toilet Smells

If your toilet rocks, even a little, it can break the wax ring seal underneath. That lets sewer gas escape from the drainpipe, making your toilet stench. So first, check if the toilet is loose. And I know it sounds bad, but you’ll need to smell around the base. Unfortunately, if it smells, the toilet will likely need to be removed, because the seal is probably broken. You’ll need to shut off the water, flush to empty the tank, and then.. call a fitter or plumber to remove it safely, because they can handle this job properly. Once it’s out, the fitter will check the wax ring. If it looks crushed, dried out, or worn, it should be replaced with a new one before reinstalling the toilet. And yes, the same toilet can usually be reused. It just needs to be reset properly with a fresh seal. 

2. Dry Drain Traps (Smelly Bathroom Drain)

Smelly drains are one of the most common reasons for bad odors. If you haven’t used a sink or shower in a while, the water in the trap may have evaporated. Each drain has a U-shaped pipe (called a P-trap) that holds water to block sewer gases from coming back up. But when that water evaporates, those gases can sneak in and cause that strong bathroom drain odor. 

Pour a few cups of water down the drain to refill the trap. And for rarely used sinks or tubs, do this every few weeks. Moreover, you can add a thin layer (1 mm should be enough) of mineral oil on top of the water in the trap. It slows down evaporation and is a better option than vegetable oil because it won’t go rancid. And don’t use RV antifreeze unless it’s pure propylene glycol. Most types contain ethanol, which evaporates fast, just like water. 

Missing or Faulty Trap Seal Primer

Some drains (especially floor drains)  have a trap seal primer. It’s a small device that drips water into the floor drain every time you use nearby plumbing, like your laundry sink. Some are connected to the faucet supply, some use a hose from a cold water line, and some have an air gap to protect your water system from contamination.

If the primer’s missing, disconnected, or just never installed (which happens more often than you’d think), you’ll need to follow the steps above. Or install a rubber trap guard. It’s a simple flap that lets water in but blocks gas from coming back up.

Trap primers aren’t always easy to spot or hook up, so if you’re unsure what you’re looking at, it’s best to ask someone at your local hardware store. Or call in a fitter to help you sort it.

3. Low Toilet Flange After Renovation

If you’ve tiled the floor, the flange (that pipe mount under the toilet) might now sit too low. That makes it hard to get a proper seal with the wax ring. To fix it, you’ll need a repair flange (also called a flange extender) to raise the height. Use longer bolts and make sure the flange sits level with, or just a bit above, the finished floor. And whatever you do, don’t stack more than two wax rings. It won’t seal right, and you’ll just end up with the same problem again. If you want more tips on applying bathroom sealant, check out our full guide covering application, materials, and maintenance.

4. Smelly Overflow Drain

That little hole near the top of your sink? It’s the overflow drain, and it can collect gunk and smell bad. Pour a bit of mouthwash into it to help kill bacteria, then rinse it through with clean water. If the smell keeps coming back, try using a thin brush or pipe cleaner to loosen any buildup inside.

5. Mildew and Black Mould in Bathroom

Mould thrives in dark, humid spaces, especially when humidity stays above 60% and the temperature is around 60°F or higher. If moisture gets into the walls and there’s no airflow, the studs can stay damp for days. And if they stay damp, they won’t dry out on their own. Check the tiles, corners, and edges of your bathroom for mould or mildew. If you see any, clean it right away using a mould remover and a good scrub. To stop it from coming back, keep the space well-ventilated and clean it weekly. A dry bathroom is much harder for mould to grow in. For a full breakdown on how to remove mould from walls and other bathroom areas and prevent it from growing back, check out our guide. 

6. Backed-Up Septic Tank

If multiple bathroom drains smell, your septic tank might be full. Unfortunately, this isn’t something you can fix on your own. You’ll need to call a septic service to pump the tank and clear the outflow lines. It usually costs around $300, depending on your area. If you haven’t had it pumped in a few years, now’s the right time to do it. Because leaving it too long can cause serious damage inside the house. 

7. Inactive Water Heater (Rotten Egg Smell)

If you rarely use hot water, bacteria can build up inside the tank, making water smell like rotten eggs. To fix it, turn the water heater up to its maximum temperature to kill off the bacteria, then flush the tank a couple of times. If that doesn’t work, you may unfortunately need to replace the heater.

8. Clogged Shower or Sink Drains

Smelly shower and sink drains can be caused by hair, soap, and gunk blocking them. Lift the drain cover and check inside, because it might be full of black sludge. For cleaning, you can use baking soda and vinegar. Pour in baking soda, then a 50/50 mix of hot vinegar-water. Cover the drain to trap the fizz and let it work inside the pipe. After a few minutes, scrub everything with a stiff brush. And then pour boiling water down the drain to flush it out. You can also use a drain stick (a cheap plastic tool with barbs) to pull out clumps of hair and gunk. And don’t forget to clean the underside of the drain stopper too. 

9. Blocked Roof Vent Pipe

Most homes built in the last 50 years use PVC vent pipes that run through the roof. Older homes might have copper or steel vents instead. Some remodels use air admittance valves (also called vacuum breakers) hidden in the wall. They’re designed to let air in, not out. If those valves fail, they can release sewer gases into your bathroom. If your vent pipe is on the roof, it can also get blocked by leaves, debris, or even bird nests. That sends the gases back into the house instead of letting them escape. You’ll need to call a plumber to check or clear the vent safely.

10. No Backflow Preventer in Low-Lying Homes

If your home is lower than the street or downhill from others, install a backflow preventer. Without one, a blocked sewer line can push raw sewage back into your house. This can flood bathrooms with sludge if the line backs up. The fix is quite simple: a backflow valve which your plumber can easily install. 

11. Yes, Poor Bathroom Ventilation Causes Sewer Smell in Bathroom

Bathrooms get humid fast. And without airflow, that moisture just sits there, giving mould and mildew the perfect place to grow (and smell awful). Open the window when you can, and always leave the shower doors open after use. If you’ve got an exhaust fan, run it for at least 30 minutes after every shower. And if you don’t have one, it really is worth installing a bathroom ventilation fan. We also put together an article on simple ventilation tips to help keep your bathroom fresh and mould-free.

What Not to Do About That Sewer Smell in Your Bathroom

As I mentioned at the beginning of the blog, don’t just spray air freshener or light candles, hoping the smell will just go away. Avoid pouring harsh chemicals down the drain without knowing what you’re dealing with. Some can damage your pipes or make things worse. Also, don’t ignore a sewer smell in the bathroom just because everything looks clean. Odors often come from hidden or hard-to-reach spots, like behind tiles, inside walls, or under the toilet. And please… don’t start ripping things out unless you’re sure what’s wrong. Because this way you could end up  turning an easy fix into a costly struggle.

When to Call a Professional

Some problems are best left to someone with tools, experience, and a license. If the sewer smell in your bathroom keeps coming back, or if it’s coming from multiple drains, your septic tank might be full. And that’s a job for a septic service. If your toilet wobbles, leaks, or smells around the base, a fitter can reseal or reset it properly.

You should also call a pro if: there’s mold you can’t reach or safely remove, you suspect a leak inside the wall or a broken vent pipe, your bathroom floods or backs up with sewage, you want to install a trap primer, backflow valve, or ventilation fan. Basically, if it’s beyond your toolkit (or if you’ve tried the fixes and the smell’s still there) just reach out for help.

Did I Mention Our Maintenance Team?

At AV Modern Bathrooms, we do full renovations, and we stick around after the job’s done. Our maintenance team is here if anything ever goes wrong. The first home visit is free, renovations start at £3,600, and every full bathroom comes with a free LED mirror. We also have a showroom in St. Helens if you’d like to browse for items, check some cool bathroom designs and discuss in person with our fitters.

If you’d like to learn more about how we can work together, book your free quote today! We’d love to hear from you and help you get started on your dream bathroom!

• Call us at +𝟰𝟰 𝟭𝟵𝟰𝟮 𝟯𝟲𝟱𝟵𝟴𝟵;

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• Book your free quote directly here;

• Visit us at 𝟲𝟵-𝟳𝟭 𝗛𝗶𝗴𝗵𝗲𝗿 𝗣𝗮𝗿𝗿 𝗦𝘁𝗿𝗲𝗲𝘁, 𝗦𝘁. 𝗛𝗲𝗹𝗲𝗻𝘀, 𝗪𝗔𝟵 𝟭𝗔𝗗, 𝗨𝗞 (our main office is in St. Helens, but we proudly serve customers across the North West, North Wales, and parts of the Midlands).

Questions My Clients Ask About Sewer Smell in Their Bathroom

How to get rid of smelly drains in the bathroom?

First, pour a few cups of hot water down the drain. Then add baking soda followed by hot vinegar-water. Cover the drain to trap the fizz. After 10 minutes, scrub with a stiff brush and rinse with boiling water. You can also use a drain stick to pull out hair and gunk. And don’t forget to clean under the drain stopper.

Why does my shower drain smell if it’s not clogged?

The water in the trap may have evaporated, letting sewer gases rise up. Pour water down the drain to refill it. Also check for trapped gunk inside the drain (hair, soap, or biofilm can smell even without a full blockage).

How to clean mould off walls to get rid of sewer smell in my bathroom?

Scrub the area with a mould remover or a vinegar and water mix. Use a stiff brush or sponge. After cleaning, dry the wall completely. Open windows or use a fan to let the moisture out.