Why does my shower drain smell?

You walk into the bathroom and cop it again. That sour, musty, sometimes sewage-like whiff coming up from the shower. The water still drains fine. The shower looks clean. So what is causing the stink?

A smelly shower drain is usually one of a handful of things. Most of the time you can sort it yourself in an afternoon. Sometimes it points to a real plumbing fault and a home remedy will not touch it. Here is how to tell the difference.

What the smell tells you

The type of smell is a decent clue about what is going on below the grate.

• Musty or mildew-like means mould or biofilm in the drain.
• Rotten egg or sulfur means sewer gas getting past a dried-out P-trap.
• Raw sewage means something has properly gone wrong - a broken seal, a cracked vent, or a blockage deeper in the line.
• Stale or stagnant means hair, soap and body oils sitting in the pipe and breaking down.

If your nose is telling you it is sewage, do not put it off. That smell is a plumbing fault, not a cleaning job.

The usual suspects

A dry P-trap

The P-trap is the U-shaped bend under your shower. It holds a small pool of water that acts as a seal between your bathroom and the sewer line. If a shower has not been used for a few weeks, that water evaporates. Once the seal is gone, sewer gas rises straight up through the drain.

This is the most common cause in guest bathrooms, holiday houses and any shower that does not get daily use. Run the tap for 30 seconds to refill the trap. The smell usually clears within the hour.

If it comes straight back the next day, the trap is cracked or the vent pipe is blocked. That needs a CCTV drain inspection to find.

Biofilm coating the pipe

Biofilm is a slimy layer of bacteria, soap scum and body oils that builds up on the inside walls of every shower drain over time. It looks like a clear or grey jelly and it stinks. Biofilm can carry Salmonella and E. coli, which is part of why bathroom drains smell the way they do.

You can usually feel it if you push a finger or a brush a couple of inches down the grate. If it comes back coated in slime, that is your problem. A drain brush and an antimicrobial spray will shift most of it.

Mould

Showers are dark, warm and wet. Mould loves those conditions and grows fast inside drain housings, behind grates and inside the pipe. It gives off a musty, earthy smell that gets worse when the water is running.

Surface mould around the grate wipes off with a supermarket mould cleaner. If it keeps coming back, or you can see it creeping up the silicone or grout, the moisture is getting somewhere it should not be. Get someone to take a look.

Hair, soap and body oils

Not glamorous but it is the bread and butter of why drains smell. Hair tangles with soap residue, conditioner and dead skin and forms a thick plug a few inches down the pipe. Bacteria feed on it and give off the gases you smell.

If the shower is also draining slower than usual, this is almost certainly the cause. A drain snake from Bunnings will pull most of it out. Boiling water and a plunger will shift the rest. Clear it once, smell back within a week? The build-up is further down the pipe than a manual tool can reach.

A fault in the plumbing itself

This is where it stops being a DIY job. A cracked vent pipe, a broken seal at the waste connection, a partial blockage in the sewer line, or tree roots growing into an underground pipe will all let sewer gas escape into the house. So will old pipework that has shifted over time, which is dead common in Melbourne homes built before the 80s.

You can spot it by the giveaways:

• The smell is strong and obvious, not subtle.
• More than one drain in the house smells or drains slowly.
• You hear gurgling from the drain when water runs elsewhere in the house.
• You have already cleaned the drain and refilled the trap and it has made no difference.

If any of those apply, stop trying to fix it from the top. The cause is further down and you need proper diagnostic gear.

What to try first

Before calling anyone, work through these. They cover the easy bit.

1. Run the tap for 30 seconds. Refills the P-trap if it has dried out.
2. Pour a kettle of boiling water slowly down the drain. Loosens soap scum and biofilm.
3. Pull off the grate and clean what you can reach with a drain brush and antimicrobial spray.
4. Use a manual drain snake to pull out any visible hair or gunk.
5. Wait 24 hours and see if the smell returns.

If the smell is gone, stick a mesh strainer over the drain to stop it building up again and you are done. If it returns within a few days, the cause is past where a home tool can reach.

When it is time to call a plumber

The shower drain is connected to the same network as your toilet, your kitchen, and ultimately your main sewer line. A persistent sewage smell can mean the system is not venting properly, which leads to blockages backing up into the house. Worth getting on top of.

Give us a call if:

• The smell is sewage or rotten eggs and does not clear after refilling the trap.
• More than one drain in the house is affected.
• The shower drains slowly or gurgles.
• You have cleaned everything you can reach and the smell keeps coming back.
• There is water staining on the ceiling below the bathroom (which usually means a hidden leak).

We carry high-pressure water jetters and CCTV cameras on every truck. We find the cause, show you the footage, and most of the time we fix it on the spot.

With over 25 years sorting out Melbourne’s drains, we have seen every version of this one. Call us on 1300 303 247 or fill in our online contact form. We service all of Melbourne from our Oakleigh and Essendon depots and we do not muck around.

For step-by-step fixes you can try yourself, read our companion guide on how to fix your smelly shower drain.

Frequently Asked Questions

Why does my shower drain smell but the water drains fine?

The drain does not have to be blocked to smell. Biofilm and bacteria coat the inside of the pipe and release gas even when water flows through normally. A dry P-trap in a shower that does not get daily use will also smell without affecting drainage. Clean the drain with a brush and antimicrobial spray, and run the tap for 30 seconds to refill the trap.

Why does my shower smell like rotten eggs or sulfur?

A rotten egg smell almost always means sewer gas. The P-trap has dried out and the water seal that normally blocks gas from rising has gone. Run the tap for 30 seconds. If the smell does not clear within an hour, the trap may be cracked or the vent pipe blocked. Either way, you need a plumber.

Why does my bathroom smell like sewage even after cleaning?

If cleaning has not worked, the smell is coming from somewhere you cannot reach. Usually a broken seal at the waste pipe, a cracked vent, a blockage further down the line, or tree roots in the underground pipework. A CCTV drain inspection will show exactly where the fault is.

Can a smelly shower drain make you sick?

The bacteria in biofilm and mould can cause respiratory irritation, headaches and stomach issues over time. Sewer gas in small amounts is unpleasant but not dangerous. In larger amounts it can cause dizziness and nausea. Either way, it is not something to live with.

Is it OK to pour bleach down a smelly drain?

Bleach kills surface bacteria but does not shift biofilm properly, and it damages rubber seals and older pipework. Enzyme-based drain cleaners work better for regular upkeep because they actually eat the organic build-up rather than mask the smell. For a serious problem, get a plumber in instead of escalating the chemicals. You will save your pipes in the long run.

How do I stop my shower drain from smelling in the future?

Stick a mesh strainer over the drain to catch hair. Pour a kettle of hot water down the drain once a week. Use an enzyme-based cleaner once a month. That keeps biofilm from forming and stops the smell coming back.

Why does my shower drain smell worse in summer?

Heat speeds up bacterial growth and dries P-traps out faster. A smell that has been mild all year often gets stronger in January and February for both reasons. Running the tap for 30 seconds twice a week in summer keeps the trap topped up.