A smelly bathroom drain is one of the most common problems homeowners deal with, and in most cases it is something you can fix yourself in under an hour. This guide covers the five most common causes of bathroom drain smell, how to diagnose which one you are dealing with, and what to do about it.
What causes a bathroom drain to smell?
The smell coming from a bathroom drain is almost always sewer gas. That gas enters your bathroom when something breaks the water seal in your drain trap. The trap is the curved section of pipe beneath the drain that holds a small amount of water at all times. That water is what keeps sewer air out of your bathroom.
There are five main reasons a bathroom floor drain smells:
- A dry trap caused by a drain that has not been used for a while
- A blocked or partially blocked drain
- Mould or biofilm buildup on the drain¹ cover or inside the channel
- A cracked or damaged drain trap
- Hair and soap scum trapped inside the drain body
How to diagnose the problem
Before you reach for the baking soda, take a moment to work out which of these is causing the smell. It will save you time and point you to the right fix.
Run the tap for thirty seconds and let the water drain. If the smell disappears almost immediately, the trap was dry. This is common in bathroom floor drains that are not used daily, such as a guest bathroom or a laundry floor waste.
If the water drains slowly or backs up, you have a blockage. If the drain flows freely but the smell persists, the issue is more likely mould, biofilm, or a build-up inside the drain channel itself.
Fix 1: Dry trap
A dry trap is the simplest fix there is. Pour two cups of water directly into the drain and wait a few minutes. If the smell is gone, that was the problem. In low-use bathrooms, this can happen within a week or two of the drain sitting idle.
To prevent it recurring, pour a cup of water into the drain once a week if the bathroom is not in regular use. You can also pour a small amount of cooking oil on top of the water once a month. The oil sits on top of the water in the trap and slows evaporation significantly.
Fix 2: Blocked drain
A partial blockage, usually hair and soap scum lodged just below the drain cover, is the most common cause of drain smell in bathrooms that are used daily.
Start by removing the drain cover and clearing any visible build-up by hand. Then try this method using items you likely have at home:
- Pour half a litre of hot water into the drain.
- Add one cup of baking soda and leave it for ten minutes.
- Add one cup of white vinegar. The fizzing reaction helps break up soap scum and organic build-up.
- After the fizzing settles, use a plunger with five to six firm pushes.
- Flush with another litre of hot water.
Repeat the process once if the blockage does not fully clear on the first attempt. Avoid using chemical drain cleaners routinely. They can damage older pipe joints and should be kept as a last resort before calling a plumber.
Fix 3: Mould and biofilm on the drain
If your drain flows freely and the trap is wet but you can still smell something, check the drain cover and the inside of the channel. A thin layer of black or grey biofilm builds up inside drain bodies over time, particularly in warm, humid bathrooms. It feeds on hair, soap residue and skin cells, and it smells.
Remove the drain cover and clean it with a mould-removing bathroom spray and a soft brush. For a stainless steel strip drain, run warm water over the channel body and wipe the interior with a cloth dampened with white vinegar. Avoid abrasive scrubbers on stainless steel surfaces as they can scratch the finish and make future build-up harder to remove.
Once clean, flush the whole drain with hot water. For ongoing prevention, a quick spray of bathroom cleaner and a rinse once a week is enough to stop biofilm from taking hold.
Fix 4: Hair and soap scum inside the drain body
Strip drains and linear drains collect hair along the full length of the channel, which is part of their design advantage for drainage, but it also means hair can accumulate deep inside the body rather than just at the surface.
Remove the drain cover and use a flexible drain cleaning brush or a folded cloth to clear hair from the inside of the channel. For a wedge wire drain, lift the grate section fully and clean the tray beneath. Doing this every few weeks significantly reduces both smell and the risk of slow drainage developing.
Fix 5: Cracked or damaged drain trap
If you have tried everything above and the smell persists, the trap itself may be cracked or have a poor seal at the connection point. This allows sewer gas to bypass the water in the trap entirely. This is not a DIY repair. Call a licensed plumber to inspect and replace the trap.
How to stop drains smelling in the first place
A few minutes of maintenance each month prevents the problem from developing:
- Clear hair from the drain cover after every shower
- Run a hot water and baking soda flush once a month
- Pour water into any drain that is not used regularly
- Clean the drain cover and channel body every few weeks
The quality of the drain itself makes maintenance easier. A tile insert strip drain or wedge wire drain in 316L stainless steel has no plastic fittings to crack or perish, no crevices where bacteria can hide in a plastic frame, and the grate or tile insert lifts out cleanly for access. That combination means less build-up and faster cleaning when you do it.
When to call a plumber
Call a licensed plumber if the blockage does not clear after two attempts with the baking soda method, if your drains are gurgling after use, if multiple drains in the bathroom smell at the same time, or if the smell returns within a day or two of cleaning. These are signs of a deeper issue in the drain system that needs professional attention.
For advice on choosing the right bathroom drain for your renovation or new build, get in touch with the Strip Drains team. All drains are manufactured in Sydney and custom sized to suit your project.