Why does my drain smell? (And how to fix it without panic.)
A bad smell in the bathroom, kitchen, or utility room doesn't always mean there's a full blockage.
Often it's something much simpler, something most people overlook.
Over time, everyday things like shampoo, soap, toothpaste, fats, and cooking oils create a thin layer inside the pipes.
When the water slows down or dries out, that smell escapes straight into the room.
But smells can also come from bigger system problems like:
- A gully outside that's partially blocked
- A vent pipe that's not working properly
- Rainwater entering a foul drain
- Sewage backing up very slowly (before you even see water)
In some homes:
- Treatment plant gases escape through cable ducts
- Septic tank vent pipes are missing or broken, so smells travel through the kitchen gully
- The vent line is blocked, so smells find the easiest route inside the property
A key sign people miss.
A strong, sour smell can be the first sign that surface water is entering your treatment plant.
A healthy, serviced treatment plant should never smell outside.
If it does, occasionally or all the time, it's a warning something isn't working properly.
The good news?
Most smelly drains are easy to fix if you catch them early.
The 5 main causes of smelly drains.
1. Dry trap (the most common cause).
The water inside the U-bend or bottle trap evaporates. No water equals no seal, so smell rises straight up.
Where this happens most:
- Spare bathrooms
- Unused sinks
- Downstairs toilets
- Garages and outhouses
Fix: Run water for 20 seconds. Toilets: one flush restores the seal.
2. Gully outside blocked with fat or silt.
Kitchen gullies often smell before they block.
Signs to look for:
- Smell around the outside drain
- Water sitting or rising slowly
- Flies gathering by the gully
Fix: If water sits on top instead of draining, it needs a proper clean.
3. Organic build-up inside the pipe.
Hair, soap, and oils create a sticky film that traps smell.
You may notice:
- Gurgling
- Slow draining
- Smell only when draining
A proper clean removes the film, and the smell disappears instantly.
4. Ventilation problem.
If the stack pipe isn't pulling air correctly, the smell comes inside instead of going out.
Typical signs:
- Smell worse after flushing
- Smell after a bath empties
- Smell depending on wind direction
5. Surface water entering foul drains.
Heavy rain can push dirty water backwards through:
- Unsealed chamber lids
- Cracked pipes
- Broken joints
You often smell the problem before you see it.
When to call a professional.
Ring if:
- The smell keeps coming back
- Water is slow or backing up
- The smell appears after heavy rain
- You smell it stronger near outside manholes
- You suspect cracked or damaged pipes
At ASL Limited:
- The pipe is cleaned properly (not a quick poke-through)
- It's camera-surveyed straight after
- The exact cause is identified
- It's stopped from returning
Smells are early warning signs. Catch them now and prevent bigger damage later.
Need advice straight away?
📞 Call ASL Limited on 0800 181 684. The cause will be found and fixed properly, safely and cleanly.
We cover Surrey, Berkshire, Hampshire, West Sussex and South West London.
Related services.
To learn more about how ASL Limited can help with smelly drains:
