Why does my drain smell? Causes and how to fix it

Persistent bad smells from a drain almost always come down to one of four causes. The good news is you can usually narrow it down in 10 minutes.

1. A dry trap

Every sink, bath, shower and floor gully has a 'trap' — a U-bend that holds a small plug of water between your room and the sewer. If a fitting isn't used for a few weeks the water evaporates and sewer gas comes straight up.

Fix: Run the tap or pour a jug of water down every drain in the property for 30 seconds. If the smell goes away within a few hours, that was the cause.

2. A blocked or undersized vent (soil stack)

Soil pipes are vented to the outside air — usually via a pipe that runs up past the roof. When a fitting drains, that vent lets air in to replace the water. If the vent is blocked (leaves, bird's nest, capped during a loft conversion) the next time something drains, your nearest U-bend gets sucked dry. You get a gurgle, then the smell.

Tell-tale sign: The toilet gurgles when you run the shower or the kitchen sink.

Fix: Inspect the vent at roof level, clear any blockage. A drainage engineer can do this safely with the right access kit.

3. A partial blockage

A build-up of fats, hair, soap and food in the pipe traps decomposing organic matter. You may still get water away — slowly — but the smell hangs around.

Fix: A drain machine or jetter clears it properly. Household chemical drain cleaners often shift the worst of the smell for a week or two but don't remove the actual blockage.

4. A cracked or displaced pipe

If the smell is outside (around a manhole, soakaway or under a downstairs floor) the pipe itself may be leaking. This is most common with old clay or pitch-fibre runs.

Fix: A CCTV survey shows you exactly where the defect is, and what your repair options are.

When to call us

If running every tap doesn't fix it within 24 hours, you almost certainly need a drainage engineer. We can usually diagnose the cause on the first visit.