Why Is There a Bad Smell Coming From My Bathroom Drain?

A bad smell from a bathroom drain is most commonly caused by a dried-out trap, biofilm (organic build-up) inside the pipe, or a blocked/faulty vent allowing sewer gas to escape — all three have different, mostly straightforward fixes.

Dry trap (the most common cause)

Every drain — basin, bath, shower — has a U-shaped trap underneath that holds a small amount of water, which forms a seal blocking sewer gas from coming back up the pipe. In drains that aren't used often (a guest bathroom, an ensuite used only occasionally), this water can evaporate, breaking the seal and letting smells through. This is especially common with shower and floor drains that don't get used daily.

Fix: simply run water down the drain for a minute to refill the trap. If the smell disappears and only comes back after another period of disuse, this confirms a dry trap rather than anything more serious.

Biofilm build-up

Soap scum, skin cells, hair and general residue build up inside pipes over time, forming a biofilm that can smell musty or sulphurous, particularly in shower and bath wastes. This is a maintenance issue rather than a fault.

Fix: a regular pour of a proprietary drain cleaner, or a simple mix of baking soda followed by white vinegar and then hot water, can shift mild build-up. For a drain that hasn't been cleaned in years, a plumber with a proper drain rod or jetting equipment may be needed for a thorough clean.

Blocked or faulty vent

Waste pipes rely on ventilation (either an internal air admittance valve or an external vent stack) to let air in as water drains, preventing trap seals being pulled dry by siphoning. A faulty air admittance valve or a blocked external vent can cause gurgling, slow draining, and smells, because the system isn't venting properly.

Fix: this usually needs a plumber to inspect and, if faulty, replace an air admittance valve (a relatively inexpensive part) or clear an external vent stack.

Other, less common causes

• A cracked or poorly sealed pipe joint, particularly under the floor, letting sewer gas escape from somewhere other than the trap itself.
• A genuinely blocked drain further down the system, where waste is sitting and decomposing rather than draining away — often accompanied by slow draining, not just smell.

Common mistakes

• Masking the smell instead of fixing the cause — air fresheners and drain gels can hide the symptom while the underlying trap or vent issue remains.
• Assuming it's always the toilet — basin, bath and shower drains are just as likely a source, and it's worth checking each individually.
• Ignoring a smell that keeps returning quickly after running water — a fast-returning smell suggests a vent or pipe fault rather than a simple dry trap.

When it might not be the drain at all

Occasionally a bathroom smell is actually coming from a different source — a failing extractor fan duct, a nearby damp patch, or even a drain outside the bathroom entirely, with the smell simply being noticeable in the bathroom because of airflow patterns. If you've addressed the trap, biofilm and vent possibilities and the smell persists, it's worth checking whether it's genuinely coming from the drain itself by getting close to each fitting individually, or asking a plumber to help pinpoint the source rather than continuing to guess.

Prevention going forward

For drains used infrequently, running water through them roughly once a week prevents the trap drying out in the first place. A monthly light clean with baking soda and vinegar (avoid mixing with bleach-based products) keeps biofilm from building up to the point of causing noticeable smell, and is a cheap, simple habit that avoids the problem recurring.

If the smell is accompanied by visible mould rather than just odour, see our guide to why your bathroom is growing mould, which covers ventilation-related causes more broadly, and our extractor fan regulations guide if poor ventilation seems to be part of the problem.

What about the toilet specifically?

A smell that seems to come specifically from around the base of the toilet, rather than a basin, bath or shower drain, points to a different likely cause: the wax or rubber seal (the pan connector) between the toilet base and the soil pipe. If this seal has perished or the toilet has ever rocked slightly on its fixings, sewer gas can escape at the base rather than through the trap. This isn't something to leave — a failed pan connector can eventually let water leak onto the floor too, and refitting or replacing the seal is a job for a plumber.

When to call a plumber

Most bathroom drain smells resolve with the simple fixes above, but if you've tried running water, cleaning with baking soda and vinegar, and the smell persists after a few days, it's worth getting a plumber to investigate rather than continuing to guess. A call-out to diagnose a persistent smell typically costs £60-£100 for the visit, with any further work (replacing an air admittance valve, clearing a vent, or resealing a toilet connector) priced separately once the cause is confirmed.