When a bathroom starts to smell unpleasant, the drain is often the source. The most frequent reason is the gradual build up of organic matter such as hair, toothpaste, soap scum and skin oils that accumulate inside the waste pipe. Another common issue is when the water inside the P trap dries out. This water usually acts as a barrier to stop gases from the sewer system escaping into the room. Without it, odours pass freely through the plughole. These can vary from mouldy or musty smells to a distinct sulphuric or eggy scent.
This guide from 24hrs Drainage explains what causes drain odours, how to fix them with proven methods and when to seek professional support. Our engineers support homes and landlords across the UK and have years of experience diagnosing persistent drain smells.
Often linked to hydrogen sulphide gas escaping from a dry or faulty P trap or blocked waste vent.
Usually caused by mould, mildew or stagnant water in the pipe system or overflow cavity.
Typically caused by decomposing organic material including soap, skin cells, hair and dental products.
Persistent smells indicate more than poor hygiene. They can highlight issues such as microbial build up, standing water, damaged seals or ineffective venting. These issues affect indoor air quality, increase humidity and over time may lead to visible damp or leaks.
Bathroom pipes collect soap lather, shampoo, body oils, and dead skin every time we use the sink or shower. These deposits stick to the inside of the pipework and start to break down. This creates biofilm, which is a sticky layer of bacteria that produces a foul smell.
Pro Tip: Always run water weekly in lesser-used bathrooms to maintain the water seal in the P trap and prevent odours from entering the room.
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The curved P trap pipe below your sink or shower holds a small amount of water. This water blocks sewer gases. If a drain is not used for several days, especially in spare bathrooms, the water evaporates and sewer odours pass through. In some cases, using pink RV antifreeze that is safe for drains can help reduce evaporation when sinks go unused for extended periods.
Damp pipes without regular flushing encourage mould colonies and biofilm. These release spores and gases that smell damp, earthy or fungal. Overflow pipes and junctions are common hiding spots.
Drain flies breed in stagnant water and slimy waste stuck inside pipes. If you see small flying insects around your plughole, there may be a layer of decomposed matter in the pipe. This will need cleaning out to stop the cycle.
If your bathroom drain has a slow flow or gurgles when you flush another tap, the venting may be blocked. Drainage systems rely on airflow through a soil vent pipe. If that vent becomes blocked or is missing, gases may escape through the water trap.
Leaks around the P trap or loose joints can allow odours to escape, even when water is present in the trap. Use a torch to inspect for any moisture or stains under the sink. A cracked P trap or faulty washers should be replaced immediately to restore the gas seal.
In some cases, especially if the smell is strongest near the toilet, the wax ring that seals the base may be leaking. A loose or cracked ring can let sewer gas escape even if the toilet does not visibly leak. Rock the toilet gently. If it moves, the seal may be compromised and need replacing.
Check for any gurgling noises or slow water flow. Shine a torch down the drain to look for surface level residue or hair. The smell may be stronger near the plughole, the basin overflow or underneath the sink where joints are connected.
The overflow hole found in most bathroom sinks is often ignored during cleaning. It can harbour damp, soap deposits and bacteria that produce odour. Use a narrow brush or toothbrush and clean it with a disinfectant or diluted bleach.
If water takes longer than usual to clear, the pipe may be narrowed by residue or blocked by debris. This slows flow and allows standing water to collect which becomes stale.
Pour freshly boiled water from a kettle directly into the plughole. This helps to soften any grease or product residue stuck to the sides of the pipe.
Pour half a cup of bicarbonate of soda into the drain. Follow this with one full cup of white vinegar. Let the mixture bubble and fizz for at least ten minutes before flushing with more boiling water.
Scrub gently using a soft brush soaked in white vinegar or diluted bleach. This removes odour causing bacteria. Rinse thoroughly.
Lift out the plug or pop up stopper and clean any buildup of hair or sludge underneath. This area is often one of the worst for odour.
Enzymatic cleaners use natural bacteria to digest organic build up without damaging the pipes. Use regularly to prevent smells returning. These are suitable for homes with septic tanks.
Use this only if odours persist and mould is suspected. Pour bleach carefully into the drain and let it sit for at least one hour. Rinse thoroughly with boiling water. Always ventilate the room.
If the smell is strongest after a bathroom has not been used, run the water for at least two minutes to refill the P trap. This restores the gas seal.
If you suspect a deeper blockage, use a cup plunger or a drain auger to break it up. Manual tools work better than pouring chemicals repeatedly.
Pro Tip: Use enzyme drain cleaners at night before bed so they can work undisturbed for several hours, giving better breakdown of organic matter.
Run hot water through the sink each week. Pour bicarbonate of soda and vinegar monthly. Use enzyme products regularly to digest organic waste.
These sit inside or over the plughole to catch hair and debris before it enters the drainage system.
While kitchen grease is a known issue, some bath products contain oils that can cling to pipes. Rinse well and use less oil based products if smells return.
Open a window or use an extractor fan to remove excess moisture. Damp rooms speed up bacterial growth in pipes and overflow openings.
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Use rubber gloves and safety glasses. Always ventilate well. Keep a torch, wire brush, cup plunger and zip tool for regular use.
Yes, in many cases. Bicarbonate of soda and vinegar clean without corroding pipework. They are safe for weekly or monthly use.
Use thick bleach or drain gels only for stubborn smells that do not respond to natural methods. Avoid overuse as these products can damage seals and older pipes.
If smells persist despite cleaning or come from multiple drains, the issue may be in the main waste line or vent system. Leaks, gurgling sounds or backflow also suggest deeper problems.
You can arrange a same day inspection with 24hrs Drainage who operate across the UK and offer CCTV drain surveys and full diagnostics.
| Smell | Common cause | What to do |
|---|---|---|
| Rotten egg | Dry P trap, blocked vent | Run tap, inspect vent pipe |
| Musty | Mould or damp | Clean overflow, use bleach or enzymes |
| Sweet or sour | Waste build up | Use bicarb and vinegar, clean plughole |
Why does the smell return even after cleaning?
There could be hidden mould or a partially dry trap. Run water weekly and clean overflow areas too.
What’s the best method to clean naturally?
Bicarbonate of soda with vinegar is reliable and safe for most homes. Flush with hot water afterwards.
Will bleach damage my pipes?
If used frequently, yes. Use occasionally and never mix with other products.
What are the small flies near my sink?
Drain flies are signs of stagnant water and decomposing waste inside the pipe. Clean thoroughly to remove breeding ground.
Can poor airflow affect smells?
Yes. Bathrooms without ventilation allow moisture to build up which supports mould and bacterial growth.
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