How do I get rid of a smelly drain?

Drain smells have three common causes and none of them is a blockage. Biofilm coats the inside of the plughole and overflow; a dry trap lets drain gases up; and decomposing material in the trap produces its own smell. Each has a different fix.

What you'll need

• Bottle brush or old toothbrush
• Hot water and detergent
• Bicarbonate of soda and vinegar (optional)
• Gloves

Step by step

1. Check whether the drain is actually slow as well as smelly.Slow and smelly means a blockage. Draining normally but smelly means biofilm or a dry trap.
2. For a rarely-used sink or shower, run the water for a full minute.The U-bend holds a water seal that blocks drain gases. It evaporates if unused, and refilling it fixes the smell immediately.
3. Scrub the plughole itself with a brush and hot detergent.The underside of the grate and the first few centimetres of pipe hold a slimy biofilm. This is the most common cause and the most overlooked.
4. Unscrew and clean the overflow.The overflow connects to the waste just below the plughole and grows its own biofilm. A bottle brush reaches it.
5. Flush with a kettle of hot — not boiling — water and washing-up liquid.Detergent cuts the grease that holds the biofilm. Boiling water can distort plastic traps.
6. For maintenance, use bicarbonate of soda followed by vinegar, then flush.Effective as routine maintenance on light build-up. It will not clear an established blockage.
7. If the smell persists, remove and clean the trap.Bucket underneath, unscrew the collars by hand. Expect grey sludge in a kitchen and hair in a bathroom.
8. For a smell affecting the whole house, check the soil vent pipe.A blocked or damaged vent pipe causes drain smells throughout and is not a sink-level problem.

Tips

• Kitchen drains smell mainly from fat. Scraping and wiping pans before washing prevents most of it.
• Running water down rarely-used drains monthly keeps the trap seal intact and prevents the smell entirely.
• Avoid caustic drain cleaners for smell — they are for blockages, and they leave a hazardous liquid in a pipe you may need to open.

Common mistakes

• Assuming a smell means a blockage — Most drain smells are biofilm or a dry trap, and clearing a blockage that does not exist achieves nothing.
• Forgetting the overflow — It joins the waste below the plughole and grows its own biofilm. It is the most commonly missed part.
• Pouring bleach down for the smell — It does little to biofilm, and it must never be combined with any acidic cleaner already in the drain.

Questions people ask