Why does my drain smell — and how to get rid of it for good
Drain smells almost always have one of six causes. Work through them in order and you'll find yours.
First, identify the smell
A stale, musty, slightly sour smell at a single plughole is biofilm — a bacterial layer inside the waste pipe. Unpleasant, harmless, easy to fix.
A sharp sewage or rotten-egg smell, especially one that comes and goes, means foul air is escaping into the property from the drainage system. That's a fault in the trap seal or the pipework and it needs finding, not masking.
Cause 1 — a dry trap
Every fixture has a water-filled trap that seals foul air out. In a spare bathroom, an unused utility sink or a shower that's been out of use, that water evaporates in a few weeks and the seal is gone.
The fix costs nothing: run the tap for thirty seconds. In a fixture you rarely use, add a capful of cooking oil after refilling — it floats on the water and slows evaporation for months.
Cause 2 — biofilm in the waste pipe
Soap, fat and skin cells build a slimy layer on the inside of the waste pipe. It smells stale and it returns if you only pour bleach down, because bleach runs straight past it.
Strip and clean the trap, scrub the visible pipe with a bottle brush, then use soda crystals with hot water weekly. Enzyme treatments work well too and are kinder to old pipework than caustic products.
Cause 3 — a blocked or silted outside gully
The gully outside the kitchen takes sink and washing machine waste. When it silts up with fat and grit, the trap under the grid stops working and you get a strong smell near the back door, worse in warm weather.
Lift the grid, scoop the silt out by hand, and flush with hot water and soda crystals until you can see the water level moving through the trap properly. If it refills and doesn't drain, the gully outlet is blocked and needs clearing.
Cause 4 — a damaged or displaced underground pipe
A cracked pipe, an open joint or a collapsed section lets foul air — and often water — escape into the ground. The smell is usually strongest outside, near a particular part of the garden, and often comes with damp patches, lush grass over the drain run, or rat activity.
This is the serious one. A CCTV survey identifies the defect and grades it, so you repair the actual fault rather than guessing. Patch lining fixes most localised defects without digging.
Cause 5 — a failed air admittance valve
Where a soil stack terminates inside — in a loft or a boxed-in cupboard — an air admittance valve lets air in but should keep foul air from coming out. They stick open as they age.
Symptom: a sewage smell in an upstairs cupboard, loft or bathroom that gets worse when someone flushes elsewhere in the house. Replacing the valve is a quick, cheap job.
Cause 6 — rats in the drain
Rats get into drainage through defective pipework and disused connections. Scratching sounds, a persistent smell and droppings near the drain are the signs. The smell here is a symptom, not the problem — the defect that let them in is.
A CCTV survey finds the entry point and a drain seal or repair closes it. Baiting alone doesn't solve it because the hole is still there.
What not to do
Don't mask it with air fresheners while the cause continues — a foul-air leak inside a building shouldn't be lived with. Don't pour caustic soda down repeatedly; it won't touch biofilm on the pipe wall and it damages older pipework and seals.
And don't assume it's yours to fix. If the smell traces back to a shared drain or a run past your boundary, that's usually the water company's, and they'll investigate free. Our free drain responsibility checker will tell you in about thirty seconds.
Getting it diagnosed
We investigate drain odours with cameras and, where needed, smoke testing to trace exactly where foul air is escaping. You get a written report and a plain answer on what needs doing.
DrainFixer is a small family-run firm covering East Grinstead and 20 miles around, 24 hours a day. Call 0330 1331 347.
Frequently asked questions
Why does my drain smell like rotten eggs?
A sulphur or rotten-egg smell means foul air is escaping the drainage system — usually a dried-out trap, a silted gully, a failed air admittance valve or a cracked underground pipe. Start with the traps, then check the outside gully.
Why does my drain smell worse in hot weather?
Heat accelerates bacterial activity in biofilm and silt, and speeds evaporation of trap seals in unused fixtures. A gully or trap that's borderline all year becomes obvious in summer.
Does bleach get rid of drain smells?
Rarely for long. Bleach runs straight past the biofilm coating the pipe wall. Soda crystals with hot water, or an enzyme treatment, break the film down properly — and neither damages older pipework the way caustic products can.
Could a drain smell mean a broken drain?
Yes. A persistent outdoor sewage smell with damp patches, unusually lush grass over the drain run or rat activity often means a cracked or collapsed pipe. A CCTV survey confirms it and grades the defect for repair.