Drains
Why does my drain smell?
A smelly drain usually means one of four things: a dried-out trap letting sewer gas into the room, a build-up of biofilm and food waste in the pipe, a broken or missing seal around the drain, or a fault in the sewer line itself. Running water, cleaning the waste and checking seals fixes most cases within minutes.
That smell coming up from the drain has one of four causes, and you can narrow it down in about five minutes. Terry gets the smelly drain call every week somewhere in Western Sydney, and more than half the time the fix costs nothing at all.
1. A dried-out trap
Every drain in the house has a water trap, the U-shaped bend that holds a plug of water and blocks sewer gas from rising into the room. If a drain is rarely used, that water evaporates. Guest bathroom showers, laundry floor wastes and second toilets are the classic offenders, especially after a hot Sydney summer or a few weeks away on holidays. The fix is pouring a jug of water down the drain to refill the trap. A splash of cooking oil on top slows future evaporation. If the smell disappears within a day, that was the whole problem.
2. Biofilm and gunk
Kitchen sinks and bathroom basins grow a slimy coating of soap, grease, skin cells and bacteria on the pipe walls. It smells musty or faintly rotten, and it is strongest when water first runs. The test: put your nose near the plughole itself. Clean the trap and the first section of pipe, scrub the plughole and the overflow opening with a bottle brush, and flush with hot water and detergent. Bicarb and vinegar freshen things up but do not remove a thick film, mechanical cleaning does.
3. A broken or missing seal
Sewer gas can also leak around a drain rather than through it. Common spots are the rubber seal where a toilet pan meets the floor, cracked trap nuts under sinks, and floor wastes where the puddle flange has failed. The clue is a smell that persists even when the trap is full of water, often stronger in one corner of the room or at floor level. These fixes involve pulling fittings apart and resealing them, so most people hand this category to a plumber.
4. A sewer problem underneath
This is the one that matters. A persistent sewage smell inside or outside the house, especially combined with gurgling drains, slow fixtures or damp patches in the yard, points to a partial blockage or a cracked sewer line letting gas and sometimes wastewater escape. A blocked vent pipe causes similar symptoms: the sewer needs to breathe through the vent on your roof, and when it cannot, it pulls air back through the water traps and drags smells in with it. An overflow gully outside that smells constantly is another flag that the line below is not moving properly. This category needs a camera inspection to diagnose, because guessing at underground problems is expensive.
Work through the list in that order, cheapest first. Refill the traps, clean the waste, sniff around the seals, and only then start thinking about the sewer. Most smells never make it past step one. If yours does get to the sewer stage, Terry is happy to give advice over the phone for free before anyone commits to anything.
Frequently asked questions
- Why does my drain smell like rotten eggs?
- The rotten egg smell is hydrogen sulfide, produced by bacteria in sewage and decomposing organic gunk. It usually means sewer gas is getting into the room, most often through a dried-out trap in a rarely used drain. Pour a jug of water down the offending drain first. If the smell survives a full trap, look at seals or the sewer line.
- Why does my floor waste smell when it never gets used?
- Because it never gets used. The water in the trap below the grate evaporates over a few weeks, especially in warm weather, and once the seal is gone sewer gas flows straight up. A jug of water restores the seal instantly, and a teaspoon of cooking oil floated on top slows the next evaporation right down.
- Will bleach get rid of a smelly drain?
- Bleach masks the smell for a day or two but does not remove the biofilm causing it, and it should never be mixed with other cleaners in a confined pipe. A bottle brush, hot water and detergent do the actual work. If a cleaned drain still smells, the source is a seal or the sewer, and no amount of bleach reaches either.
- Is sewer gas in the house dangerous?
- At the levels found in a home with a dry trap it is unpleasant rather than acutely dangerous, but it is not something to live with. In confined spaces, concentrated sewer gas can be harmful and even flammable, and a persistent smell often signals a blockage or broken pipe developing. Treat it as an early warning and fix the entry point.
- Why is the drain smell worse in summer?
- Three reasons: traps evaporate faster in the heat, bacteria producing the odour are more active in warm pipes, and warm air rising through the house draws more air up from the drains. Sydney summers dry out unused floor wastes in as little as two to three weeks, so holiday absences often end with a smelly laundry.
Need a hand from a real plumber?
Terry's been sorting plumbing across Berala and Western Sydney since 1986. Call 0415 274 542 for a straight answer and a fixed price.
NSW Licence L10959 · 40+ years experience