Why Does My Drain Smell?

You walk past the laundry and catch it. A sour, eggy smell that wasn’t there last week. You pour some bleach down, it fades for a day, and then it’s back.

Drain smells get worse as Sydney warms up, which is why we field so many of these calls in spring. Here are the five real causes, roughly in order of how often we find them.

1. A dry P-trap

Under every fixture is a U-bend that holds a small amount of water. That water is a seal — it physically blocks sewer gas from rising up the pipe into your home. It is the simplest and most elegant part of your plumbing.

If a fixture goes unused, that water evaporates. The seal breaks, and the drain starts breathing sewer gas into the room. Spare bathrooms, laundry tubs, floor wastes and holiday homes are the classic offenders.

The test: run the tap for thirty seconds. If the smell disappears within the hour and stays gone, that was it. For a floor waste with no tap above it, tip a couple of litres of water down instead.

2. Grease, soap and hair coating the pipe

This is the most common cause in a drain that gets used daily. Fat, soap scum, shampoo and hair build a film on the inside of the pipe. The drain still runs, so nothing looks wrong — but that film is organic matter, and bacteria feed on it.

Kitchen sinks and shower drains are the usual sites. The smell is less sewer-like and more stale or sour.

The test: if the smell is strongest right at the plughole and you can see dark gunge on a bent wire or an old toothbrush, you’ve found it. A kettle of hot (not boiling) water with a squirt of dish soap, poured slowly, shifts a surprising amount. Boiling water can damage PVC joints and crack older fittings, so keep it just off the boil.

3. A partial blockage holding waste in place

When something narrows the pipe, waste stops flowing away cleanly and sits there rotting. The smell arrives before the blockage becomes obvious, which makes it a useful early warning.

Look for the smell alongside slow drainage, gurgling after the washing machine empties, or water rising slightly before it clears. Those three together mean a blockage is forming. Our guide on what actually causes blocked drains covers the usual culprits.

Clearing a partial blockage properly means removing the buildup from the pipe wall, not just punching a hole through it. That is what high-pressure jet blasting does — it scours the pipe back to its full diameter, which is why the smell stays gone instead of returning in a month.

4. A blocked vent pipe

Your drainage system needs air. Vent pipes run up through the roof so that when water goes down, air can follow it — otherwise the falling water siphons the seal straight out of your traps.

Block that vent with leaves, a nest or a possum, and two things happen: traps get sucked dry, and sewer gas finds its way back through the fixtures.

The clue: several drains smelling at once, and gurgling from one fixture when you use another — flush the toilet and the basin burbles. That pattern points up at the roof, not down at the plughole, and it is a plumber’s job.

5. A damaged sewer line

The least common cause, and the one that matters most. A cracked or displaced pipe under the house or yard lets sewer gas escape into the soil and up through the slab, floorboards or garden.

Signs worth taking seriously: a smell outdoors near the same patch of ground, a lawn area that stays soggy or unusually green, recurring blockages in the same line, or a smell inside that no amount of drain cleaning touches.

This needs a camera down the line to confirm. Guessing is expensive — see our guide on relining versus replacement for what the repair options look like once the fault is found.

What to try first, in order

1. Run every tap for thirty seconds, including the ones you never use, and tip water down any floor waste. Wait a few hours. This alone solves a good share of calls.

2. Clean the plughole. Pull the grate, clear the hair and gunge you can reach, and flush with hot soapy water.

3. Note the pattern. One drain or several? Any gurgling? Slow drainage? Worse after the washing machine runs? Write it down — it genuinely shortens the diagnosis.

4. Skip the chemical drain cleaner. It rarely fixes a smell, it damages older pipes, and it makes the drain hazardous for whoever opens it next.

If the smell survives all that, the cause is inside the pipework rather than at the opening, and it needs someone with a camera and a jetter.

FAQs

Why does my drain smell worse in summer?

Warmth speeds up the bacteria feeding on the film inside your pipes, so the same buildup smells far stronger. Water in unused traps also evaporates faster, so a drain that behaved all winter can start letting gas through once the weather turns.

Is a smelly drain dangerous?

An occasional whiff from a dry trap is a nuisance, not a hazard. A persistent sewer smell means gas is entering the house where it should not, which can point to a damaged pipe or a venting fault. Strong, constant, or paired with gurgling and slow drainage — get it looked at.

Will bleach fix it?

Rarely, and it can make things worse. Bleach runs straight past the grease film where the smell actually lives, and does nothing at all for a dry trap, a blocked vent or a cracked pipe. Hot soapy water and a physical clean address more causes for less risk to your pipes.