If the drain smells like sewage but the water goes down fine, you do not have a blockage. You have a broken seal. Every drain in your house holds a small amount of water in a U-bend, and that water is the only thing separating your rooms from the sewer. When the smell arrives, the seal has gone.
Which is why good drainage is not the reassurance it feels like. A blockage restricts water. A lost trap seal restricts nothing at all. It just opens a path.
Quick answer
- Rarely used fixture: the trap has dried out. A bucket of water fixes it today.
- Fixture you use daily: something is pulling the seal out. Usually the vent.
- Kitchen sink, sour rather than sewage: biofilm under the grate, not the pipe.
- Rotten egg smell away from any drain: stop and read the gas section below first.
First, Rule Out a Gas Leak
This section is short, and it goes first for a reason.
Natural gas has no smell of its own, so an odorant is added to it deliberately, and that odorant smells like rotten eggs or sulphur. Sewer gas contains hydrogen sulphide, which smells like rotten eggs or sulphur.
They are genuinely easy to confuse, and the consequences of getting it wrong run in one direction only.
Treat it as a possible gas leak if: the smell is not strongest at a drain, you notice it away from the kitchen or bathroom, it is present throughout the house, or it appears near a gas appliance, meter or hot water unit.
If any of that fits: do not use light switches or anything electrical, open doors and windows, get everyone outside, and call the gas emergency line. Do not investigate it yourself.
If the smell is clearly strongest at a specific drain and disappears a metre away from it, it is a plumbing problem, and the rest of this page applies. For anything gas-related afterwards, that is licensed gas fitting work.
What the Trap Actually Does, and Why Yours Stopped
Under every fixture there is a bend in the pipe that stays permanently full of water. Waste passes through it, air does not, and sewer gas cannot get past it. It is not a comfort feature. It is the only barrier there is.
A trap fails in three ways: the water evaporates, something siphons it out, or the pipe has a fault letting gas in somewhere other than the trap. Each has a different fix.
Cause One: The Trap Has Dried Out
The most common cause by a wide margin, and the cheapest to fix.
Trap water evaporates. In a fixture used daily that never matters, because it is topped up constantly. In one used rarely, it does. A guest bathroom, a laundry tub, a floor waste in a room nobody enters, a holiday period with the house empty. A Sydney summer accelerates it considerably.
The fix takes two minutes. Pour half a bucket of water down every drain in the house you do not use weekly, including floor wastes. Then do it again in a fortnight. If the smell goes and stays gone, that was it.
If you have found the culprit, it is worth making it routine. A drain that dried out once will dry out again on the same schedule.
Cause Two: Something Is Siphoning the Seal Out
If the smell is at a fixture you use every day, evaporation is not the explanation. Something is actively pulling the water out of the trap.
That is almost always a venting problem. Every drain needs air to move as freely as the water does, and a vent pipe running through the roof is what allows it. When the vent is restricted by leaves, a nest, or a renovation that capped or built over it, water draining elsewhere creates suction that equalises by dragging water out of the nearest trap.
The tell is that it comes and goes: the seal gets pulled down, refills next time you use the fixture, gets pulled down again.
There is usually a noise attached to it before the smell arrives. If you also hear gurgling, a shower that gurgles but drains fine is the same fault a stage earlier, and what the gurgling means depends on which fixtures are involved.
Cause Three: The Smell Is Not Coming Through the Trap at All
If the traps are full and the smell persists, gas is entering somewhere else.
A cracked or damaged pipe in a wall cavity, under a floor or beneath a slab. The smell often seems to come from the drain because that is the nearest opening, but it is escaping upstream of it.
A missing or improperly fitted trap on a washing machine standpipe or a fixture added during a renovation. It happens more often than it should, and it produces a constant smell that never improves no matter how much water you pour.
A perished seal at the pan connector behind a toilet. Common in older homes and often mistaken for a bathroom that simply smells.
None of these can be diagnosed from above the floor. A camera run locates the fault and marks the depth and distance, which is what turns it into a drain repair you can actually price.
The Kitchen Sink Exception: Sour Is Not Sewage
Kitchen sinks get their own section because they are usually a different problem wearing the same description.
A sour, stale, food-waste smell is organic build-up. Fat and food residue coat the inside of the waste, the underside of the grate and the rim of the overflow, and bacteria do the rest. It is unpleasant. It is not sewer gas.
Clean the grate underside and the plug hole physically, then clear the trap below it. Pouring things in will not remove a coating; you have to scrub.
If your kitchen smells and also makes noises when the dishwasher runs, the two are related, and what the gurgling means there is worth reading.
Poured water down everything and it still smells?
Call R&G on 0420 146 747. That rules out the cheap explanation, which means something is either pulling the seal out or getting past it. $0 call-out fee, fixed price before any work starts.
Is Sewer Gas Actually Harmful?
At the concentrations found in a house with one failed trap, it is unpleasant rather than dangerous. Headaches and nausea are the usual complaints, and they resolve when the smell does.
The reason to deal with it is what the smell tells you: there is an open path between your home and the sewer. Cockroaches are the most common thing to accept that invitation, and a persistent drain smell with a cockroach problem in the same room is very often one fault rather than two.
It also tends not to stay still. A vent problem that empties one trap will empty others, and the smell spreads room by room while everything continues draining perfectly.
Frequently Asked Questions
Why does my drain only smell sometimes?
Intermittent smell points at a seal that keeps getting pulled down and refilled, which means a venting problem rather than evaporation. It often correlates with using a different fixture, so notice whether it follows a shower or a washing machine cycle.
Will bleach or drain cleaner fix a smelly drain?
It masks it for a day or two. If the cause is a dry or siphoned trap, there is nothing for a chemical to act on, because the pipe is clear. If it is kitchen biofilm, the coating needs removing physically rather than dissolving in place.
Why does it smell worse in summer, or after rain?
Heat speeds up evaporation from traps and makes gases more noticeable. After rain, a line taking in stormwater through a crack can push air back through the system. Both are worth mentioning to a plumber, because they narrow down the cause.
Can I just keep pouring water down it?
If it is a rarely used fixture, yes, and that is a perfectly reasonable permanent answer. If you are topping up a drain you use every day, you are managing a symptom on a schedule, and the underlying fault is still there.
Find out what the smell is actually telling you
If a bucket of water fixes it, we will tell you that on the phone and it costs you nothing. If it does not, something is pulling your trap seals out or getting past them, and that is worth finding properly. $0 call-out fee, fixed price before any work starts, across the North Shore, the Hills and Western Sydney. See how we handle blocked drains across Sydney.
