Sewage Smell in House? How to Find the Source in SEQ

A sewage smell in house drains is gas from the sewer getting past the water that’s meant to block it. In Brisbane, Logan, Ipswich and Gold Coast homes, the cause is usually a dry trap, the water-filled bend under a fixture nobody’s touched in weeks. Water from the tap refills it in seconds.

Plenty of houses need nothing more. When the smell comes back within a day or two, the toilet seal and the vents are next.

Key takeaways

• Check fixtures nobody uses first, especially floor wastes.
• A toilet seal can leak gas for months with the floor around it bone dry.
• Gurgling plus a smell: a venting fault or a partial blockage in the main line.
• Hot water that smells of rotten eggs is an anode problem inside the unit.
• Write down when an on-and-off smell turns up. Rain and the air con point to different faults.

Where does a sewage smell in house plumbing come from?

Every drain in the house runs to the public sewer, and the air in that sewer would come straight up the pipes if nothing stopped it. What stops it is a few centimetres of water sitting in each trap.

The trap under a basin is the U-bend in the cupboard. Toilets and floor wastes have them too, moulded into the pan or sitting under the grate. Let any of them dry out, and sewer gas comes up into the room.

The vent stops draining water from sucking the traps empty. It also releases the gas above the roofline.

Damaged pipe is the third way in. In older Brisbane suburbs, that often means a cracked earthenware sewer pipe under the house, though it’s rarely the first thing to check.

Is a dry trap behind the smell?

Most of the time. A drain that hasn’t had water through it for a few weeks has lost its water seal and is an open pipe to the sewer.

Nine times out of ten, the search starts here. The guest bathroom, the spare shower and the laundry floor waste are the usual suspects, because nobody thinks to run them.

Of those, the floor waste causes the most trouble. Nothing drains into it during a normal day, so the only water it gets comes from mopping or an overflowing shower, and a sewage smell in bathroom floor wastes can go on for weeks before anyone thinks to look at the grate in the corner. Rental properties sitting vacant between tenants, and holiday units, end up with smelly drains for the same reason, and it tends to be noticed on the first day someone moves back in.

Hot weather shortens the timeline. With an SEQ summer and the air con drying out the house, a trap that would hold for months in winter can be dry in a few weeks.

Gas from a dry trap moves through the house. A laundry floor waste downstairs can be behind a smell that seems strongest in an upstairs hallway.

Before spending a cent on a call-out or a gas detector, run water for 30 seconds through every basin, shower, bath and sink in the house and pour a bucket into each floor waste. Toilets need a flush as well. After that, a fixture that sits unused needs topping up every couple of weeks, not once a year.

If a bathroom drain smells like sewage again two days after it’s been run, evaporation isn’t the problem. Something is pulling the water out.

Why do the drains gurgle when the house smells?

Gurgling is air being dragged through a trap. Poor venting does it, and so does a partial blockage in the main line.

A flush in one toilet can siphon the trap in another bathroom, and then that bathroom smells even though nobody has used it. The reverse is common as well. A gurgling toilet while the shower drains is one example.

Where the venting has gone wrong, it’s normally one of these:

• Leaves or a bird’s nest blocking the roof vent. The gas comes back down and shows up as a sewer smell in house ceilings and cupboards.
• An air admittance valve stuck open under a sink. These one-way valves get fitted where a vent can’t go through the roof, and a jammed one leaks gas straight into the cupboard. Cheap to replace.
• A vent cut short behind new plaster.
• Traps left off or drains run straight during building work.

Renovations cause a lot of this. A sewage smell in house extensions, or in the new downstairs bathroom of a built-in Queenslander, usually traces back to venting done badly. AS/NZS 3500.2 is specific about how fixtures have to be trapped and vented. Roof vents are a working-at-height job for a plumber with the right access gear.

If several fixtures gurgle together, forget the vents for a moment. A partial blockage in the main line pushes gas up through every drain in the house before anything overflows. Smell plus slow drains everywhere means a blockage.

Can a toilet leak gas without leaking water?

Yes. The seal between the pan and the sewer pipe lets gas out long before water.

Once the traps are full and nothing gurgles, check this next. Seals supplied in the box with a new toilet are often thinner than ones bought separately. Some toilets, once lifted, have no seal at all.

When a toilet smells like sewage around its base and the pan rocks slightly as someone sits down, the seal has gone, and every bit of movement breaks it further. A sewage smell in the bathroom air that’s strongest at floor level near the pan points the same way.

Resealing means lifting the pan. That’s a licensed plumber’s job.

What does it mean when the sewage smell in the house comes and goes?

An intermittent smell means gas is only getting out under particular conditions, and those conditions are the best lead on where it’s coming from.

By the time a plumber arrives, it has often cleared. A week’s record of when it happens, including the weather and whether a fan or the air con was running, is the most useful thing to hand over.

Why does the dishwasher or washing machine smell like sewage?

Dishwasher smells mostly come down to how the drain hose was installed.

A dishwasher smells like sewage when wastewater from the sink can run back down its hose and sit in the bottom of the machine between washes. Manufacturers’ installation instructions call for the hose to loop up to the underside of the benchtop before it drops to the sink waste. Installers skip that loop more often than they should, particularly when the hose only just reaches. The fix takes a cable tie. Food caught in the base filter gives off a similar smell and comes out in a couple of minutes.

Front loaders hold water in the rubber door seal, and a load left damp overnight gives them a sour smell. The other reason a washing machine smells like sewage is an outlet hose pushed too deep into the standpipe behind it. Each time the machine drains, the hose siphons that trap dry.

What else can smell like a sewer?

Hot water units, fridge drip trays, basin overflows and overcharging batteries all get blamed on the drains.

Rotten-egg hot water with normal cold water is the anode. That’s a metal rod in the tank that corrodes to protect it, and bacteria react with a spent or wrong-type anode to make the smell. A plumber swaps it in one visit.

Lead-acid backup batteries in older solar and UPS setups give off rotten eggs when they overcharge (a job for an electrician), and the drip tray under a fridge can smell just as bad, so both get reported as a sewage smell in house plumbing.

The overflow on a bathroom basin hardly ever has water through it. Soap and grime build up in the channel behind the hole for years until someone runs a bottle brush and hot water through, and because it doesn’t look like a drain, it gets left off the list when people go hunting for smelly drains. Hair and soap under a shower grate turn into a similar grey slime. In plenty of homes, that slime is the only reason a bathroom drain smells like sewage.

Damp, musty smells are moisture in a wall or floor, and you should go to a leak detection specialist.

When should you call a plumber for sewage smell?

A smell still there a few days after the traps are refilled and the toilet seal is checked needs a camera down the line. With sewage coming up through a floor waste, or every drain in the house slowing at once, ring the same day.

Drain cleaner hides smelly drains for a day or two. The cracked pipe underneath is still there afterwards, and it shows up on a CCTV drain inspection, where a pan-and-tilt camera goes through the pipe and records the whole run. The camera head has a locator built in. Above ground, the drainer traces that signal and marks the exact spot of a crack or root mass before any digging or relining.

A camera can miss gas escaping from a joint that looks intact, and a smoke test covers that. Smoke goes into the drainage system and should only come out of the roof vent. Anywhere else it appears is the leak.

Old cast iron and earthenware crack inside walls and leak gas without any water damage. A sewage smell in house walls with no wet patch still needs testing. Around Logan and Ipswich, the reactive clay moves as it wets and dries, and earthenware joints pull apart with it.

In Queensland, drainage repairs are regulated under the Plumbing and Drainage Act 2018 and have to be carried out by a QBCC-licensed plumber.

Where the smell began straight after plumbing work, the plumber who did that work should be the first call, as something may have been left open or uncapped. A smell the neighbours have noticed as well usually comes from the sewer main, and that’s for the water authority to fix. Urban Utilities covers Brisbane and Ipswich, while Logan Water and the City of Gold Coast look after their own areas.

What if the backyard smells like sewage?

Start with the overflow relief gully. Most houses have one outside, a grated fitting set lower than the floor of the house so that sewage spills into the yard before it can back up indoors if the line blocks. Its water seal dries out the same way an indoor trap does, and a cracked or missing grate leaves the gas nothing to stop it.

Further out, older suburbs have earthenware lines under the yard with roots from mature jacarandas and figs in the joints. Many SEQ backyards also have a public sewer main and maintenance hole running through them. A smell around that is the authority’s to fix.

Still smelling it after the basics?

If every trap is full and the sewage smell in house plumbing hasn’t shifted, we’ll find where it’s getting in. We’ll put a camera down the line and show you the footage before quoting any repair. Book a CCTV inspection or call 1300 453 884 for a same-day call-out across Brisbane, Logan, Ipswich and the Gold Coast.