Drainage systems are designed so sewer gas can only leave through the vent above your roof. If you can smell it inside, something in that sealed system has been bypassed. Air freshener and drain cleaner treat the symptom; they don't close the gap.
1. A dried-out trap
Every fixture has a water-filled trap holding gas back. In rarely used spare bathrooms, laundry tubs and floor wastes, that water evaporates in weeks — especially over a hot Illawarra summer. Run the tap or pour a bucket of water into the floor waste and the smell usually disappears within the hour. This is the single most common cause we find.
2. A blocked or restricted vent
If the vent pipe on the roof is blocked by leaves, a nest or a collapsed cowl, the system can't breathe. Instead of drawing air through the vent, a draining fixture pulls water out of nearby traps, and gas pushes through. The giveaway is a gurgling sound from a basin or floor waste while a nearby fixture drains, followed by a smell.
3. A partial blockage in the line
Waste and grease sitting in a partly blocked drain generate odour on their own and disrupt the airflow that keeps traps sealed. Slow drainage plus smell nearly always means a partial blockage — CCTV and jetting resolve both.
4. Failed seals at the toilet pan
The pan connector seal between the toilet and the drain can perish or crack, particularly on older suites and after a bathroom renovation where the pan was reset. Gas escapes at floor level around the base of the pan. Silicone around the outside doesn't fix it — the connector needs replacing.
5. The overflow relief gully outside
The gully near the house is a safety device: it releases a sewer surcharge outside rather than inside. If it's set too low, buried under garden mulch or has a damaged grate, you get odour around the house and sometimes indoors through nearby windows. It must sit above the lowest indoor fixture and stay clear.
6. Shower and floor waste build-up
Hair, soap and skin build a biofilm inside the shower waste that smells sour rather than sewer-like. Clearing the waste properly and flushing the trap fixes it. Persistent odour after cleaning points to a waste that isn't trapped correctly — common in older conversions.
7. Pipework faults and cracked drains
Cracked earthenware, an open disused connection left uncapped during a renovation, or a broken vent inside a wall lets gas into the building cavity, where it appears in odd places like a wardrobe or a hallway. Smoke testing and CCTV find these; guesswork doesn't.
A safety note that matters
Sewer gas is not harmless: hydrogen sulphide and methane at concentration are hazardous, and a persistent indoor smell means an ongoing breach. Separately, if the smell is rotten eggs but sharper and you have gas appliances, treat it as a possible gas leak instead — turn the gas off at the meter, ventilate, don't touch switches, and call us immediately.
How we trace it
We check every trap and floor waste, water-test the dry ones, inspect the vent and overflow relief gully, CCTV the line for partial blockages and cracks, and where the source is hidden in a wall or subfloor we smoke-test the drainage to make the escape point visible. You get a written scope naming the exact failure and the repair, not a bottle of drain cleaner.
Frequently asked questions
- Why does my bathroom smell like sewer?
- Usually a dried-out trap in a rarely used fixture or floor waste, a blocked roof vent, a partial blockage in the drain, or a failed toilet pan connector seal. Each has a distinct pattern of symptoms a plumber can identify in one visit.
- Is sewer smell in the house dangerous?
- It shouldn't be ignored. Sewer gas contains hydrogen sulphide and methane, and a persistent indoor smell means gas is escaping the sealed drainage system through a fault that needs fixing.
- Why do my drains gurgle and then smell?
- Gurgling means the system can't draw air through the vent, so it siphons water out of nearby traps instead. Once a trap loses its water seal, gas comes straight through. The vent or a partial blockage is the usual cause.
- Will drain cleaner fix a smelly drain?
- Rarely, and it can damage older pipework and seals. Odour caused by a dry trap, a blocked vent, a failed pan seal or a cracked drain is unaffected by chemicals.
- Why does the smell come back every summer?
- Traps in unused bathrooms, laundries and floor wastes evaporate faster in hot weather. Pouring a bucket of water into each floor waste monthly prevents it.
- What is an overflow relief gully and why does it smell?
- It's the outside grate designed to release a sewer surcharge away from the house. If it's buried, damaged or set too low it can allow odour around the home, and it must be kept clear to work correctly.
- Do you find hidden drain smells across the Illawarra?
- Yes — CCTV inspection and drainage smoke testing throughout Shellharbour, Shell Cove, Warilla, Albion Park, Oak Flats, Dapto, Kiama and Wollongong.