A persistent sewer smell in or around your home is unpleasant, potentially unhealthy, and often misdiagnosed. Many homeowners assume they need their drains relined to fix the odour. Sometimes that’s true, but often the source is something simpler. This guide walks through the common causes of drain odour, which ones drain relining fixes, and which need a different approach.
The quick answer
Drain relining fixes drain odour when the smell is caused by a cracked or damaged pipe wall leaking sewer gas into the ground or into a sub-floor space. It doesn’t fix odours caused by dried drain traps, broken floor waste fittings, blocked vents, or behaviour (what goes down the drain). Correct diagnosis is essential, a CCTV inspection and sometimes smoke testing are the right diagnostic tools.
Where sewer odour comes from
Sewer gas is primarily hydrogen sulphide (H₂S), produced by anaerobic bacteria breaking down organic waste. It smells like rotten eggs at low concentrations and is detectable at very low parts-per-million. At high concentrations it’s toxic, but residential drain odour is almost always at trace levels, making it unpleasant rather than dangerous.
The gas exits the sewer system wherever there’s an opening. Normally, it’s controlled by:
- P-traps in every fixture (the water seal prevents gas coming up through the drain)
- Vent pipes on the roof that release gas to atmosphere
- Sealed pipe joints that don’t allow gas to escape underground
When any of these three systems fail, odour problems result.
Odour causes that drain relining DOES fix
1. Cracked or broken pipe body leaking sewer gas
If a sewer pipe has a crack that allows sewer gas to escape into the surrounding soil, that gas can migrate through the soil and enter buildings through:
- Sub-floor spaces (in older homes on stumps)
- Cracks in concrete slabs adjacent to the pipe
- Utility penetrations through slabs or walls
- Sub-floor vents
A relined pipe seals the crack permanently. Sewer gas can no longer escape. This is the clearest case where drain relining directly fixes an odour problem.
2. Joint deterioration allowing sewer gas egress
Deteriorated mortar joints in terracotta or AC cement pipes allow gas to escape in the same way as a crack. A continuous liner spanning all joints eliminates all gas escape points.
3. Odour from root intrusion in a blocked or partially blocked pipe
If roots have created a partial blockage in a sewer pipe, sewage accumulates behind the blockage. Anaerobic decomposition in stagnant sewage produces elevated H₂S. The gas can then escape through any joint gap and migrate to surface. Relining (after clearing the roots) addresses both the blockage and the joint integrity.
Odour causes that drain relining DOES NOT fix
1. Dried P-traps
Every drain fixture, basin, sink, bath, shower, floor waste, has a P-trap that maintains a water seal. If a drain hasn’t been used for weeks (common in holiday homes and spare bathrooms), the water seal evaporates and sewer gas can come up through the drain.
Fix: Run water through every drain to refill the trap. Add trap primer to floor wastes in infrequently used areas.
2. Blocked or inadequate vent pipes
Sewer vent pipes run up through the roof and release sewer gas above roof level. If a vent is blocked (birds nesting in it, debris, tree contact), gas backs up into the drain system and can find exit points indoors.
Fix: Clear or replace vent pipe. Sometimes addition of an air admittance valve is appropriate.
3. Cracked or broken fixture components
A cracked toilet pan, broken floor waste grate seal, or damaged shower drain fitting allows gas to escape at the fixture rather than through the pipe. These are localised fixture problems, not pipe problems.
Fix: Replace the fixture component.
4. Grease and organic build-up in kitchen drain lines
Grease-lined kitchen drain pipes generate odour from the anaerobic decomposition of organic material on the pipe walls. This is especially common in drain lines that aren’t regularly flushed with hot water.
Fix: Hydro jet cleaning of the kitchen drain lines. Relining a grease-laden drain without thorough cleaning first doesn’t solve the odour issue.
5. Odour from the overflow relief gully (ORG)
An ORG without a functional water seal or with a dried-out trap can allow sewer gas to escape to the yard.
Fix: Check and service the ORG. See our ORG guide.
Diagnostic flowchart for drain odour
| Question | Yes answer | No answer |
|---|---|---|
| Smell only in specific room? | Localised fixture, trap or vent issue | Proceed to next question |
| Smell after weeks of non-use? | Dried trap, run water | Proceed to next question |
| Smell outside near drain line? | Pipe crack/joint failure, CCTV | Proceed to next question |
| Smell only after rain? | Stormwater infiltration or ORG issue | Proceed to next question |
| Smell in sub-floor space? | Pipe crack under house, CCTV | Other sources, check appliances |
The diagnostic tools: CCTV and smoke testing
CCTV inspection
A CCTV inspection maps the pipe interior and identifies cracks, joint gaps and root intrusion. It won’t tell you if a particular joint is leaking gas, but it will show whether the structural conditions for gas leakage exist.
Smoke testing
Smoke testing is the definitive diagnostic tool for sewer gas leaks. A smoke-generating machine is connected to the sewer system at one access point. Harmless white smoke is pumped into the pipes under light pressure. Smoke exits wherever the system has a breach, including cracked pipes, faulty fittings, failed seals and blocked vents. See our smoke testing drain leaks guide for more.
When is relining the right answer for odour?
Book drain relining for odour problems when:
- CCTV confirms cracks or severely deteriorated joints in the sewer pipe
- Smoke testing has confirmed that the sewer pipe itself (not fixtures or vents) is the smoke escape point
- The property is older than 30-40 years and the pipe material is clay or AC cement (high probability of joint deterioration)
- The odour is in the yard or sub-floor space above the pipe line (indicating external pipe gas egress)
Don’t book relining for odour problems until you’ve ruled out the simpler causes (dried traps, vent issues, fixture problems). These can be checked and resolved quickly and cheaply.
Odour management in holiday homes on the Central Coast
Holiday properties that sit idle for months between lets are particularly prone to trap-drying odour problems. When the property is vacant:
- Toilet and basin traps lose their water seal over 3-4 weeks of non-use
- Floor waste traps (in laundries, shower areas) can dry in as little as 1-2 weeks
- The ORG in the yard may also lose its seal
Before guests arrive, run every tap and flush every toilet. For properties that sit vacant for extended periods, trap primer devices can be installed to maintain the water seal passively.
Cost guide for odour diagnosis and repair
| Service | Typical cost |
|---|---|
| Drain CCTV inspection | $250, $500 |
| Smoke testing (residential) | $350, $700 |
| P-trap replacement (fixture) | $150, $400 |
| Vent pipe clearing | $200, $500 |
| ORG service | $200, $600 |
| Hydro jet cleaning (odour-causing build-up) | $250, $500 |
| Drain relining (if confirmed as fix) | $3,500, $9,000 |
Frequently asked questions
I can smell sewer gas but only at night, why? Temperature differentials between day and night affect sewer gas movement through soil. Gas tends to concentrate in low-lying areas and rise when the soil warms. If you notice the smell more at night or early morning, it’s consistent with a ground-level gas escape point (cracked pipe or joint gap).
My plumber jetted the drains and the smell improved for a while, then came back, why? Jetting temporarily removes the build-up of material that was generating odour. But if the underlying issue is a cracked pipe leaking gas, jetting doesn’t fix that. The build-up returns, and so does the smell. CCTV to check the pipe structure is the next step.
Can drain relining in one section fix an odour coming from another section? Only if the relined section is where the gas escape is occurring. Relining is targeted at specific defect locations identified by CCTV or smoke test. Relining the wrong section won’t fix the odour.
Is sewer gas from drain pipes a health hazard? At the concentrations typically experienced in residential drain odour situations, sewer gas is unpleasant but not hazardous. Very high concentrations (e.g., in enclosed spaces with significant sewage decomposition) can be dangerous. If the smell is very strong in an enclosed area, ventilate and seek professional assessment quickly.
Does drain relining eliminate odour inside the pipe? The liner itself doesn’t generate odour. The smooth epoxy surface resists scale and organic build-up slightly better than old clay, which may reduce odour generation over time. But the main odour benefit of relining is sealing the escape points, not eliminating the odour within the pipe.
Have a persistent drain odour you can’t track down? Book a CCTV inspection or smoke test.