Every drain in a building has a trap holding a small plug of water, and that water is the only thing between you and the sewer. When you smell sewer gas indoors, something has broken that seal — and finding which one is a process of elimination rather than a guess.
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How a contractor actually diagnoses it
- They ask which room, and whether the smell is constant or intermittent. Intermittent usually means a trap; constant usually means a vent or a break.
- They check for unused fixtures. A guest bathroom or floor drain that has not run in weeks has an evaporated trap.
- They check the vent stack. A blocked or frozen vent pulls traps dry through siphonage every time water runs.
- If neither explains it, they camera the line looking for a break under the slab or in a wall.
Three ways this actually fails
A dry trap
The most common and the easiest. Pour a couple of litres of water down the unused drain and wait a day.
A blocked or failed vent
Without air entering the system, draining water siphons the traps empty. Symptom: the smell arrives right after using another fixture.
A broken line under the slab or in a wall
Constant smell, no obvious source, sometimes with a damp patch or unexplained high water use. This is the expensive one and the one a camera is for.
Signs this is urgent rather than annoying
- A constant sewer smell with no dry-trap explanation
- Smell accompanied by damp flooring, a warm spot on a slab, or unexplained water use
- Any smell of gas rather than sewage — that is a different emergency, leave the building and call the gas utility
What it costs
Published third-party ranges shown so you can sanity-check a quote. Not our prices, not an estimate, not a quote.
| Job | Published national range | Source |
|---|---|---|
| Drain inspection | $125–$500 (camera) | HomeGuide |
| Standard drain cleaning | $147–$352 | Angi |
| Call-out / trip fee | $50–$200 | HomeGuide |
Published national ranges, shown for educational context only. These are not quotes and they are not our prices. Sources disagree with each other — that spread is real. Only the contractor who sees your line can price your job.
Full cost guide, by fixture and by method →
Questions people ask
How do I find drain odour diagnosis near me?
Call the number on this page and give us your town or ZIP code. We match it against partner coverage and connect you with an independent licensed contractor who works that area. We do not charge you anything — the contractor sets their own price.
Why does my basement smell like sewage?
Most often a floor drain whose trap has evaporated because nothing has run into it for months. Pour a bucket of water down it and see whether the smell goes within a day. If it does not, the next suspects are the vent and the line itself.
What is a dry trap?
The U-bend under every fixture holds water that blocks sewer gas. If the fixture is unused, that water evaporates and the barrier disappears. It is the single most common cause of indoor sewer smell and it costs nothing to rule out.
Why does the smell come and go?
Intermittent smell usually means a trap is being siphoned empty when another fixture drains — which points at the vent, not at the drain you can smell.
Can a smell mean a broken pipe under the floor?
Yes, and that is the case worth taking seriously. A constant smell with no dry trap and no vent fault, particularly alongside damp flooring or a warm spot on a slab, justifies a camera.
Is sewer gas dangerous?
It is unpleasant at low concentrations and hazardous at high ones, and it can contain methane and hydrogen sulphide. Persistent indoor sewer gas is worth resolving rather than tolerating.
Will bleach fix it?
No. Bleach masks the smell for hours and does nothing about the broken seal producing it.
What about a smell only from the kitchen sink?
That is usually organic build-up in the disposal or the tailpiece rather than sewer gas — different smell, different fix, much cheaper.