A trap that has dried out
Every waste fitting holds a small amount of water that seals the pipe. A spare room basin, a utility sink or a floor gully that nobody uses will evaporate that seal in a few weeks, and the drain then vents straight into the room.
Run every tap and flush every toilet in the property, including the ones you never use, then wait a day. If the smell goes, that was it.
A trap being siphoned empty
If a smell appears just after the washing machine empties or someone upstairs runs a bath, the surge of water is pulling the seal out of another trap. You will often hear a gurgle at the moment it happens. That is a venting fault rather than a blockage, and it needs the pipework looking at.
A partial blockage holding waste
Waste sitting in a pipe that should be running clear will smell, particularly in warm weather. The clue is a smell that comes with slow drainage rather than on its own. Clearing the run removes the source.
A defect in the buried pipework
- A cracked pipe or open joint under a floor or a path.
- A chamber with a broken or missing cover seal.
- A redundant pipe that was capped badly during past work.
- A soil vent terminating too low or too near a window.
When to stop guessing
If the smell survives running every outlet and there is no slow drainage to explain it, the source is very likely below ground or inside a wall. A camera survey finds it without opening anything up, and the footage shows exactly where the defect sits so a repair can be priced properly.
Questions we get asked
Why does the smell only appear at night?
Cooler air and a still house make it easier to notice, and if nothing has run for hours a weak trap seal has had time to drop.
Will bleach get rid of a drain smell?
Only briefly, because it masks rather than removes the cause. If bleach works for a day and the smell returns, something structural or a lost water seal is behind it.
Is a drain smell harmful?
Persistent sewer gas indoors is unpleasant and should not be ignored, so it is worth tracing rather than living with.
Can you find the source without digging?
Almost always. A CCTV survey follows the run and shows defects, standing water and broken connections without excavation.