Bad Smells From Your Drains: Causes and Fixes

The usual suspect: a dry trap

Every fixture holds a water plug in its trap that blocks sewer gas. A shower unused for a fortnight, a washing machine standpipe, a floor gully, any of them can evaporate dry and let the smell in. Run water in every fixture for thirty seconds; if the smell fades in a day, that was it.

Failed seals and vents

A smell that returns when it rains points at a damaged seal or a blocked soil vent pushing gas back through the weakest trap. The wax seal under a toilet, a cracked collar on the stack, or a bird's nest in the vent are the classics. These need finding, not Febreze.

When it's the drain itself

A persistent smell plus slow drainage plus lush patches in the lawn is the signature of a cracked or collapsed run leaking into the ground. A CCTV survey settles it in an hour and gives you the exact position and depth, which is the difference between a patch repair and digging up half the garden.

A dry trap is the usual answer

Every fixture has a water seal in its trap, and that water evaporates in an unused guest bathroom, a rarely used shower or a spare WC. Once it goes, sewer gas comes straight up. Running the tap for thirty seconds refills it. If a smell appears after a holiday, this is almost always what it is.

When it is the vent, not the trap

A soil stack vents above roof level so pressure changes do not siphon traps empty. If the vent is blocked by a nest or has been badly capped during a loft conversion, draining one fixture pulls the seal out of another, and the symptom is a gurgle followed by a smell. That is a stack problem, not a cleaning problem.

Smells that mean something is broken

A persistent smell outside near a chamber, or damp ground with an odour, suggests a cracked or displaced run leaking below ground. Inside, a smell in a room with no plumbing in it can mean a soil pipe leaking within a wall or floor void. Both warrant a camera rather than another bottle of drain cleaner.