Why Your Drains Smell and How to Stop It
In short
Drain smells usually mean a water seal has failed or waste is sitting where it should not. The common causes are a dried-out or siphoned trap in a little-used appliance, grease and biofilm coating a kitchen waste pipe, and a silted external gully. If the smell survives all three fixes, there is normally a physical fault such as a cracked pipe or a failed connection, which a CCTV survey will locate.
A drainage system should be sealed against smells by water. Every appliance has a trap holding a small plug of water that stops sewer gas coming back into the room, and the soil vent pipe lets the system breathe at roof level instead of through your plughole.
When you can smell drains, one of those defences has failed. Working out which one takes a few minutes and usually costs nothing to fix.
A dried-out or siphoned trap
The most common cause, especially in a spare bathroom, utility room or an unused shower. If an appliance goes unused for a few weeks the water in the trap evaporates and the pipe is open to the drain.
Run every tap and flush every toilet in the house for a minute, including ones you rarely use. If the smell disappears, that was it — run them monthly. Washing machine and dishwasher standpipes are often the culprit; a self-sealing waste valve solves it permanently.
Grease and biofilm in the waste pipe
A sour, food-like smell from a kitchen sink is usually a coating of grease and food residue on the pipe wall rather than a blockage. It smells worst in warm weather.
Clean the trap out by hand, then keep on top of it with hot water and washing-up liquid weekly, or bicarbonate of soda and vinegar monthly. If it returns quickly the build-up is further down and needs jetting.
Blocked or silted external gully
Outside smells near a kitchen wall almost always come from the gully — the small grated drain the sink and washing machine discharge into. Leaves, silt and fat build up in the trap below the grate.
Lift the grate, scoop the debris out with a gloved hand or trowel, and flush it through with a bucket of hot soapy water. Check the water level drops away freely; if it does not, the drain below is blocked.
When it is a defect, not housekeeping
Persistent sewage smell that comes back after all of the above usually means a physical fault: a cracked or displaced underground pipe, a failed connection at the soil stack, a missing or damaged inspection chamber seal, or a redundant capped drain left open under the floor.
Rats are the other sign worth taking seriously — they get into houses through broken drains, so a rat problem plus a drain smell is worth a CCTV survey. A survey identifies the exact location and depth of the defect so it can be patch-lined or repaired without digging up the whole run.
Still not resolved? CCTV Drain Surveys can help
Find out exactly what's going on inside your drains, without guessing. Our engineers are available 24 hours a day with no call-out charge, and you get a fixed price before any work starts.