Smell of Drains in House — Diagnose & Fix
Sewer smell in a UK home is always one of five causes. Here is the diagnostic order.
Step-by-step method
1. Dried-out trap
An unused sink, shower or floor gully evaporates its water seal in weeks. Pour 1L water down every drain to refill traps.
2. Failed air admittance valve (AAV)
AAVs sit on soil stack top and let air in without letting gas out. They fail closed (partial vacuum siphons traps) or open (gas escapes). Replace with McAlpine MRAA — £22.
3. Cracked or displaced soil pipe
Common at solvent joints in loft runs. Smoke test finds them. Replacement £180–£450 depending on access.
4. Blocked or partially blocked drain
Solids trapping gas and forcing it back up. Rod or jet the drain.
5. Rat entry
Rats chew through push-fit waste connectors. Sign: droppings + smell in bathroom. Fit metal rat-blockers.
Frequently asked questions
Is sewer gas dangerous?+
In small doses, mainly unpleasant. Concentrated hydrogen sulphide is toxic. Persistent strong smell = fix within a week.
Why do I only smell drains when it rains?+
Blocked external drain surcharging back up. Rod from a manhole toward the property.