Bad smell from kitchen sink: traps, fats and venting
Kitchen sink smells are usually fats and food caught in the trap or the branch pipe — but sometimes it's the vent.
A smelly kitchen sink is almost always one of these three things.
1. Fat and food in the trap
The most common cause. Fats from cooking and washing-up congeal in the U-bend under the sink and trap food particles, which then rot.
Fix:
- Place a bucket under the trap
- Unscrew the two plastic nuts on either end of the U-bend
- Clean it out, rinse, refit
Takes 10 minutes. If the smell goes within a day, you've found it.
2. Partial blockage in the branch pipe
If the trap looks clean but the smell continues, the blockage is further down — usually in the horizontal branch pipe between the trap and the soil stack.
Fix: A flexible kitchen drain snake (cheap, available at any DIY shop) can clear most of these. For stubborn or compacted fat, a van-pack jet will clear it properly — around £140–£180 + VAT.
3. Vent issue
If the sink gurgles when emptied — and especially if other fittings (washing machine, dishwasher, toilet) gurgle when the sink drains — your soil vent is compromised. Sucked-dry traps then let sewer gas back into the house.
Common causes:
- Soil vent capped during a loft conversion without an air admittance valve
- Bird's nest at the top of the vent
- Disconnected branch under the floor
Fix: Diagnose first, then either clear the vent or fit an air admittance valve (AAV) on the soil stack. AAV installation around £180–£280 + VAT.
What not to do
- Pouring kettle water down the sink doesn't melt out built-up fat — it just pushes it further down to congeal somewhere worse
- Bleach masks the smell for a day, doesn't fix the cause