One fixture, or several?
- One sink, bath or basin slow — almost always the trap or the branch pipe immediately after it. A householder job.
- Several fixtures slow at once, or a downstairs toilet backing up when an upstairs one is flushed — the shared soil stack or the underground drain. That needs someone with the right kit.
- Water rising in an external gully or manhole — the run to the sewer. Not a plunger problem.
- Smell of sewage indoors — a dry or damaged trap, or a broken pipe. Worth investigating properly rather than masking.
What actually works on a single fixture
- Clear the visible stuff — hair from the plughole, the strainer, the overflow.
- Try a plunger properly: block the overflow with a wet cloth first, otherwise you are just pushing air around in a circle.
- For a grease-heavy kitchen sink, a good handful of soda crystals followed by hot water from the tap will often shift it. Leave it to work rather than rinsing straight away.
- Undo the trap under the sink with a bucket beneath it and clean it out by hand. It looks unpleasant and it fixes the majority of kitchen and basin blockages outright.
- Check the external gully the waste discharges into — they silt up with leaves and grease and are easy to clear.
Why caustic drain cleaner is usually the wrong answer
It sometimes works, and it comes at a cost. Strong caustic products can damage older pipework and seals, and they generate heat, which does plastic waste pipes no favours.
More to the point, if it does not clear the blockage you now have a pipe full of caustic liquid — which is genuinely dangerous for whoever opens that trap next, and that is usually you or the engineer you call an hour later. If you have already poured some in, say so before anyone starts work.
When repeated blockages mean something else
A drain that blocks in the same place every few months is not dirty, it is wrong. Almost always it has insufficient fall, an unnecessary bend, or a section that has dropped.
This is common where a kitchen or bathroom has been moved from its original position — very frequent in older terraced housing, where the waste has been extended across a room to reach the original stack. Rodding it clears it for a while. Correcting the run fixes it permanently, and if you are clearing the same blockage three times a year it is usually the cheaper option.
Common questions
Is boiling water safe to pour down a drain?
Be careful with it on plastic waste pipes and never use it on a blocked toilet, where the temperature change can crack the ceramic. Hot water from the tap plus soda crystals is safer and works about as well on grease.
Who is responsible for a blocked drain?
Generally you are responsible for the pipes within your property boundary, and the water company for the shared sewer beyond it. If more than one property is affected, ring the water company first — it may not be your bill at all.
Why does my sink keep blocking in the same place?
Because the pipe run is the problem rather than the debris. Insufficient fall, an extra bend or a dropped section will catch material indefinitely. Rodding clears it, correcting the run fixes it.
Related guides
Burst pipe: what to do in the first ten minutes Why does my boiler keep losing pressure?
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