How to Unblock a Sink (Kitchen or Bathroom)
In short
Bail the standing water out, clear the plughole, then soften the blockage with hot (not boiling) water and washing-up liquid before plunging with the overflow blocked. If that fails, take the trap off and clean it — that is where most sink blockages sit. If more than one appliance is draining slowly, the problem is in the drain rather than the sink.
Most blocked sinks are caused by the same two things: congealed fat and food waste in the kitchen, hair and soap scum in the bathroom. Both build up slowly in the trap or the first few feet of waste pipe, which is good news — that part of the system is the easiest to reach without tools.
Work through the steps below in order. If water is backing up into more than one appliance, or coming up outside, stop and read the last section first, because that is a drain problem rather than a sink problem.
Step 1: clear the standing water and check the plughole
Bail out as much standing water as you can into a bucket so you can see what you are doing. Remove the plug and strainer and pull out anything sitting in the plughole — in bathrooms, a surprising amount of hair sits within the first inch and can be hooked out with a bent piece of wire or a cheap plastic drain snake.
Step 2: hot water, washing-up liquid and a plunger
For a kitchen sink, a kettle of hot (not boiling — boiling water can damage plastic traps and seals) water with a good squeeze of washing-up liquid will soften a fat blockage. Leave it 10 minutes, then run the hot tap.
If it is still slow, use a cup plunger. Block the overflow with a damp cloth first, otherwise you are just pushing air around. Cover the plughole completely, then plunge with short sharp pushes for 20 seconds. In a double sink, block the second plughole too.
Step 3: bicarbonate of soda and vinegar
A cup of bicarbonate of soda followed by a cup of white vinegar, left for 20 minutes and flushed with hot water, will break down light grease and smells. It is not powerful enough for a solid blockage, but it is safe for pipes, cheap, and worth trying before anything harsher.
Step 4: clean the trap
The U-bend under the sink catches most blockages and takes about ten minutes to clean. Put a bucket underneath, unscrew the two collars by hand, lift the trap out, and clean it through with hot water and an old bottle brush. Check the seals are seated properly when you refit it and run the tap to check for drips.
If the trap is clean but the sink still will not drain, the blockage is further along the waste pipe or in the drain itself.
What not to do
A few household fixes cause more damage than the blockage did.
- Avoid caustic drain crystals in a fully blocked sink — the chemical sits in standing water, generates heat and can damage plastic pipes and burn you when you dismantle the trap.
- Never mix drain products; some combinations release chlorine gas.
- Do not pour fat or cooking oil down the sink to "flush it through" with hot water — it cools and sets a few metres along the pipe.
- Do not force a metal rod down a plastic waste pipe; it is easy to punch through a joint.
When it is not just the sink
If the kitchen sink, bath and toilet are all draining slowly, or gurgling when another appliance is used, the blockage is in the shared soil pipe or the underground drain, not the trap. Lift the nearest external inspection chamber cover: if it is full of water, the blockage is downstream of it and needs jetting.
That is a job for a drainage engineer with high-pressure jetting equipment. If the same sink blocks every few weeks after being cleared, ask for a CCTV survey — it usually means grease build-up further down or a defect holding waste back.
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