How do I clear a blocked sink trap?

Taking the trap apart without flooding the cupboard, what actually causes the blockage, and preventing the next one.

Difficulty: beginner
Time: 30 min
Read: 3 min
Safety: caution

Short answer

Put a bowl underneath, unscrew the trap by hand, and clean it out. Most kitchen blockages are congealed fat and food; bathroom ones are hair and soap. Neither needs chemicals.

The trap is designed to be removable, and clearing one by hand is faster, cheaper and safer than any chemical.

Safety

Never use chemical drain cleaner and then dismantle the trap — caustic liquid trapped in the pipe will splash out and cause serious burns. If you have already poured chemicals down, flush thoroughly with cold water and wear gloves and eye protection. Do not mix different drain products, as some combinations produce dangerous gases.

What you'll need

A bowl or bucket and old towels
Rubber gloves
A bottle brush or a piece of wire

Step by step

Clear the cupboard and put a bowl underneath.The trap holds water by design and it will all come out. Towels round the bowl for what misses.
Do not use chemicals first.If you dismantle a trap containing caustic drain cleaner it splashes out. If chemicals have already been used, flush thoroughly with cold water before touching anything.
Unscrew the trap by hand.Plastic traps have hand-tight connectors. If they are stiff, grips with a cloth to protect the plastic — never a spanner directly on the nut.
Note which way it goes back.A photograph before dismantling. Traps have washers that must sit the right way round and it is easy to reassemble wrongly.
Clean it out completely.A bottle brush or a bent piece of wire. Kitchen traps fill with congealed fat and food; bathroom ones with hair and soap scum bound together.
Check the pipe beyond the trap.If the trap was clear, the blockage is further along. A drain snake or a length of stiff wire reaches into the waste pipe.
Check the washers before reassembling.Perished or displaced washers are the commonest cause of a leak after a trap has been cleaned. Replace them if they are hard or misshapen.
Hand-tighten only.Plastic connectors seal on the washer, not on force. Over-tightening cracks the thread and causes a permanent leak.
Run water and check for leaks.Fill the sink and release it so the joint sees a proper flow, then check with dry tissue around every connection.
Prevent the next one.A sink strainer, and never pouring fat down the drain. Fat solidifies as it cools in the pipe and is the single biggest cause of kitchen blockages.

Tips

Photograph the trap before dismantling. Washer orientation matters and is easy to get wrong.
Hand tight only. Plastic traps seal on the washer, not on force.
Never dismantle a trap after using chemical drain cleaner without flushing thoroughly first.

Common mistakes

Reaching for chemical drain cleaner first — It often fails to shift a solid blockage, and it then sits in the pipe as a caustic hazard for whoever opens the trap. Dismantling takes ten minutes and works.
Tightening plastic connectors with a spanner — They are designed to seal on a rubber washer at hand pressure. A spanner cracks the plastic thread, which then leaks permanently.

If it doesn't work

Trap was clear but sink still slow

Cause: Blockage further along the waste pipe — Fix: Use a drain snake through the open pipe, or check the outside gully.

Leak after reassembling

Cause: Washer misplaced or perished — Fix: Dismantle, check every washer is seated correctly and the right way round, replace any that are hard.

Gurgling but draining

Cause: Partial blockage or a venting problem — Fix: Clear the trap, and if it persists the soil vent may be partially blocked.

Smell from the sink

Cause: Trap dry, or biofilm in the pipe — Fix: Run water to refill the trap. For biofilm, a bacterial drain treatment works better than bleach.

Blockage recurs regularly

Cause: Fat in the kitchen, hair in the bathroom — Fix: Fit a strainer, and dispose of fat in the bin rather than the sink.

Questions people ask

Do I need a plumber to clear a trap?

No. Modern plastic traps unscrew by hand and are designed to be cleaned. It is one of the most straightforward plumbing jobs there is.

Are chemical drain cleaners a good idea?

Generally no. They frequently fail on solid blockages, they damage some pipework over time, and they leave a caustic hazard in the pipe for anyone who then opens it.

Why does my sink smell even when it drains?

Either the trap has dried out, letting drain air through, or there is biofilm in the pipe. Running water fixes the first; a bacterial drain treatment addresses the second.