How to Unblock a Sink: Plunger, U-Bend and Prevention Tips

A sink that drains slowly, gurgles or sits full of grey water is usually telling you the same thing: something's built up in the pipe just below the plughole. Most of the time it's within reach, and you can clear it with a plunger, a simple waste pipe cleaner or by taking off the trap underneath.

I've based this guide on step-by-step advice from Toolstation, Admiral, the housing association Places for People and Thames Water, plus safety advice from the London Fire Brigade. Start with the gentlest method and only move on if it doesn't work.

Why kitchen and bathroom sinks block differently

Toolstation says slow drainage usually means debris such as grease or hair has built up in the U-bend or bottle trap. Which one depends on the room:

Sink	Usual culprits	Where the guidance comes from
Kitchen sink	Fat, oil and grease, food scraps, coffee grounds	Thames Water and Admiral
Bathroom basin	Hair, soap and toothpaste build-up	Places for People and Toolstation

Knowing the likely cause helps you pick the right tool. A waste pipe cleaner suits hair, while fat in a kitchen pipe often needs the trap taken off and cleaned.

Plunging a blocked sink the right way

A plunger's the first thing Toolstation and Admiral suggest. The trick is the seal:

1. In a bathroom basin, take out the pop-up plug or stopper first, as Places for People suggests.
2. Block the overflow hole with a wet cloth. Admiral explains this creates an airtight seal so the pressure goes down the pipe.
3. Have enough water in the basin to cover the plunger head. Places for People suggests at least half of it.
4. Press the plunger firmly over the plughole and pump up and down.
5. Run the water to check. It may take several attempts, so repeat if needed.

Using a waste pipe cleaner or drain snake on a sink

If plunging hasn't worked, Toolstation suggests a waste pipe cleaner: push the corkscrew end down the drain, then push and pull it through the pipe. It's especially handy for hair, which catches on the corkscrew and comes out with it. Admiral describes using a drain snake or auger the same way, pushing it round the U-bend to break up the blockage or bring it back up, then flushing through with cold water.

How to clean the U-bend under the sink

If the water still won't go, the blockage is often sitting in the trap, the bend in the pipe directly under the plughole. Here's the method, combining the Toolstation, Admiral and Places for People steps:

1. Clear the cupboard under the sink and put down an old towel.
2. Turn off the water at the stopcock. Admiral notes it's usually under the kitchen sink.
3. Put a bucket or washing-up bowl under the trap and wear rubber gloves.
4. Take a photo or make a note of how the trap fits together so you can rebuild it the same way.
5. Unscrew the fittings slowly. Water will come out, so let it drain into the bucket. Keep your face away from the trap as you loosen it.
6. Keep track of any rubber washers or seals.
7. Clean the trap in hot soapy water with an old toothbrush or pipe cleaner, as Toolstation suggests.
8. Refit it exactly as it came apart, with all the washers in place, then turn the water back on and check for leaks.

Toolstation says if the sink still drains slowly after this, the blockage is deeper in the pipes and it's time to seek professional advice.

Bicarbonate of soda and vinegar for a slow sink

This is one of the most commonly suggested home methods, and Admiral describes it as a gentler alternative to harsh chemicals. Admiral's version is a teaspoon of bicarbonate of soda down the plughole, followed by a cup of white vinegar, then flushing through with warm water after a few minutes. Places for People suggests it for a slow drain rather than a full blockage. Treat it as worth a try, not a guaranteed fix.

Sink unblocking safety: boiling water, bleach and chemicals

• Skip the boiling kettle and the bleach. Thames Water advises against pouring boiling water or bleach down the drain, saying it only moves the problem further down the pipes. Admiral adds that bleach isn't designed to break down blockages.
• Never mix products. The London Fire Brigade says not to pour chemicals into the same container or use one cleaner straight after another, as it may create dangerous gases.
• Ventilate. Open a window whenever you use any chemical product, as both Admiral and the London Fire Brigade advise.
• If one unblocker fails, stop. The London Fire Brigade suggests calling professional tradespeople rather than reaching for a second product.
• Read the label. If you do use a chemical drain cleaner, follow the product's own instructions exactly. Toolstation notes they can be harsh on pipes and the environment, so try manual methods first.

Keeping your sink clear with the bin it habit

Thames Water's advice for kitchens is to keep fat, oil and grease out of the sink altogether. Its tips include:

• Scrape fats and oils into the bin before washing up.
• Let surplus cooking oil cool, then bin it or take it to a local recycling centre.
• Don't pour milk, gravy, yoghurt or soup down the sink.
• Put coffee grounds in the bin or compost rather than the drain.
• Fit a basket strainer in the plughole to catch food scraps.

In the bathroom, Admiral suggests a strainer to catch hair, and Toolstation suggests cleaning the pipework monthly to keep things flowing.

More home fixes and where to share them

Sinks and toilets often block for related reasons, so if the loo's misbehaving too, here's how to unblock a toilet safely. And while you're on household jobs, it's worth knowing how to bleed a radiator before the cold weather sets in.

There's also a page of plumbing publications that welcome how-to articles if you'd like to write one yourself.

Frequently asked questions

Should I cover the overflow when plunging a sink?

Yes. Admiral says blocking the overflow with a wet cloth creates an airtight seal, so the pressure from the plunger goes down the pipe instead of out through the overflow.

Can I pour boiling water down a blocked sink?

Thames Water advises against pouring boiling water or bleach down the drain, as it only moves the problem further down the pipes. Try a plunger or clean the trap instead.

Does bicarbonate of soda and vinegar unblock a sink?

It's a commonly suggested home method for a slow drain. Admiral describes it as a gentler alternative to harsh chemicals, but for a full blockage a plunger or cleaning the trap is more reliable.

What's the U-bend under a sink?

It's the bent section of pipe directly under the plughole, also called the trap. Toolstation says grease and hair often build up there, and you can unscrew it to clean it out.

When should I call a plumber for a blocked sink?

Toolstation says if the sink still drains slowly after cleaning the trap, the blockage is deeper in the pipes and you should seek professional advice.