How to unblock a sink without damaging the pipes

First, work out where the blockage is

Before you reach for a plunger, spend a minute on the pattern. It tells you whether this is a job for the sink itself or for the drain it joins. Run a little water in each nearby fitting and watch what happens.

If only one sink is slow, the blockage is almost always in its plug hole, trap or the short waste pipe behind it. If the sink, the dishwasher and the washing machine are all affected, the shared kitchen waste is the likely problem. If a toilet, shower or outside gully also backs up, the blockage is in the underground drain, and nothing you do at the sink will clear it.

What you need

You do not need specialist kit for a sink blockage. A cup plunger designed for flat surfaces works better on a sink than the flanged type made for toilets.

• Rubber gloves and a bucket or washing-up bowl
• A sink plunger
• A damp cloth to block the overflow
• Washing-up liquid and a kettle
• Old towels for the cupboard floor
• Optionally, a hand-operated drain snake or auger

Step 1: clear the plug hole and pop-up waste

Scoop out standing water into a bucket so you can see what you are doing. Lift out the strainer or plug. Bathroom basins often have a pop-up plug that unscrews or lifts out, and hair and soap build up on its stem. Pull out whatever you can reach with gloved fingers or a bent piece of wire, then run a little water to see if it drains.

Step 2: plunge it properly

Plunging only works if the pressure has nowhere else to go. Press a wet cloth firmly over the overflow slot. On a double-bowl sink, put the plug in the other bowl and have someone hold it down. If a dishwasher drains into the sink waste, clamp or pinch its flexible hose for the moment you plunge, so you are not pushing dirty water back into the machine.

Leave a few centimetres of water in the sink to cover the plunger cup. Seat it over the plug hole, then push and pull sharply ten to fifteen times without breaking the seal. Lift it off and see if the water drops. Two or three rounds are often enough for a soap, grease or food blockage close to the sink.

Step 3: flush with hot water and washing-up liquid

In kitchens, the usual culprit is cooking fat that has set inside the trap and pipe. A good squeeze of washing-up liquid followed by a kettle of hot water can soften it enough to move. If your waste pipes are plastic, let the kettle cool for a few minutes first; very hot water poured repeatedly can soften plastic joints.

This works well on partial blockages and less well on a sink that is completely full. If the water just sits there, go straight to the trap.

Step 4: clean out the trap

The trap is the U-shaped or bottle-shaped fitting directly under the plug hole. It holds water to stop drain smells coming up, and it is where most sink blockages collect. Clear the cupboard, lay towels down and put a bowl underneath before you touch it.

Unscrew the plastic nuts by hand. They are usually hand-tight; if one is stiff, wrap it in a cloth before gripping it with pliers so you do not crack it. Empty the trap into the bowl, rinse it out in another sink or outside, and check the rubber washers are in place and undamaged. Refit it hand-tight, run the tap for a minute and check every joint for drips.

If the trap is clear but the sink still will not drain, the blockage is in the pipe beyond it. A hand snake can be fed in gently from the trap connection, but do not force it through bends or into old pipework.

Be careful with chemical drain unblockers

Shop-bought drain unblockers can clear light blockages, but they are strong chemicals. Read the label, wear gloves and eye protection and ventilate the room. Never mix two products, and never pour one onto a sink full of standing water, because you then have a bowl of caustic liquid that you cannot plunge or dismantle safely.

If you have already used a chemical and the sink is still blocked, tell the plumber which product and how much. It changes how they approach the trap and protects them from splashes.

When it is not the sink at all

Some blockages that show up at a sink are really drainage problems. The signs are water rising in the sink when the washing machine empties, gurgling when other fittings drain, a bad smell from the plug hole, or an outside gully that overflows. Clearing the sink trap will not help any of these.

Recurring blockages are also worth investigating rather than clearing over and over. A sink that blocks every few weeks may have a poorly graded waste pipe, a build-up of fat further along, or a partly blocked drain outside. A drainage engineer can rod or jet the waste and, where needed, look inside with a camera to find out why.

If the toilet is the fitting that has backed up, the method is different: a toilet needs a flanged plunger or a toilet auger, and our guide on how to unblock a toilet explains both.

Stop it happening again

Thames Water asks households to keep fat, oil and grease out of sinks, because it sets hard in pipes and sewers. Let cooking fat cool and put it in the bin, scrape plates before washing up, and use a sink strainer to catch food. In bathrooms, a simple hair catcher over the basin or shower waste saves a lot of trouble. Running hot water after washing greasy pans does not stop fat setting further down the pipe.

Call a plumber for a sink that will not clear

Loyal Plumbers clears blocked sinks, kitchen wastes and drains across London and everywhere inside the M25, with 10 vans working 24 hours a day. If the sink is still blocked after cleaning the trap, several fittings are affected, or water is coming up outside, call us or send an enquiry.

Tell us which fittings are slow, whether you have used a chemical unblocker and whether the outside gully is overflowing. Our call-out charge is £60, and the engineer explains what clearing the blockage involves before starting.