How to unblock a kitchen sink — every method, in the right order
Fat, food and soap scum cause almost every kitchen sink blockage. Here's how to clear one properly without wrecking your pipework.
Why kitchen sinks block more than anything else
A kitchen sink takes a daily load nothing else in the house does: cooking fat, oil, milk, coffee grounds, rice, pasta starch and washing-up soap. Warm fat pours away as a liquid, then cools inside the waste pipe and sets on the pipe wall like candle wax. Each wash adds another layer until the bore of a 40mm pipe is down to the width of a pencil.
That's why kitchen blockages come back. Clearing the water through is not the same as clearing the pipe. If you only punch a hole through the grease, you'll be doing it again in a fortnight.
Step 1 — Boiling water and washing-up liquid
Squeeze a generous amount of washing-up liquid into the plughole, then pour a full kettle of boiling water down slowly, in stages, so the heat has time to work rather than flashing straight past the blockage.
The detergent emulsifies the fat and the heat softens it. Repeat twice. This alone clears a genuinely surprising number of slow kitchen sinks. Do not do this if you have plastic push-fit waste that is already loose, or if the sink is completely full of standing water — the boiling water won't reach the blockage.
Step 2 — Soda crystals, not caustic unblocker
A mug of soda crystals followed by a kettle of boiling water is the safest chemical route for a kitchen sink. Soda crystals saponify fat — they literally turn grease into soap so it washes away.
Avoid caustic soda-based drain unblockers as a first resort. They generate heat, can distort plastic waste pipes, corrode older copper and lead runs, and if they fail they leave a caustic pool that anyone opening the trap afterwards has to deal with. Never mix two different drain products.
Step 3 — Plunge it properly
Most people plunge a kitchen sink wrong. Block the overflow slot with a damp cloth first — otherwise all your pressure escapes out of it and nothing moves.
Leave a couple of inches of water in the bowl so the cup seals, press down to expel the air, then pump firmly fifteen to twenty times keeping the seal. It is the sharp pull-back stroke that shifts the plug, not the push. If you have a double bowl, block the second plughole too.
Step 4 — Strip the trap
Under the sink is a U-bend or bottle trap, and this is where the blockage sits maybe seven times out of ten. Put a washing-up bowl underneath, unscrew both plastic collars by hand, and lift it out.
Empty it into a bin bag — never back down the drain. Scrub the inside with a bottle brush until you can see clean plastic all the way round. Check the rubber washers are seated the right way and refit hand-tight only; over-tightening cracks the collar. Run the tap and check for drips.
Step 5 — Snake the waste run
If the trap was clean, the blockage is further along the horizontal waste pipe, usually where it turns to go through the wall. Feed a hand snake (drain auger) in through the open trap connection, turning the handle as you go, until you feel resistance.
Work it back and forth rather than forcing it, then withdraw it slowly — you want to pull the grease plug out, not compact it. Flush through with hot water and detergent afterwards to take the residue with it.
When it isn't the sink at all
Stop DIY and call someone if: the sink gurgles when the washing machine drains, water backs up into the sink when you use another fixture, the outside gully by the kitchen wall is standing full, or you can smell drains inside the house.
Those are signs the blockage is in the underground drain, not the waste pipe, and no amount of plunging will touch it. It needs high-pressure jetting, and often a camera to find out why it blocked in the first place.
How to stop it coming back
Never pour cooking fat, oil or meat juices down the sink — wipe the pan out with kitchen roll into the bin first. Scrape plates before washing. Fit a cheap plughole strainer for coffee grounds and food debris. Once a month, run soda crystals and a kettle of boiling water through as maintenance.
If you're a food business, this is not optional — grease trap servicing and planned jetting are what keep you out of a shutdown.
Get it cleared properly
DrainFixer is a small family-run drainage firm covering East Grinstead and 20 miles around, 24 hours a day. If your kitchen sink is beyond the trap we'll jet the waste run and the underground drain and camera it to show you what caused it.
Call 0330 1331 347 any time, day or night.