Blocked Kitchen Sink? Causes, Fixes and When Fat Has Won

Reviewed by the DCI Drainage team · Last updated September 2026

The kitchen sink works harder than any other drain in the house, and it takes the worst diet: cooking fat, food scraps, coffee grounds and dishwasher grease, day after day. So when it stops draining, the cause is rarely a mystery. This guide covers the fixes that genuinely work, in the order worth trying them, how to clean the trap without flooding the cupboard, and the honest signs that fat has won and the pipe needs a professional clean rather than another bottle of unblocker.

What actually blocks a kitchen sink

Understanding the cause matters, because it decides which fix works. The overwhelming majority of kitchen sink blockages are fat, oil and grease. Fat goes down the plughole warm and liquid, then cools and sets on the pipe walls a metre or two along, where you cannot see it. Every wash adds a thin layer, and every layer catches food particles, coffee grounds and rice on its way past. Over months the pipe narrows like a furred kettle element, until one day a normal bowl of washing up water simply has nowhere to go.

The rest of the list is shorter: food scraps that went down in one lump, a build-up in the trap (the U shaped bend under the sink that holds water to keep smells out), a clogged washing machine or dishwasher connection sharing the same waste run, and occasionally an object, a bottle top or a teaspoon, sitting in the trap catching everything that follows. Where the blockage sits changes the symptom. Water that will not leave the bowl at all usually means the trap. A sink that drains slowly and gurgles usually means the pipe run beyond it.

Fixes in the right order

1. Hot water and washing up liquid

Start with the method that attacks the actual cause. Bail standing water out into a bucket, squirt a generous amount of washing up liquid down the plughole, then slowly pour in a kettle of hot water. Let a boiled kettle stand for a minute first: very hot water softens plastic waste pipes and trap seals. The detergent breaks the surface of the fat and the heat softens it. Wait ten minutes and run the hot tap. For a partial blockage, two or three rounds of this often restores full flow.

2. Plunge with the overflow blocked

A flat-bottomed sink plunger works on kitchen sinks, with one trick most people miss: block the overflow, the slot near the rim, with a wet cloth. Otherwise every push simply blows air out of the overflow instead of pressing on the blockage. Put a few centimetres of water in the bowl, seal the plunger over the plughole, and plunge firmly for 20 seconds. On a double sink, block the second plughole too. Follow a successful plunge with the hot water and detergent treatment so the loosened fat leaves the pipe rather than resettling further along.

3. Clean the trap

If plunging fails, the blockage is very likely sitting in the trap itself, and the trap unscrews by hand. The full steps are below.

4. Rod the waste pipe gently

With the trap off, you can feed a flexible drain snake or a length of curtain wire into the waste pipe where it enters the wall. Work it gently back and forth to break up soft fat. Do not use force: kitchen waste pipes are plastic, push-fit in places, and a joint knocked apart inside a wall or floor is a far bigger job than a blocked sink.

Cleaning the trap, step by step

This is a ten minute job and needs no tools beyond a bucket. Put the bucket under the trap, the U or bottle shaped fitting directly below the plughole. Unscrew the two large plastic collars by hand, turning anticlockwise, and let the trap come away with its water. Tip out the contents, and expect an unpleasant paste of grease and food; this is normal and exactly what you are here for. Clean the trap out thoroughly with hot soapy water and an old washing up brush, check the rubber sealing washers are seated and undamaged, then screw the collars back hand-tight. Run the tap and watch the joints for a minute. A slow drip usually means a washer has folded; undo, reseat, retighten. Do not use a wrench on plastic collars, hand-tight is correct.

While the trap is off, shine a torch into the standpipe and the wall pipe. If you can see fat lining the pipe like candle wax, the trap was only part of the story, and the run beyond it is on its way to blocking too.

When fat has won

Here is the honest part. Everything above deals with the trap and the first stretch of pipe, which is as far as DIY reaches. When fat has been going down a sink for years, it does not sit in one convenient lump. It coats the whole waste run, sometimes all the way to the outside gully, and it sets hard. You will know fat has won when the sink blocks again within a few weeks of each clear, when unblocker products make no lasting difference, when the drain smells greasy however much you clean, or when the outside gully that takes the kitchen waste is scummy and slow. At that point, more plunging just pokes temporary holes through a pipe-length problem, and it is the same pattern we describe in our guide to drains that keep blocking.

The proper fix is drain jetting, which uses high pressure water to strip the set fat off the full bore of the pipe and flush it away, rather than drilling a hole through the middle of it. You can read exactly how it works in our guide to what drain jetting is. The same visit can deal with any other internal blockage in the house, and the fix for a blocked toilet or a backed-up gully is often the same appointment. Cleared properly, a fat-lined kitchen run stays clear for years rather than weeks.

Keeping it clear for good

Prevention with kitchen sinks is unusually simple, because the cause is so consistent. Four habits do almost all the work. Never pour fat, oil or grease down the sink: let pans cool and wipe them into the bin, and pour used oil into a container for the bin or the recycling centre. Fit a plughole strainer and empty it into the bin, not the sink. Scrape plates before rinsing, because dishwashers and hot rinses send softened food fat down the same pipe. And once a week, send a squirt of washing up liquid and a kettle of hot water down the plughole last thing, so any film of grease moves on before it sets. If your household cooks a lot, an occasional professional maintenance clean of the kitchen run is cheaper than the emergency alternative; that is part of our drain cleaning and maintenance service.

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