blogTL;DR: You turn on the tap and the water rises instead of draining away. A blocked kitchen sink is one of the most common drainage jobs we handle across Cornwall, and in most cases you can unblock a kitchen sink yourself without calling anyone out. The cause is nearly always the same: grease, fat, and food waste that cools and hardens inside the pipe.
Before you reach for a bottle of chemical drain cleaner, which we would advise against and will explain why, it helps to work through the safe methods in the right order. Start with the gentlest, then move to the more hands-on if you need to.
This guide covers what blocks a kitchen sink, the tools you need, a step-by-step method to clear it, how to stop it happening again, and the point at which the blockage needs an engineer.
Why Does a Kitchen Sink Get Blocked?
A kitchen sink gets blocked when fat, oil, grease, and food waste wash down the plughole, cool, and stick to the inside of the pipe. Over time this build-up narrows the pipe until water can no longer pass. Soap scum, coffee grounds, and rice make it worse.
Fats, oils, and grease (known in the trade as FOG) are the main culprit. They pour down as a liquid, then cool and set solid further along the pipe. According to Water UK, leftover cooking fats and oils poured down the kitchen sink are a leading cause of the fatbergs that block pipes and drains.
The scale is significant. There are roughly 200,000 sewer blockages and pollution incidents across England and Wales each year, and up to 75% of them are caused by fat, oil, and grease, according to local authority guidance.
The common causes come down to a short list:
- Fat and cooking oil that hardens as it cools
- Food scraps, especially rice, pasta, and coffee grounds that swell or clump together
- Soap and detergent scum that binds loose debris
- Small objects and packaging that slip down the plughole
Older Cornish properties add another factor. Many have clay or pitch-fibre pipe runs that are rougher inside than modern plastic, so grease grips more easily and blockages form faster. Rural homes on septic systems face the same issue underground, where fat and food waste reduce how well the tank works. If your blockage keeps returning, it is worth reading our guide to blocked drains in Cornwall for the likely deeper causes.
What You Will Need Before You Start
You can clear most kitchen sink blockages with tools you already have at home: a cup plunger, a bucket, rubber gloves, hot water, washing-up liquid, and soda crystals. For a stubborn blockage further down the pipe, a drain snake (also called a plumber’s auger) is worth keeping in the cupboard.
- A cup plunger, the flat type, not the toilet flange plunger
- A bucket and old towels to catch water
- Rubber gloves
- Soda crystals (washing soda), or baking soda and white vinegar
- A drain snake or plumber’s auger for deeper blockages
- A wrench or pair of grips if you plan to open the U-bend
Lay the towels down and keep the bucket within reach. Clearing a sink is a wet job, and a little preparation keeps the mess contained.
How Do You Unblock a Kitchen Sink Step by Step?
To unblock a kitchen sink, first bail out any standing water, then work through the methods from gentlest to most hands-on: hot water and washing-up liquid, soda crystals, plunging, and finally cleaning the U-bend. Most blockages clear before you reach the last step.
Step 1: Clear the Standing Water
If the bowl is full, bail the water into a bucket first. The methods below only work if they can reach the blockage, and a plunger needs the pipe, not a full sink, to build pressure. Scoop out what you can, then lift the strainer and remove any visible food or debris from the plughole by hand. Wear gloves.
Step 2: Flush With Hot Water and Washing-Up Liquid
For a mild grease blockage, this often does the job. Squeeze a generous amount of washing-up liquid down the plughole, then pour a kettle of hot, not boiling, water after it. The detergent helps break down the fat and the heat softens it.
Skip boiling water. Northumbrian Water warns that boiling water poured down the sink can carry grease further along the pipe, where it cools and sets again. Very hot water can also damage plastic pipe joints and seals. Hot tap water, or a kettle left to stand for a minute, is safer and works just as well.
Step 3: Use Soda Crystals or Baking Soda
Soda crystals are a British staple for grease and cost very little. Pour a mugful down the plughole, follow with a kettle of hot water, and leave it for a few minutes.
If you prefer baking soda, pour one cup down the drain, follow with one cup of white vinegar, and cover the plughole for ten minutes while it fizzes. Flush through with hot water afterwards. This works on light build-up but will not shift a solid blockage on its own.
Step 4: Plunge the Sink
Fill the bowl with enough water to cover the cup of the plunger. If you have a double sink or an overflow hole, block the second plughole and the overflow with a wet cloth so the pressure has nowhere to escape. Place the plunger over the plughole, press down to form a seal, then pump firmly up and down ten to fifteen times. Pull away sharply on the last push, and repeat two or three times if the water starts to drain.
Step 5: Clean the U-Bend
If the blockage is still there, it is most often sitting in the U-bend, the curved section of pipe under the sink. Put the bucket underneath to catch the water, then unscrew the two fittings on either side of the bend by hand or with grips. Tip the trapped water and debris into the bucket, clear out any grease or food by hand, rinse the section, and screw it back on. Check the seals sit flush so it does not leak.
Step 6: Use a Drain Snake
For a blockage deeper in the pipe, feed a drain snake into the plughole or the open U-bend and turn the handle to push it through. When you feel it catch, work it back and forth to break the blockage up, then flush the pipe with hot water. If the snake will not reach, or the blockage will not move, the problem is likely further down the drain run and needs professional equipment.
Should You Use Chemical Drain Cleaners?
We would not recommend chemical drain cleaners for a blocked kitchen sink. The caustic chemicals can damage older pipes, harm septic systems, and often only burn a narrow hole through the blockage rather than clearing it, so the problem returns within weeks.
Here is what they actually do. Most work by generating heat to dissolve the blockage, and that heat is the problem. It can soften and warp plastic pipes, corrode metal fittings, and crack the clay or pitch-fibre pipework found in many older Cornish homes. On a septic system, the same chemicals kill the bacteria the tank relies on to break down waste.
Even when a cleaner does clear the water, it often bores a narrow channel straight through the grease rather than removing it. The pipe stays coated, so it blocks again, and you are left buying more of the same. A blockage cleared properly stays cleared.
If repeated chemical use has already damaged a pipe, Drain Pipe Relining can repair it from the inside, with no digging required.
How to Stop Your Kitchen Sink Blocking Again
The simplest way to stop a kitchen sink blocking is to keep fat, oil, and food waste out of it. Let cooking fat cool and set, then scrape it into the bin. Fit a strainer over the plughole to catch scraps, and run hot water through the drain after washing greasy pans.
- Pour cooled cooking oil into an old jar or container and put it in the bin, never down the sink
- Scrape plates and pans into the food waste caddy or bin before washing up
- Fit a plughole strainer and empty it regularly
- Run hot water and a little washing-up liquid through the drain once a week
- Keep coffee grounds, rice, and pasta out of the sink, as they swell and clump
These habits matter because a blocked pipe inside your property boundary is your responsibility to fix. Thames Water puts the cost of clearing a blocked home pipe at more than £200. Prevention costs far less than a call-out.
If your home runs on a septic tank, the same rules protect the system underground. Regular Septic Tank Maintenance keeps grease and solids from building up where they cause the most damage.
When Should You Call a Professional?
Call a professional if the blockage keeps coming back, if more than one drain in the house is slow at the same time, if water backs up when you use the washing machine or dishwasher, or if there is a bad smell you cannot clear. These signs point to a blockage deeper in the drain, not in the sink itself.
When several drains slow down together, the blockage is often in the shared pipe that carries wastewater away from the house, not in the sink trap. No amount of plunging will reach it.
This is where professional equipment comes in. High-Pressure Jetting scours the full width of the pipe and clears grease, roots, and debris a drain snake cannot touch. A CCTV Drain Survey sends a camera through the line to pinpoint the exact cause and location, so the repair is done once, properly.
Clearing a Blocked Kitchen Sink: The Short Version
Most blocked kitchen sinks come down to grease and food waste, and most clear with the steps in this guide. Work through them in order:
- Bail out standing water and clear the plughole
- Flush with hot water, washing-up liquid, and soda crystals
- Plunge, then clean the U-bend if the water still will not drain
- Keep chemical cleaners well away from your pipes
If the blockage keeps returning, or more than one drain is slow, the cause is deeper in the system and needs an engineer. Clear Stream covers the whole of Cornwall with no call-out fee, a one to two hour response, and a 5-Year Guarantee on every repair. For a blockage that will not clear, call our team now on 01872 222555 or visit clearstreamdrainage.co.uk.
Frequently Asked Questions
Why Is My Kitchen Sink Draining Slowly but Not Fully Blocked?
A slow drain means grease and food waste have narrowed the pipe without sealing it completely. It is an early warning. Clear it now with hot water, washing-up liquid, and soda crystals before it hardens into a full blockage.
Can I Pour Boiling Water Down to Unblock a Kitchen Sink?
Hot water helps soften grease, but boiling water is best avoided. It can move melted grease further along the pipe where it sets again, and it can damage plastic pipe joints and seals. Use hot tap water, or a kettle left to stand for a minute, instead.
Does Baking Soda and Vinegar Actually Unblock a Sink?
It works on light grease and food build-up. The reaction helps loosen the residue, and a hot water flush carries it away. For a solid or deep blockage it will not be enough, and you will need a plunger or a look inside the U-bend.
Why Does My Kitchen Sink Have a Bad Smell?
A smell often comes from food and grease trapped in the U-bend, or from a dry trap letting sewer air back up. Clean out the U-bend and run water through it. If the smell returns quickly, there may be a build-up further down the pipe.
How Much Does It Cost to Unblock a Kitchen Sink?
It depends on where the blockage sits and how it is cleared. A blocked pipe inside your property is your responsibility, and Thames Water estimates clearing one at more than £200. Clear Stream charges no call-out fee and gives you a fixed-price quote before any work starts, so you know the cost upfront.
Can a Blocked Kitchen Sink Clear Itself?
A minor slow drain can sometimes ease as water works through it, but a true blockage will not clear on its own. Grease and food waste keep building until the pipe is fully sealed. It is better to clear it early than wait for it to flood the sink.
