INTRODUCTION
A kitchen sink draining slowly is nearly always caused by fat, food waste and soap scum hardening inside the trap or waste pipe under the sink. In hard water areas like London, limescale sticks to that layer and makes it worse. Most slow draining kitchen sinks clear with hot water and washing up liquid, a plunger, or by unscrewing and cleaning the trap by hand. If water backs up in both bowls, or the problem returns within a week of clearing it, the kitchen sink blockage sits further down the waste pipe and needs a plumber.
Water sits in the bowl a second or two longer than it used to. Then a second or two becomes a minute. Then you’re standing there watching a grey puddle sink away while the kettle boils.
A kitchen sink draining slowly is one of the most common jobs our plumbers get called out to across South London, and it almost never happens overnight. It builds up over months. That’s good news, because catching it early usually means a twenty minute fix that costs nothing.
What causes a kitchen sink to drain slowly?
A slow drain and a completely blocked drain are the same problem at two different stages. The water is still getting through, just not easily. Something has narrowed the pipe.
Fat, oil and grease
This is the main one. Cooking fat goes down the plughole as a warm liquid and cools inside the waste pipe, where it sets into a waxy layer on the pipe wall. Every wash after that adds another thin coat. Rinsing with hot water doesn’t remove it, it just pushes the fat a bit further along before it cools again.
Anything oily counts here: roasting tin drippings, leftover curry, salad dressing, the film on a pan after frying bacon.
Food waste and coffee grounds
Rice, pasta, tea leaves and coffee grounds don’t dissolve. They swell, settle into the bend of the trap, and catch everything that comes down after them. Coffee grounds are the worst of them because they’re heavy and they clump.
Limescale and soap scum
South London sits in a hard water area. Calcium in the water reacts with washing up liquid and leaves a chalky scum that bonds to the fat already on the pipe. If you’ve got limescale on your kettle element or crusting around your taps, the same thing is happening inside the waste pipe where you can’t see it.
A blocked trap
The trap is the U-shaped or bottle-shaped fitting directly under the plughole. It holds a small amount of standing water to stop drain smells coming back up into the kitchen. That standing water also means anything heavy that goes down the plughole settles there first.
Roughly speaking, if only your kitchen sink is slow and every other drain in the house is fine, the trap is the first place to look.
A problem further down the waste pipe
If the trap is clean and the sink still drains slowly, the blockage is beyond it. That could be a build-up at a bend in the pipe, a section of pipework with too little fall on it, or a partial blockage where the kitchen waste joins the main soil stack. This is where DIY tends to run out of road.
Old or damaged pipework
Older London properties often have a mix of pipe materials joined together over decades. Rough joints, corroded sections and pipes that have sagged all give waste something to catch on. If a pipe has split you’ll usually get damp under the sink alongside the slow drainage, which is a burst pipe repair job rather than a blockage.
How do you work out where the blockage is?
Run through these before you touch anything:
- Fill the bowl, then release it. Does it drain slowly the whole way, or start fine and then stall?
- If you have a double sink, does the second bowl fill up when you empty the first? That points to a shared blockage past the trap.
- Check the bathroom sink, shower and toilet. If more than one is slow, the problem is the main drain, not your kitchen.
- Open the cupboard under the sink and look for damp patches, drips or a white crust on the pipe joints.
- Listen. A gurgling noise as the water goes down usually means air is being pulled through a partial blockage.
If everything in the house is draining badly at once, stop. That’s a main drain issue and it can back up fast. Our emergency plumbing team handles those.
How do you fix a slow draining kitchen sink?
Work through these in order. Start gentle.
Hot water and washing up liquid
Squeeze a generous amount of washing up liquid straight down the plughole, then pour a kettle of hot water after it. The detergent breaks down the fat and the hot water carries it away. Repeat two or three times.
Use hot, not boiling, if you have plastic waste pipes or a plastic trap. Boiling water can soften the joints.
Bicarbonate of soda and white vinegar
Tip about half a cup of bicarbonate of soda down the plughole, follow it with a cup of white vinegar, and leave it to fizz for fifteen minutes. Then flush with hot water. It’s mild, it’s cheap, and it works on light grease and soap scum. It won’t shift a solid blockage.
Clean the trap out by hand
This is the one that fixes most slow kitchen sinks, and it’s less horrible than it sounds.
Put a bucket underneath, unscrew the two collars either side of the trap by hand, and lower it into the bucket. Everything in there comes out with it. Clean it with a bottle brush and hot soapy water, check the rubber washers are still seated properly, and screw it back on. Run the tap and check for drips.
Plunger
Block the overflow with a wet cloth first, otherwise you’re just pushing air around. Fill the bowl with enough water to cover the plunger cup, then plunge with short sharp pushes rather than one big shove. On a double sink, block the second plughole too.
Drain snake
A hand-turned drain snake or auger goes past the trap and physically breaks up what’s sitting in the pipe. Feed it in slowly and turn as you go. Don’t force it around a bend, because a snake can crack an older waste pipe or push the blockage into a tighter section.
Which method should you use?
| Method | Works on | Time | Skill needed | Watch out for |
| Hot water and washing up liquid | Light grease | 10 minutes | None | Don’t use boiling water on plastic pipes |
| Bicarbonate and vinegar | Grease, soap scum, smells | 20 minutes | None | No use on solid blockages |
| Cleaning the trap | Food waste, hair, grease in the bend | 20 to 30 minutes | Low | Refit the washers correctly or it will drip |
| Plunger | Blockages just past the trap | 10 minutes | Low | Seal the overflow first |
| Drain snake | Blockages deeper in the waste pipe | 30 to 60 minutes | Medium | Can damage older or corroded pipework |
| Plumber with drain rods or a jetter | Anything beyond the waste pipe | Same day | Professional | Cost |
Do chemical drain unblockers work?
They do, on grease. They’re caustic and they eat through a fat layer quickly.
We’d still put them last. They damage older metal pipework and some plastic joints over time, and if they don’t clear the blockage you’re left with a sink full of standing caustic liquid that whoever comes next has to deal with. If you do use one, follow the contact time on the bottle, open a window, wear gloves, and never mix it with another product.
When should you call a plumber?
Get someone in if:
- The sink is still slow after you’ve cleaned the trap
- Water comes back up into the bowl, or the second bowl fills when you empty the first
- The same blockage returns within a week or two
- More than one drain in the house is slow
- There’s a smell of sewage rather than stale food
- You can see damp, staining or dripping under the sink
That last one matters more than people think. A slow drain with a wet cupboard usually means the waste pipe has a leak as well as a blockage, and the chipboard base of a kitchen unit will swell and crumble long before you notice.
Our kitchen repair specialists in South London deal with blocked waste, leaking traps and water damaged units, and our wider plumbing services cover the drain work behind them. If you’re planning a new kitchen anyway, our kitchen fitters in South London will set the waste run up with the right fall on it so this stops being an annual problem.
How do you stop it happening again?
- Pour cooking fat into an old jar and bin it once it sets
- Scrape plates into the bin before washing
- Put coffee grounds in the food waste caddy, not the plughole
- Fit a cheap plughole strainer and empty it
- Once a month, flush the drain with a kettle of hot water and washing up liquid
The monthly flush is the one that makes the difference. Two minutes, and it keeps the fat layer from setting hard.
CONCLUSION
If your kitchen sink is draining slowly, giving off unpleasant odours, or has become completely blocked, professional assistance can resolve the issue quickly and help prevent costly plumbing damage.
For homeowners across London, Plumr is one of the trusted plumbing specialists for kitchen sink unblocking, blocked kitchen drain clearing, kitchen sink drain cleaning, and complete kitchen plumbing services. Their experienced plumbers use professional equipment to remove stubborn blockages, restore proper drainage, and ensure your kitchen plumbing is working efficiently—all while delivering reliable service that meets current UK plumbing standards.
FAQs
- Why is my kitchen sink draining slowly but not blocked?
The pipe has narrowed rather than closed. A layer of cooled cooking fat, soap scum and limescale on the inside wall of the waste pipe leaves a smaller channel for water to get through. It drains, just slowly. Left alone it will block completely.
- Can I pour boiling water down my kitchen sink?
Only if you have metal waste pipes. Most modern kitchens use plastic waste pipes and a plastic trap, and repeated boiling water can soften the joints and cause leaks. Hot tap water with washing up liquid does the same job safely.
- Why does my other sink fill up when I empty the kitchen sink?
Both bowls share a waste pipe past the trap, so a blockage in that shared section pushes water back up the path of least resistance. It means the blockage is beyond the trap, and cleaning the trap on its own won’t fix it.
- How much does it cost to unblock a kitchen sink in London?
It depends on where the blockage sits and how long the plumber is on site. Clearing a trap or a short waste run is a quick job. A blockage in the main drain that needs rodding or jetting costs more. Ask for a price before work starts, and get a video quote if you’d rather not have someone out for something you could clear yourself.
