Stop Kitchen Drain Blocking | The FOG Field Guide

Fat doesn't behave the way most people think it does. You pour it down the sink hot, it goes liquid, it disappears. Job done. But a metre or two into your waste run, the pipe cools. The fat cools with it. It sticks to the pipe wall, sets, and the next lot sticks to that. Every roast dinner, every fried egg, every wipe of a greasy pan with a bit of hot water is adding another fraction of a millimetre to a ring of congealed fat that's slowly strangling your drain.

This is the slow blockage. Not the dramatic sudden block of a foreign object. The kind where the kitchen sink drains a little slower in October than it did in June, and by Christmas you're standing in a puddle every time you run the tap.

Here's how to get ahead of it.

Understand What's Actually Happening in Your Pipes

The technical term is FOG: fat, oil and grease. Drainage engineers talk about it constantly because it's behind the majority of kitchen drain calls we attend, domestic and commercial alike.

The trap under your sink is the first place it accumulates. That U-bend holds a permanent pool of water to stop sewer gases coming back up. It also catches everything that cools quickly. Even with a bottle of hot water poured after it, the grease is already coating the plastic before it reaches the trap. The trap itself narrows over months into something barely wider than a pencil.

Beyond the trap, the waste run to the stack or the inspection chamber is usually a 40mm or 50mm plastic pipe running horizontally, sometimes with a slight fall, sometimes barely any. Fat loves a shallow-fall pipe. It sets, it layers, and it doesn't shift on its own.

In older properties in Eton or Old Windsor with clay drainage, the joins between pipe sections can snag grease even faster. A rough join is a ledge. The fat catches, hardens, and starts building a plug.

The Field Guide: Check These in Order

Start at the plug hole. Remove the strainer. Look at the underside. If there's a grey-white waxy film on it, that's solidified fat. That's also what the inside of your pipe looks like. Clean the strainer with hot water and washing-up liquid, and note how bad the build-up was.

Time your drain. Fill the sink to about a quarter full. Pull the plug and watch. A clear kitchen drain empties in under 15 seconds. If yours takes 30 or more, there's restriction already. This isn't a blockage yet. It's a warning.

Check under the sink for drips. When a trap starts to block and water backs up, the pressure finds the weakest joint. Look for white mineral deposits or watermarks on the pipe fittings, signs that water has been sitting or weeping.

Don't pour boiling water as a fix. This one matters. Boiling water does soften fat. It also just moves it further down the pipe where it re-sets, often at the junction with the main stack or at a bend. You haven't cleared it, you've relocated it. The block that follows is harder to shift.

Don't use caustic drain cleaner as a routine habit. Once, when it's already blocked, with care. Regularly? You're softening the fat momentarily and you're also attacking the rubber seals in your fittings. A leaking trap joint is a worse problem than a slow drain.

Buy a sink plunger and use it monthly. Not the disc type. A cup plunger with a fold-out rubber flange. Block the overflow hole with a damp cloth, fill the sink slightly, and give it 10 to 12 firm strokes. This shifts early-stage FOG build-up before it hardens fully. Three quid from any hardware shop. Worth every penny.

The one piece of kit that actually helps: a grease trap. For domestic use, a small in-line grease interceptor fits under the sink between the trap and the waste run. For commercial kitchens, a properly sized grease trap is not optional. More on that below. Domestic interceptors cost £30 to £80. They catch and hold grease before it enters the pipework, and they're cleanable.

For the trap itself: remove and clean it twice a year. Turn off the water. Put a bowl under the U-bend. Unscrew the two slip-joint nuts by hand. The trap will drop. Tip the contents into the bowl. What you see is what's been going into your drain. Wipe the inside of the trap with a cloth. Refit. Do this in spring and autumn and your trap will never silently throttle your sink.

A Word on Habits That Actually Work

Wipe every greasy pan with kitchen roll before washing it. Every time. The roll goes in the bin, the grease goes with it. That one habit reduces FOG entering your drain by more than any chemical product will.

Let cooking water with fat in it cool in the pan until the fat solidifies. Spoon or scrape the solid fat into a container. A tin, a jar. When it's full, it goes in the general waste bin. Councils across the SL4 area have been pushing this for years. It works.

If you're using a dishwasher, scrape plates before loading. The fat and food debris that runs off in the wash cycle still ends up somewhere in your drainage system.

Commercial Kitchens: Different Scale, Same Problem

If you run a kitchen on a high street unit or in a trading estate in Dedworth or Spital, the FOG problem isn't a minor inconvenience. A blocked kitchen drain in a commercial kitchen shuts service. In busy periods, that's a direct revenue hit.

Commercial grease traps need to be emptied and serviced on a schedule, typically every one to three months depending on kitchen output. A restaurant producing 50 covers a night will fill a trap far faster than a café doing breakfasts twice a week. If you're not sure what size trap you need or when it was last serviced, a CCTV drain survey on your kitchen waste run will show you exactly what's building up and where.

High-pressure water jetting clears FOG build-up in commercial waste runs quickly and completely. It's the only method that actually removes the material rather than pushing it along. A planned jetting session twice a year costs far less than an emergency call-out during Friday evening service.

Reactive works when you have a single-site café and a simple drainage layout. Planned preventative maintenance beats reactive every time when you have a kitchen producing volume, multiple runs, or an older drainage system with clay pipes or awkward falls.

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If your kitchen sink is draining slower than it should, or you're in a commercial kitchen that's overdue a drain check, Drainsco covers Windsor and the surrounding SL postcodes with engineers available around the clock. No call-out fee. A clear price before any work starts. Call to talk through what you're seeing and we'll tell you honestly whether it needs jetting, a trap clean, or a CCTV survey to find what's further down.