The short answer
FOG stands for fat, oil and grease, the cooking residues that pour down the sink as warm liquid and then cool and congeal inside the pipe. As it hardens it sticks to the pipe walls and traps food scraps, wet wipes and other debris, gradually narrowing the drain until it blocks. On a larger scale in the sewers, FOG combines with wipes to form fatbergs, the solid masses water companies spend large sums clearing. The fix is prevention: never pour fat, oil or grease down the drain; let it cool, then bin or recycle it.
Fat, oil and grease is one of the leading causes of blocked drains in UK homes and a major problem for the public sewers. Understanding how it builds up makes it easy to avoid.
At a glance
- FOGFat, oil and grease
- How it blocksCools and congeals on pipe walls
- TrapsFood scraps, wipes, debris
- Large scaleForms fatbergs in sewers
- PreventionCool then bin, never pour away
How FOG builds up
When you wash up after cooking, fat from the pan, oil from a roasting tin and grease from plates go down the sink as a warm liquid. It seems to disappear, but inside the cooler drain it solidifies. Each time more is added, a fresh layer sets onto the pipe wall, like wax building up inside a candle mould. Over weeks and months the pipe's internal diameter shrinks.
The congealed fat is also sticky, so it captures anything else passing through: coffee grounds, rice, pasta, food scraps and, most damagingly, wet wipes and sanitary items. These bind into the fat and accelerate the narrowing. Eventually the drain can no longer cope and backs up, often showing first as a slow-draining kitchen sink and a foul smell.
From household blockage to fatberg
Multiply one kitchen's FOG by every home connected to a sewer and the scale of the problem becomes clear. In the public sewers, congealed fat combines with non-flushable items, especially wet wipes, to form fatbergs: solid, rock-hard masses that can grow to fill a sewer and cause widespread flooding. Water companies across the UK spend heavily each year clearing them, and the cost ultimately feeds into customer bills.
The items that bind into fatbergs are exactly the things that should never be flushed or poured away. Knowing what belongs in the bin rather than the drain is the single most effective way to prevent both household blockages and sewer fatbergs.
| Item | Where it should go |
|---|---|
| Cooking fat and oil | Cool, then bin or recycle |
| Pan grease and dripping | Wipe with kitchen roll, bin it |
| Wet wipes (even 'flushable') | Bin, never flush |
| Food scraps | Food waste caddy or bin |
| Sanitary items and cotton buds | Bin, never flush |
What to keep out of drains to prevent FOG blockages. For guidance only.
Preventing and dealing with FOG
Prevention is straightforward. Let fat and oil cool in the pan, then scrape it into the bin or pour it into a container to dispose of with general waste; some councils accept used cooking oil for recycling. Wipe greasy pans and plates with kitchen roll before washing up so less grease reaches the drain. Fit a sink strainer to catch food scraps, and only ever flush the three Ps: pee, paper and poo.
If FOG has already built up, a slow kitchen sink is the early warning. Pouring boiling water down the sink can shift a thin, fresh layer, and a plunger may help, but established fat deposits often need mechanical clearing. A drainage engineer can use a high-pressure water jet to cut through and flush out hardened grease, which is more effective than chemical drain cleaners on a fat blockage.
Caustic-based drain cleaners can dissolve some grease but should be used with care, following the safety instructions exactly, and are not a long-term answer if FOG keeps being poured down the sink. The reliable fix is to stop fat entering the drain in the first place.
Spotting and removing an established FOG blockage
FOG builds up slowly, so the warning signs appear gradually. A kitchen sink that drains a little slower each week, a faint greasy smell from the plughole, and gurgling as water struggles past the narrowed section all point to fat coating the pipe. Because the kitchen sink is usually the first fixture to suffer, it acts as an early-warning gauge for grease buildup before the drain blocks completely.
Once a deposit is established, gentle methods rarely clear it fully. Boiling water and washing-up liquid may loosen the surface but tend to move softened fat downstream, where it cools and re-sets in a less accessible spot. Cleaning the U-bend trap under the sink removes grease collected there, but fat further along the waste pipe stays put. The most thorough removal is high-pressure water jetting by a drainage engineer, which scours the pipe walls back to a clean bore rather than just punching a hole through the deposit.
After clearing, the only way to keep the drain clear is to change what goes into it. Scraping plates, wiping greasy pans before washing up, fitting a strainer, and disposing of fat and oil in the bin or for recycling all keep FOG out. Enzyme-based maintenance products can help digest light grease over time and are gentler than caustic cleaners, but no product compensates for fat being poured down the sink in the first place.
Frequently asked questions
Are 'flushable' wet wipes safe for drains?
No. Despite the label, most wipes do not break down like toilet paper and are a leading component of fatbergs. They should always go in the bin, never down the toilet.
Can boiling water clear a fat blockage?
It can help shift a thin, recently-formed layer of grease, but it rarely clears an established, hardened deposit. Repeated use also risks pushing the fat further down to re-set elsewhere.
How do I dispose of used cooking oil?
Let it cool, then pour it into a sealable container and put it in your general waste, or take larger amounts to a recycling centre that accepts cooking oil. Never pour it down the sink or an outside drain.
Sources & further reading
Figures on this page are typical UK ranges drawn from published cost guides and are intended as guidance, not a quotation.