UncategorizedTL;DR: Pouring fat or grease down the sink is one of the fastest ways to block a domestic drain. Fat cools within seconds of leaving the pan, sticks to pipe walls, and hardens into a solid mass that traps everything passing through. South West Water clears thousands of avoidable blockages every year across Cornwall and Devon. Bin your fat, and call a drainage specialist if your sink is already draining slowly.
Almost every blocked kitchen drain we clear in Cornwall starts the same way: someone poured fat or grease down the sink. It seemed harmless at the time. The pan was hot, the fat was liquid, and it disappeared down the plughole without any fuss.
That fat did not go away. It travelled a few metres into a cold pipe, cooled, and stuck fast. Water UK research found that 27% of UK adults admit to pouring cooking fats and oils down the sink, and that sewer blockages cost the UK up to £200 million each year, with more than 3,000 homes flooded as a result.
Most people have no idea it is a problem until wastewater backs up into the sink. By then the repair bill has already been decided. This guide explains what fat actually does inside your pipes, why Cornwall properties are particularly exposed, and how to dispose of cooking fat properly.
What Happens When You Pour Fat Down the Sink?
Fat poured down a sink cools rapidly once it enters the drain and changes from a liquid back into a solid. It coats the inside of the pipe in a layer that thickens with every wash-up. That layer narrows the pipe, catches food particles and coffee grounds, and eventually seals the drain completely.
The process is quicker than most homeowners expect. A pipe under a Cornish kitchen sits at ground temperature, which for most of the year is under 12°C. Cooking fat starts to solidify well above that.
There is a chemical reaction at work too. Fats react with calcium in wastewater to form a hard, soap-like deposit, a process known as saponification. Saponification is what turns soft grease into a substance with the consistency of candle wax, then eventually concrete. It does not rinse away. It has to be cut, scraped or jetted out.
The fats causing the damage are not exotic. They are the ones in your kitchen right now:
- Roasting fat and meat juices from the tray
- Frying oil, including olive oil and vegetable oil
- Butter, lard, ghee and dripping
- Cream, custard, gravy and sauces
- Salad dressings and mayonnaise
- Melted cheese scraped from a pan
Liquid at the point of disposal does not mean liquid at the point of arrival. That distinction is the whole problem.
Hot Water and Washing-Up Liquid Do Not Solve It
Flushing fat with boiling water and detergent does not remove it from the drainage system. It moves the fat further down the pipe, where it cools anyway and hardens somewhere less accessible. The blockage still forms. It just forms in a spot that costs more to reach.
Detergent emulsifies grease temporarily, breaking it into droplets suspended in water. Those droplets separate out again within metres as the water cools. What was a manageable deposit under the sink becomes a deposit in the underground run to the main sewer, or in a shared drain serving several properties.
The same applies to running the hot tap for longer, tipping fat down an outside gully, or pouring it into a toilet. Every route leads to the same cold pipe.
Why Slow Drainage Is a Warning, Not an Inconvenience
A sink that empties slowly is telling you the pipe has narrowed. Grease build-up rarely announces itself with a sudden blockage. It creeps: a slightly slower drain in March, a gurgle in June, a full backup on the day you have guests.
Homeowners who act at the slow-drainage stage pay for a straightforward clearance. Those who wait pay for an emergency callout, and sometimes for damage to the pipe itself.
Fatbergs Start in Ordinary Kitchens
A fatberg is a large solid mass inside a sewer, formed when fats, oils and grease bind with wet wipes, sanitary products and other unflushable items. Fatbergs make headlines when they weigh 100 tonnes. They begin as a few tablespoons of roasting fat from a domestic sink.
Water UK points out that sewers were only ever designed to carry human waste, wastewater and toilet paper. Everything else is an intruder, and fat is the glue that holds the intruders together.
The consequences reach further than a blocked pipe:
- Sewage backing up into kitchens, bathrooms and gardens
- Pollution of watercourses, beaches and bathing waters
- Pump station failures at treatment works
- Repair costs recovered through customer water bills
South West Water reports that around one in five blockages in the South West are caused by fat, oil and grease combining with wet wipes, and that blockages of this type are behind roughly 60% of internal flooding incidents. Internal flooding means raw sewage inside a property. That is the endpoint of the pan you rinsed out last Sunday.
Cornwall Properties Are More Exposed Than Most
Cornwall’s drainage stock makes grease build-up worse than it would be in a modern estate elsewhere in England. Older clay pipework, long private runs to the main sewer, and heavy seasonal demand from holiday lets all give fat more opportunity to settle and harden.
Three factors stand out.
Old Clay and Pitch Fibre Pipework
Plenty of properties in Falmouth, Truro and Penzance still run on clay pipes laid decades ago. Clay joints and rough internal surfaces give fat something to grip. Cracks and displaced joints, common where granite and shillet ground has shifted, create ledges where grease collects first.
Long, Cold, Shallow Runs
Rural and coastal properties frequently have long private drain runs before they reach the public sewer. More pipe means more cooling time. Shallow runs in exposed coastal locations sit colder for longer, and fat sets faster in cold pipes.
Holiday Lets and Seasonal Surges
A holiday let in a Cornish coastal town might see twenty different households cook in one kitchen over a summer. Guests do not know the drainage history, and nobody is watching the sink for early warning signs. Blockages tend to surface in August, at the worst possible moment for the owner.
South West Water clears around 8,500 blockages a year at a cost of £4.5 million, and has named Falmouth, St Ives and Truro among its ten blockage hotspot areas. Local geography is not the cause. Local habits are.
If your drain is already backing up, our engineers cover the whole county for blocked drains in Cornwall, on site within one to two hours, with no call-out fee.
Can Fat Damage a Septic Tank?
Fat and grease seriously damage septic tanks. Grease floats on the surface of the tank, thickening the scum layer, and starves the bacteria that break down solid waste. A tank overloaded with fat empties more often, treats waste poorly, and can push grease into the soakaway, where it clogs the drainage field permanently.
Thousands of Cornwall properties sit off mains drainage, particularly on the Lizard, around Bodmin Moor and across the rural west. For those homes, fat down the sink is not a sewer problem. It is a problem inside an asset the owner is legally responsible for.
Signs a tank is struggling with grease include:
- Odours around the tank or drainage field
- Emptying required more frequently than every 12 months
- Standing water or soggy ground above the soakaway
- Slow drainage across multiple fixtures at once
A blocked drainage field is expensive to replace and rarely covered by insurance. Regular Septic Tank Maintenance catches grease loading before it reaches that stage.
How Should You Dispose of Cooking Fat and Oil?
Cooking fat and oil should go in the bin, not the sink. Let it cool in the pan until it solidifies, scrape it into a container or straight into your rubbish, and wipe out the residue with kitchen roll before washing up. Larger quantities of used oil can be taken to a Household Waste Recycling Centre.
The routine takes about thirty seconds:
- Leave the fat in the pan to cool and set. Do not rinse it while it is hot.
- Scrape the solidified fat into an old jar, a yoghurt pot or a takeaway tub.
- Wipe the pan with kitchen roll to lift the greasy film, then bin the paper.
- Fit a strainer over the plughole to catch food scraps.
- Wash up as normal, with a pan that is already mostly grease-free.
For larger volumes, such as oil from a deep fryer, Cornwall Council accepts up to 5 litres of cooking oil at most Household Waste Recycling Centres. There are 14 sites across the county, so there is one within reasonable reach of most homes.
Running a food business? Grease management is a legal obligation, and a grease trap is the practical answer. It is also considerably less expensive than a shutdown during peak season.
What to Do If Your Drain Is Already Blocked With Grease
A drain blocked with hardened grease needs mechanical clearance, not chemical drain cleaner. High-Pressure Jetting cuts the deposit from the pipe wall and flushes it out of the system. A CCTV Drain Survey afterwards confirms the pipe is clear and shows whether the grease has been hiding damage underneath.
Shop-bought drain unblockers are caustic. They can soften a surface layer of fat, but they rarely clear a hardened blockage, and repeated use degrades older clay and plastic pipework. Some of the worst pipe damage we see in Cornwall follows months of chemical treatment on a drain that needed jetting from day one.
Where grease has been sitting in a cracked or displaced pipe, clearing the blockage exposes the underlying fault. A CCTV Drain Survey gives you the evidence, on camera, before you commit to any repair. If the pipe is damaged, Drain Pipe Relining rebuilds it from the inside without excavating your kitchen floor or driveway.
Every repair we carry out is covered by a 5-year guarantee, and you get a fixed-price quote before any work starts.
Bin the Fat, Protect the Pipe
Fat down the sink is the single most avoidable cause of blocked drains in Cornwall homes. The cure is a jar and a sheet of kitchen roll. The alternative is an emergency callout, a damaged pipe, or sewage on your kitchen floor during peak season.
Three things worth doing this week:
- Put a container by the hob for cooling fat
- Act on slow drainage now, not when it stops entirely
- Book a survey if your drains have blocked more than once
If your sink is already draining slowly, or a blockage has taken hold, call our emergency team right now on 01872 222555. We are available 24/7, we are on site anywhere in Cornwall within one to two hours, and there is no call-out fee.
Frequently Asked Questions
Is it illegal to pour fat down the sink in the UK?
For households, pouring cooking fat down the sink is not a criminal offence, but you are responsible for blockages in your private drainage. For food businesses, the position is different: commercial kitchens have a legal duty to prevent fats, oils and grease entering the sewer, and water companies can take enforcement action against premises that cause blockages.
Can I pour fat down the sink with hot water and washing-up liquid?
No. Hot water and detergent only keep fat liquid for a few metres. The fat cools further down the pipe and hardens there instead, creating a blockage that is harder and more expensive to reach.
How long does it take for grease to block a drain?
Grease builds up gradually rather than all at once. A kitchen drain receiving fat regularly can narrow noticeably within months, with slow drainage appearing long before a full blockage. Older clay pipework blocks faster than smooth modern plastic.
Does bicarbonate of soda and vinegar clear a grease blockage?
It does not clear an established grease blockage. The fizzing reaction has no cutting power against a hardened deposit stuck to the pipe wall. Jetting is the reliable method.
Can fat damage my septic tank?
Yes. Grease thickens the scum layer in the tank, disrupts the bacteria that break down waste, and can carry through to the soakaway. A clogged drainage field is one of the most expensive septic tank failures to put right.
Where can I dispose of used cooking oil in Cornwall?
Cornwall Council accepts up to 5 litres of cooking oil at most of its 14 Household Waste Recycling Centres. Smaller amounts of solidified fat can be scraped into a container and put in your household rubbish.
What should I do if my kitchen drain is already blocked?
Stop using the sink and call a drainage specialist. High-Pressure Jetting clears hardened grease properly, and a CCTV Drain Survey confirms whether the pipe underneath is damaged. Clear Stream Drainage Solutions covers the whole of Cornwall with a one to two hour response and no call-out fee. Call 01872 222555.
