Grease Build-Up Explained

Warm cooking fat may look harmless when it leaves a sink, but it cools and sticks to the inside of the pipe. Food particles, soap and other debris then attach to the greasy layer until the opening becomes too narrow for normal flow.

Before you worry

A slow kitchen sink may be caused by the trap, a short branch line or a deeper shared drain. Note whether the washing machine, dishwasher or nearby fixtures are also affected before deciding where the problem lies.

How to recognise the problem

• The kitchen sink drains slowly.
• A stale or rancid smell comes from the waste pipe.
• Water backs up when the dishwasher discharges.
• The blockage improves briefly and then returns.
• Dark greasy residue appears in the trap or waste line.

Common causes

• Cooking oil, animal fat, butter and sauces poured into sinks.
• Food scraps passing through without a strainer.
• Soap residue combining with fat deposits.
• Long horizontal pipe runs with low flow.
• Cold sections of pipe where grease solidifies quickly.

What you can safely check yourself

• Allow grease to cool and place it in a sealed waste container.
• Clean an accessible sink strainer and trap if you can do so safely.
• Use hot tap water and ordinary dishwashing liquid only for light fresh residue, not a complete blockage.
• Stop the dishwasher if water is backing up into the sink.

What you should never do

• Do not pour additional oil or fat down the drain.
• Do not mix caustic drain products.
• Do not force a chemical-filled trap open without protective precautions.
• Do not rely on repeated boiling-water treatments for plastic fittings.

When to call a plumber

Call a plumber when the blockage is complete, recurs quickly, affects appliances, or appears deeper than the trap. Commercial kitchens and shared drain lines often need equipment capable of cleaning the full internal diameter.

How professionals solve it

The trap and branch are assessed first. A drain snake may open a local obstruction, while hydro jetting can remove grease from a longer section more thoroughly. Where the build-up is recurrent, camera inspection can reveal poor gradient or damaged pipework.

How to prevent it happening again

• Scrape plates into the bin before washing.
• Use sink strainers.
• Collect oils and fats separately.
• Run appliances according to manufacturer guidance.
• Arrange maintenance cleaning for high-use commercial kitchens.

Frequently asked questions