What Actually Happens When You Pour Fat Down the Sink

It seems harmless enough. The fat is hot, it's liquid, and it disappears down the plughole without any fuss. But what happens next is exactly why fat, oil and grease (FOG) are one of the biggest causes of blocked drains in the UK.

Fat doesn't stay liquid once it leaves your sink. As it cools inside the pipe, it solidifies, and once it starts sticking to the sides, it rarely stops there.

Below, we'll cover exactly what happens when fat goes down your drain, why it's such a widespread problem, and what to do instead.

Need help right now? Call us on 0203 870 1067 — we offer emergency drain unblocking and grease management services across the UK.

What happens to fat once it's down the drain

Hot fat flows easily, which is exactly the problem. As it travels through your pipes, it cools quickly and starts to solidify against the inner walls.

Over time, this creates a coating that:

• Narrows the pipe, reducing how much water can flow through
• Traps other debris, like food scraps, wipes and hair, which stick to the grease layer
• Builds up further with every subsequent use, growing thicker over weeks and months
• Eventually blocks the pipe completely, often at a bend or narrow section

This is how a single pour of fat becomes part of a much bigger blockage, one that usually takes far more than a plunger to shift.

The scale of the problem

FOG build-up isn't a minor issue, it's one of the leading causes of drain blockages nationally. Fat, oil and grease are estimated to be behind the majority of severe blockages reported each year, costing the water industry tens of millions of pounds annually in call-outs, repairs and network maintenance.

In extreme cases, FOG build-up in the public sewer network has created "fatbergs", solid masses of congealed fat mixed with wipes and debris large enough to block entire sections of sewer. While most household FOG build-up is smaller scale, it follows exactly the same process, just in your own pipes rather than the public network.

Signs fat has been building up in your pipes

You won't always notice a problem straight away, but a few signs point to FOG build-up:

• Water draining more slowly than it used to, even without an obvious blockage
• A greasy residue visible around plugholes or drain covers
• Recurring blockages in the kitchen sink specifically
• Unpleasant smells coming from the kitchen drain

If any of these sound familiar, there's likely already a build-up forming, even if it hasn't caused a full blockage yet.

What to do instead of pouring fat down the sink

A few simple habits prevent the problem building up in the first place:

1. Let fat cool and solidify, then bin it — pour it into a container, let it set, and throw it away rather than down the sink
2. Wipe greasy pans before washing — a quick wipe with kitchen roll removes most of the fat before it ever reaches the drain
3. Use a sink strainer — this catches food scraps that would otherwise combine with grease and speed up blockages
4. Avoid "flushing it through" with hot water — this just moves the fat further down the pipe before it cools and resolidifies elsewhere
5. Book a professional clean if you suspect build-up — this removes existing grease deposits before they cause a full blockage

For commercial kitchens

Households aren't the only source of the problem, commercial kitchens produce far higher volumes of FOG and are subject to specific rules around how it's managed. If you run a kitchen that produces significant fat, oil or grease waste, our grease management service covers everything from grease trap servicing to compliant waste disposal.

Contact us today

ICE Drainage provides drain unblocking and grease management services for homes and businesses across the UK, helping prevent FOG-related blockages before they happen.

To book a clean or ask a question, contact our team on 0203 870 1067 or contactus@icedrainage.co.uk