What Causes Blocked Drains? The Most Common Culprits

Fat and grease poured down kitchen sinks is the single biggest cause of blocked drains, followed by wet wipes and sanitary items flushed down toilets, hair building up in bathroom drains, and tree roots intruding into older pipework. Most blockages are preventable once you know what’s actually causing them.

Fat, oil, and grease (FOG)

Pouring cooking fat or oil down the sink feels harmless when it’s still liquid and hot — but it cools as it travels through the pipe, solidifying into a waxy layer on the inside walls. Over weeks and months, food particles and other debris stick to this layer, narrowing the pipe until it blocks completely. This is the most common cause we see in kitchen drains by a wide margin.

Prevention: let fat cool in the pan, then scrape it into the bin or a container rather than the sink.

Wet wipes and sanitary products

Toilet paper is designed to break down in water; almost nothing else flushable is, regardless of “flushable” labelling on the packet. Wet wipes, sanitary towels, and cotton buds don’t disintegrate — they snag on pipe joints, roots, or existing debris and accumulate into a solid mass that eventually blocks the pipe entirely.

Prevention: bin everything except toilet paper — no exceptions, regardless of packaging claims.

Hair (bathroom drains)

Hair combines with soap scum to form a sticky mass inside shower and bath drains. It happens gradually, so people often don’t notice until the drain is running very slowly.

Prevention: a simple hair-catcher over the plughole, emptied regularly, prevents almost all of this.

Tree roots

In older properties with clay pipework, small cracks or degraded joints let a tiny amount of moisture escape — which attracts tree roots seeking water. Roots grow toward and eventually into the pipe, restricting flow or cracking it further. This is more common near mature trees and in Victorian-era drainage systems, and it’s a genuine structural issue rather than something a simple unblock will permanently fix — it usually needs a CCTV survey to properly diagnose.

Limescale and mineral build-up

In hard-water areas, limescale can build up inside pipes over years, narrowing the internal diameter gradually — similar to FOG blockages but mineral rather than organic. It’s a slower process than fat build-up, so it tends to show up as a drain that’s been “a bit slow for ages” rather than a sudden total blockage.

Food waste and general kitchen debris

Even without pouring fat directly down the sink, general food scraps rinsed off plates — rice, pasta, coffee grounds, vegetable peelings — add up over time, especially combined with any grease residue already coating the pipe. None of these are dramatic on their own, but they’re a steady, everyday contributor that most households don’t think twice about.

Prevention: scrape plates into the food bin before washing up, and consider a simple sink strainer to catch solids before they go down at all.

Structural problems (collapsed or misaligned pipes)

Occasionally a blockage isn’t caused by what’s gone down the drain at all — a section of pipe has shifted, cracked, or collapsed, usually due to ground movement or age. This is less common but explains why some drains block repeatedly no matter how carefully you use them; if that sounds like your situation, it’s worth reading about who’s responsible for blocked drains since a structural fault in a shared section may not be yours to fix.

If you already have a blockage

For most FOG, wipe, or hair blockages, drain rods or jetting will clear it without any digging — see our guide on unblocking a drain without digging. If it keeps recurring despite careful use, that’s usually a sign of roots or a structural issue that needs a proper look.

We clear blocked drains across Norwich and the surrounding area, usually the same day. Get in touch if you’ve got a blockage that needs sorting.