Guides · Harrow

What Actually Causes Blocked Drains

Blockages fall into two categories: things that were put down the drain, and things that happened to the pipe. Which one you have determines whether clearing it solves the problem or whether it will happen again. This page covers both.

Fat, oil and grease

Cooking fat poured down a sink is liquid when it goes in and solid a few metres later once it cools. It coats the pipe wall, narrows the bore, and then catches everything that passes - food debris, paper, hair - until the run closes entirely. This is the single most common cause of kitchen-side blockages, and it is entirely preventable: let fat cool, and put it in the bin rather than the sink. Running hot water afterwards does not solve it, because the fat simply solidifies further along the run where it is harder to reach.

Wet wipes and what "flushable" actually means

Wet wipes are the dominant cause of serious drain and sewer blockages, and the labelling is misleading. A wipe marked flushable will pass through the toilet, which is all the claim means. It will not break down the way toilet paper does, so it travels until it snags on a joint, a bend or a root, and then catches everything behind it. The same applies to kitchen roll, cotton wool, sanitary products, cotton buds and dental floss. The rule that avoids essentially all of this: nothing goes down a toilet except the three Ps - pee, paper and poo.

Tree roots

Roots do not break into sound pipework. They find moisture escaping from a joint that has already opened or a crack that has already formed, and then grow into it, which is why root ingress is both a cause of blockage and a sign of an existing defect. Once inside, a root mass acts as a net and traps everything passing through. This is a significant factor across Harrow given the age of the drainage and the amount of mature planting - the borough's street trees and the large established gardens in Pinner, Stanmore and Harrow Weald sit directly over Victorian and inter-war clay runs. Root blockages recur on a predictable cycle unless the damaged section is repaired or lined.

Structural problems: collapse, dropped joints and displaced pipes

Clay pipework laid before the war was not designed to last a century. Joints drop as the ground settles, sections crack, and pipes can partially collapse, leaving a lip that catches debris. Ground movement contributes: parts of the borough sit on London Clay, which shrinks and swells with moisture content, and that movement over decades is enough to displace rigid pipework. Where the cause is structural, clearing the blockage buys time rather than fixing anything - the run will block again, usually within months. A CCTV survey is what distinguishes the two.

Foreign objects and building debris

Two recurring culprits. The first is objects flushed by children - toys, nappies, whole toilet rolls - which typically block close to the property and are straightforward to clear. The second is less obvious and more damaging: debris from building work. Sand, cement, tile adhesive and plaster washed into a drain during a renovation set hard inside the pipe and are extremely difficult to remove. If you are having work done, make sure the trades on site are not rinsing tools into an outside gully. It is a common cause of blockages appearing a few weeks after an extension or a bathroom refit.

Common questions

How can I tell if it is roots?

Root blockages tend to recur on a seasonal cycle, are more common in properties with mature trees within about ten metres of the drain run, and often clear easily with jetting only to return. The definitive answer comes from a CCTV survey, which shows the ingress point and the extent of the damage.

Can I be liable if my tree blocks a neighbour's drain?

Potentially, where roots from a tree on your land cause damage to a neighbour's property. In practice these situations usually turn on whether the pipe was already defective. It is worth taking proper advice rather than assuming either way, and worth resolving informally with your neighbour first.

Does a blocked drain mean I need a new one?

Usually not. Most blockages are obstructions in sound pipework and clear with rodding or jetting. Replacement or lining is only needed where a survey shows structural damage - a collapse, a badly dropped joint, or extensive root damage - and lining is often possible without excavating the run.