The short answer

Three things cause most of the drain blockages across North London. The first is older clay drainage that has had many decades of ground movement. The second is mature street trees whose roots find their way into pipe joints. The third is hard water, which slowly scales the inside of narrow pipes.

However, which one dominates depends on your street. Our general guide to what causes a drain blockage covers the basics, and this article looks at the local picture.

Root intrusion on leafy streets

Many North London roads are lined with large trees, because many were planted when the older terraces went up. Roots search for moisture. London clay holds water well, so roots follow the easiest path they can find. That path is often straight into the joint of an old clay drain a few feet down.

Once a root has a foothold, it does not stay small. It widens the crack it entered through, catches debris and narrows the bore. A clear run becomes a repeat blockage.

Rodding or jetting clears the immediate obstruction. However, if nobody seals the entry point, the roots usually return. That is why a recurring blockage on a tree-lined street is a clear sign to book a CCTV survey instead of another round of rodding. Our CCTV drain survey service shows what a survey involves, and blocked drains in Barnet is one place where trees often play a part.

Decades of clay

The second driver is the age of the pipe network. Much of the older housing around Edgware has salt-glazed clay drains. Over many years, ground movement and roots work the joints loose. Some sections bend from a true circle into an egg shape. A pipe that has lost its round shape restricts flow even before anything blocks it, because solids settle at the low point of the oval instead of washing through.

Victorian terraces in Enfield, Barnet and further south have an even older layer of drainage. Many were laid as combined drains that serve several houses on one run. A blockage on such a street can therefore affect more than one household at once. Our article on what older clay drains need in Edgware goes into detail. If a pipe has failed completely, read about collapsed drain repair and the no-dig fix and why we use drain relining.

Hard water scale narrows the bore

London’s tap water is hard, and hard water leaves limescale, which is a deposit of calcium and magnesium, inside the pipes it runs through. That includes the older clay sections, which were already narrower than a modern plastic run.

The process is slow. As a result, scale rarely shows up as a sudden blockage. Instead, a drain becomes “a bit slow” for months before it finally stops. Alternatively, it blocks again soon after you clear it.

Descaling is the fix. A jetting head strips the scale back to bare pipe, so you do more than clear whatever is stuck today. It is usually worth doing once a real blockage has been cleared.

It varies from street to street

None of this plays out the same way everywhere. For example, streets with big trees point towards roots. Older estates with clay drains point towards joint damage and pipe deformation. Busy commercial kitchens add a heavy grease load to the drains they share, which is a different problem even though the symptoms look similar from the sink. Slow draining and repeat blockages can come from any of these causes.

That is why you should read the picture for your own area, because assuming a neighbour’s blockage matches yours. If the same drain keeps blocking, read why your main drain keeps clogging. To tell a local blockage from a main-run fault, see a blocked drain pipe, branch or main run.

While the causes differ, the usual order of methods is the same, and our guide to clearing a blocked drain is the place to start. To plan ahead, read how often you should have your drains cleaned. If a blockage sits outside your boundary, who is responsible for blocked drains outside explains who to call. A macerator can also mimic a drain fault, and why a Saniflo stops flushing explains how. We cover drain unblocking in Enfield too, and our blocked drains page lists the wider service.

Frequently asked questions

Why do North London drains keep blocking? Most repeat blockages come from one of three causes. For instance, roots may have entered an old clay joint. The pipe may have aged to the point of joint displacement or deformation. Scale may be narrowing the bore. Clearing the blockage without finding the cause often means the drain blocks again within a year or two.

Is it the trees or the pipe age that causes most blockages? It depends on the street. Where mature trees line the road, roots are usually the leading cause, while in older estates with clay drains, pipe age and joint damage often matter more, even without many trees.

Does hard water really affect drains, not just kettles? Yes, because limescale forms wherever hard water runs, including inside drain pipes. It builds slowly, so it is rarely the only cause of a first blockage. Even so, it narrows older pipes over the years and makes other causes more likely to bite.

How do I know which cause applies to my drain? A CCTV survey is the reliable way to tell roots, joint displacement and scale apart, since all three cause slow draining, gurgling and repeat blockages. To talk through your own case, send the online enquiry form.