The short answer

One slow sink or toilet almost always means a branch blockage close to that fixture. Several fixtures backing up together, however, point further down the line. In that case the blockage sits in the main run or outside, and a plunger under the sink will never reach it. Knowing which one you have tells you what to try yourself and when to stop. If you would rather have it checked from the start, send the online enquiry form for a fixed price agreed before work.

Branch or main run: the quickest way to tell

Every drainage system has two parts. Branch pipes run from each sink, bath, toilet or gully. A main run then carries everything from those branches out to the sewer. The difference matters because the fix, and the urgency, change with each one.

If only one fixture is slow or blocked, the fault is almost always in its own branch pipe. If two or more fixtures are affected, the blockage is further along. For example, a downstairs toilet may gurgle when the bath empties. That means a section shared by several branches, or the main run itself, is the likely cause.

Reading the symptoms properly

A few consistent signs separate the two, because each type of blockage behaves differently:

Gurgling from one drain when you use a different fixture suggests a shared or main-run problem. Air is pushed back through the system because water cannot pass a blockage further down.
Backing up at the lowest point in the house is a strong signal of a main-run blockage. A ground-floor toilet, shower tray or basement gully is the usual place. Water finds the lowest exit it can before it reaches the obstruction.
A smell outdoors near a gully or manhole, even when everything inside seems fine, usually points to the outside drain run.
One fixture only, with no effect elsewhere, normally stays a branch issue. It is the type most likely to respond to a safe DIY attempt.

Safe first checks before you call anyone

For a slow single fixture, three checks are worth trying first, since they carry little risk:

Clear the gully grate outside. Leaves and silt build up over a gully and can restrict flow, even when nothing is blocked further down.
Use a plunger properly. Keep enough water in the basin to form a seal, and use firm, steady strokes. Our guide to unblocking a toilet without a plumber covers the method in detail.
Pour hot water and washing-up liquid slowly. This can shift light fat and soap in a kitchen pipe. Stop there, though. Chemical cleaners are a safety risk if the blockage does not clear, because the next person to work on the pipe meets the chemical. They also do very little against anything solid.

If none of this works within one or two attempts, stop. The same goes if more than one fixture is affected. Repeating the same fix with more force rarely helps. If the problem is a slow toilet in particular, see how to clear a slow draining toilet.

How professionals clear it

When DIY fails, or the signs point to a main-run or outside blockage, the usual escalation is simple, because each step has a clear limit:

Manual rodding or a drain auger comes first. It breaks up the obstruction or pulls it back through the nearest access point.
High-pressure water jetting follows where rodding alone does not clear it. Jetting scours the full bore of the pipe, so it cleans as well as clears. Our complete guide to clearing a blocked drain sets out the whole ladder of methods.
A CCTV drain survey is the step to take when a blockage keeps returning. The camera shows whether roots, a displaced joint or scale sits behind the repeat. Our CCTV drain survey service covers what is involved, and a separate article explains what changes the price of a survey.

While a plain blockage needs only clearing, a structural fault such as a cracked or sunken pipe needs more. If the survey shows one, read about collapsed drain repair and the no-dig fix. In older areas, repeat blockages are common. Why drains block so often in North London explains the main causes, and what older clay drains need in Edgware shows one local example.

Whose pipe is it, and does that change anything?

Find out early whether the pipe is yours or the water company’s. A drain that serves only your property, up to your boundary, is generally yours. Since 1 October 2011, most shared drains and private sewers outside the boundary have belonged to the water company. Across most of North London, that is Thames Water.

This matters most for outside blockages, since the cost can differ. If the smell or backing up happens at or beyond your boundary, and more than one property is affected, check with the water company first. Our article on who is responsible for blocked drains outside goes through the rules. A second guide asks who is responsible for blocked sewage.

Local help

We work across North London, so the same checks apply wherever you live. For example, you can read about blocked drains in Barnet, drain unblocking in Potters Bar and blocked drains in Southgate. Our blocked drains page describes the wider service. For a sewer pipe, see how to unblock a blocked sewer pipe. If you have a Saniflo unit, the fault may lie there instead, as this Saniflo guide explains.

Frequently asked questions

Why does my drain pipe keep blocking? Repeat blockages at the same spot usually have a cause, because bad luck rarely strikes twice. Common ones are roots at a joint, a partial collapse that narrows the pipe, or scale in older pipework. Clearing the pipe again and again does not remove the cause. A CCTV survey will show it.

Is a blocked drain pipe my responsibility? It depends on where the blockage sits, because ownership follows the boundary. Pipework inside your boundary that serves only your home is yours. Shared drains and the public sewer outside your boundary are generally the water company’s. If you are unsure, ask your water company before you pay for private work.

Is it safe to use chemical drain unblockers? Avoid them as a first resort because they do little against solid blockages or roots. If they fail, whoever works on the pipe next, including you, meets a pipe full of caustic chemical. A plunger or drain rods are a safer first try for a simple branch blockage.

Because a returning blockage rarely clears itself, send the online enquiry form and we will take it from there.