What a main sewer blockage means

A blocked main sewer line backs up the whole house. Sinks, baths and toilets all drain badly at the same time. You should deal with it quickly, because waste has nowhere else to go.

This guide covers the signs, the causes and the first steps. It also explains why most main line jobs need an engineer, since the pipe is long and hard to reach. Related guides on a sewer backup in a basement and bathtub drain backing up cover specific symptoms.

Signs of a blocked main sewer line

Several fixtures blocking together is the clearest sign. If the toilet, bath, sink and shower all back up, the main line is the likely cause. However, a single slow sink usually points to a local blockage.

Other signs include these, for example:

A toilet that bubbles when you run a sink or washing machine
Water rising in the toilet or shower after you drain another fixture
Gurgling after a flush
A sewage smell inside or outside the house
Water or damp patches near a manhole or inspection chamber
Lawn patches that stay wet or smell

A test helps, because it costs nothing. Run the sink nearest the toilet and watch the bowl. If the water level rises or bubbles, you probably have a blockage. For a wider list of signs, see how to know when your drain needs cleaning.

What causes a main sewer blockage

Damaged pipes cause many blockages because a cracked, shifted or badly jointed pipe catches waste, and the debris builds up. Ground movement, building work and heavy traffic can all harm a pipe.

Fat is another common cause. It cools, hardens and coats the pipe wall, even if you run hot water afterwards. Wipes, paper towels and sanitary products add to it, since they do not break down like toilet paper. For more, read common causes of blocked drains and things that get stuck down the drain.

Tree roots are a third cause. Roots find tiny gaps in a joint, and they grow into the pipe. Although a small root seems harmless, it traps paper and waste. In time, it can block the pipe completely, so a repair may be needed as well as clearing. Our guide to causes and prevention of blocked drainage explains more.

Where the main sewer line is

Every drain in your home leads to one main run, which then joins the public sewer or a private system. In many UK homes the run is outside, so you can reach it through inspection chambers in the garden or drive.

Some older properties have rodding points or a cap on a pipe near the house. Not every property is the same, however, so check your own layout. If your drain serves only your property, it is usually your responsibility up to the boundary. Since 1 October 2011, many shared drains outside the boundary belong to the water company. Check with your water company if you are unsure.

What to do first

Stop using water in the home. Running taps or the dishwasher adds to the backup. Therefore avoid flushing, showering or washing until the line is clear.

Next, lift an inspection chamber cover if you can do so safely, because it shows where the blockage sits. A full chamber shows that the blockage lies further down the line. An empty one suggests the problem is between that chamber and the house. Wear gloves, avoid splashes and never climb into a chamber.

Why a professional should clear it

A main line is a bigger job than a sink. A basic snake or rod may only punch a hole through the blockage. As a result, waste flows for a while and the pipe clogs again. An engineer can clear the whole pipe and find the cause. Jetting blocked drains has the detail.

If a pipe has failed, an engineer can show you what the damage looks like, although you need a camera to do so. Read how CCTV drain inspections work if you want to know more. Then you can decide on a repair based on facts.

Home steps that are safe

You can do a few things while you wait. Check the nearest chamber, keep children away and put down towels to protect floors. Avoid strong chemicals, because they can harm pipes and the person who works on them later.

While some blockages are small, others are not. For smaller ones, see how to unblock a sewage drain and how to unblock a blocked sewer pipe. Outdoor blockages are covered in blocked drain outside house.

Cost and next steps

Cost depends on access, the cause and the time needed. Therefore there is no standard figure. Our guide on how much it costs to unblock a drain explains the factors. If you are not sure the job is needed, read do you need drain clearance.

Because prices vary, we agree a fixed price before work starts, with no call-out fee. We offer 24/7 emergency call-out for urgent jobs. Send the online enquiry form to get started.

Where we work

While a toilet is often the first sign, see our blocked toilet clearance page. For other issues, browse our blocked drains services. We cover drain unblocking in Enfield, blocked drains in Barnet and blocked drains in Edgware.