1. Leaves and garden debris in gullies
Leaves, soil and moss wash into gullies and collect there, because rain carries them in. In autumn, a gully can fill in a few weeks because leaves fall fast. The water then backs up and spills onto the ground.
Clear the grate by hand with gloves on, because debris packs tight. Scoop out the silt below it. Check the gully again after heavy rain, since debris returns quickly.
2. Root ingress
Tree roots look for water, so they can enter a pipe through tiny cracks or loose joints. Inside, they grow into thick mats that catch debris. Over time the pipe blocks completely. See our guide to tree roots in drains for more.
Roots often return after a simple clear, because they stay in the soil. A camera survey can show where they enter. Relining or repair then stops the problem for longer.
3. Fat and grease from kitchen outlets
Fat poured down the sink cools in the pipe, and then it sticks to the walls. It builds up in layers until little room is left. Grease is a common cause of blocked outside drains for this reason.
High-pressure water jetting can remove it. You can read more about drain jetting on our service page. Prevention is easier: put fat in the bin, not the sink.
4. Wet wipes and flushed materials
Wipes do not break down like toilet paper, so they stay in the pipe. They catch on rough spots and trap other waste. Even wipes sold as flushable can cause blockages, so bin them. See what never to flush down a toilet.
5. Collapsed or displaced pipe sections
Old pipes can crack, sag or slip out of line, since ground moves over time. Ground movement and heavy loads can cause this. Debris then collects at the damaged spot.
A drain that keeps blocking in the same place may have this fault. A CCTV drain survey shows what is happening inside the pipe. Cleaning alone will not fix a broken pipe.
6. Silt and sediment buildup
Fine soil and sand wash in from gardens and paths, and they settle in flat sections. They settle in flat or low parts of a pipe. Given time, they form a hard layer.
Regular cleaning keeps this under control. Meanwhile, check that garden soil does not slope towards your gullies.
7. Mortar and building debris
After building work, cement, mortar and grit may enter drains, because gullies are often left open. Once set, they form hard lumps that block the flow. This is common after an extension or repair.
Cover gullies during building work, since rubble blocks them fast. If you suspect debris, a survey can find it. Hard lumps may need special equipment to remove.
8. Shared sewer blockages
Many homes share a drain with neighbours, so a fault can affect several. If several homes have problems at the same time, the blockage may be in a shared pipe. In England and Wales, the water company is responsible for many shared pipes outside your boundary.
Ask your water company to check, because they can tell you who is responsible. They can tell you who is responsible. If you are unsure, a camera survey helps show where the fault lies.
Diagnosing the cause
Start with the simple checks, because they cost nothing. Look at the gully, then the chamber, then the flow from the house. A pattern often tells you where the blockage sits.
If you want expert help, a local engineer can clear the drain and check the pipe. You can request a free quote and we will pass your details on. A CCTV drain survey can show the cause.
Related guides and local help
If you want to know more, read Drain Jetting vs Rodding: Which Method Is Right for You?. You can also read Blocked Sink: Every Common Cause and How to Fix Each One and Drain Maintenance: How to Keep Your Drains Clear All Year, because they cover closely related problems.
These guides help too: Blocked Drain Emergency: What to Do Right Now in the UK, How to Unblock a Drain: A Simple Step-by-Step UK Guide, Blocked Bath Drain: Causes, Quick Fixes and Prevention Tips, How to Unblock a Toilet: Safe Methods and When to Stop and Bathroom Drain Problems: Hair, Soap Scum and Real Fixes. Each one explains a related topic in plain English.
Need help near you? Read our blocked drain guide first, or see Blocked Drains in Edinburgh and Blocked Drains in Bristol. You can also see Blocked Toilets in Bideford, since every service page has a free quote form. We pass each request to a local drainage engineer.
