Why Does My Outdoor Drain Keep Blocking?

Need advice straight away?

Call ASL on 0800 181 684 – we’re always happy to help.

ASL’s engineers work across Surrey and the surrounding counties, where many properties feature older drainage systems. Blocked drains, root ingress, and collapsed pipework are issues we encounter regularly, whether we’re carrying out CCTV surveys or drain clearance work in rural market towns, leafy villages, or established residential streets where the infrastructure has simply been there for decades.

Why outdoor drains block more often than people realise

Surface water or roof water

Outdoor drains are supposed to be simple.

Rainwater runs in, flows through the pipework, and disappears underground.

But when something isn’t quite right, they become one of the most common causes of blocked drains around a property.

Sometimes the blockage happens suddenly after heavy rain.

Other times, it builds up slowly until smells start appearing or water begins sitting around the drain instead of flowing away.

The important thing to understand is that outdoor drains rarely block for no reason.

There is almost always a cause, and once you find the cause, you can stop it from happening again.

The most common causes of outdoor drain blockages

Leaves and debris collecting in the gully or the gully grid

One of the simplest causes is also one of the most common.

Leaves, dirt, moss and small bits of debris wash down into the gully and gradually collect at the bottom. Over time they create a thick sludge that slows the water down until the drain eventually blocks.

This is especially common where trees or hedges are close to the drain or there is a gully disguised in a flower bed.

You might notice:

• Water standing around the drain,
• A smell coming from the gully,
• Slow draining after rain.

Often this type of blockage can be cleared quickly, but if the debris is left there long enough, or the drainage is not maintained, silt may build up in the pipework or block the soakaway.

Fat and grease from kitchen pipes

This type of blockage often produces a strong smell before the drain actually blocks completely.

Rainwater entering the wrong drain

Sometimes the problem isn’t the gully itself but how water enters the system.

Surface water from driveways, patios or paths may run toward the foul sewage drain instead of away from it. When heavy rain hits, large amounts of water suddenly enter the drainage system.

If the pipes are already partially restricted, that extra water can cause the drain to back up.

Signs of this include:

• Water overflowing during rain,
• Flooding around inspection chambers,
• Smells appear after storms.

In many cases, the solution is simply to redirect surface water, properly seal inspection chamber lids, or install a concrete surround.

Pipes entering the gully are too deep

This is something we see quite often when inspecting outdoor drains.

A pipe may come through the wall of the house, run a short distance, then drop into the gully. If the pipe is installed too deep in the gully, debris can easily collect at the point where the pipe meets the drain.

Once debris starts to build up, it creates the perfect place for fat, leaves, and other material to collect.

Eventually, the pipe blocks right at the entrance to the gully.

Correct pipe positioning inside the gully makes a huge difference to how well the system works.

Cracked pipes and root intrusion

If a drain keeps blocking repeatedly, the cause may be further underground.

Tree roots are incredibly good at finding tiny openings in drainage pipes. Once inside, they grow quickly and trap debris as it flows through the pipe.

Over time, the roots form a dense mass that slows the flow until the drain blocks completely.

This type of issue is usually identified with a CCTV drain survey.

Why recurring blockages should never be ignored

Many people clear a blockage and assume the problem has been solved.

But if the drain blocks again a few weeks later, that usually means something deeper is going on.

Repeated blockages are often caused by:  

• Pitch fibre pipe breaking down,
• Misaligned pipe joints,
• Structural damage to the pipe,
• Roots entering the drainage system,
• Poor pipe gradients,
• Debris collecting in the pipework.

Without identifying the real cause, the blockage will keep recurring, exacerbating the problem, especially at the open joints, causing a bulging or subsided pipe.

This demonstration shows a pitch fibre drain pipe that has begun to deteriorate.

Over time, pitch fibre pipes can soften and deform, causing the pipe to lose its round shape. When this happens, the internal diameter of the pipe becomes restricted, slowing the flow of wastewater and allowing debris to catch inside the pipe.

As the pipe continues to break down, it can eventually collapse or lead to repeated blockages. This is why pitch fibre drains often require repair or structural lining once the damage has been identified during a CCTV drain survey.

How a professional drainage engineer approaches the problem

When our engineers attend an outdoor blockage, the goal is not simply to get the water flowing again.

We want to understand why it blocked.

A typical visit may include:

1. Clearing the blockage properly using jetting equipment,
2. Inspecting the gully and pipe entry points,
3. Carrying out a CCTV camera survey if required,
4. Identifying the root cause of the problem.

Once the cause is known, the solution becomes much clearer.

A real example from one of our jobs

We were recently called to a property where the outdoor drain blocked every few months.

The homeowner had cleared it themselves several times, but the problem kept returning.

After jetting the line and inspecting the pipe with a camera, we discovered that the issue was a pipe that entered the gully too deeply. Debris was collecting at the entrance and gradually building up until the pipe blocked.

Once the pipe was adjusted and the gully cleaned properly, the drainage system began working normally again.

Sometimes a small adjustment makes a big difference.

What you can do to reduce the risk of outdoor drain blockages

There are a few simple steps that can help prevent problems:

• Clear leaves and debris from gullies regularly,
• Avoid pouring fat or oil down the sink,
• Check that inspection chamber lids are sealed properly,
• Keep an eye on standing water after rain,
• Arrange a professional inspection if blockages repeat.

These small checks can save a lot of trouble later.

When to call a drainage professional

If your outdoor drain:

• Keeps blocking repeatedly,
• Smells strongly,
• Overflows during heavy rain,
• Drains very slowly.

It is worth having the system inspected properly.

A quick investigation can often reveal the underlying cause before the problem becomes more serious.

Need help with a blocked outdoor drain?

At ASL Limited, our engineers deal with blocked drains every day. We clear the blockage, inspect the pipework and explain exactly what is happening so the issue can be prevented in future.

📞 Call ASL Limited on 0800 181 684

We’ll help you get things flowing again – cleanly, safely and professionally.