How To Work Out Where A Downpipe Is Blocked

Key takeaways

• Blockage at the outlet makes the gutter overflow while the downpipe stays dry.
• Blockage at a bend makes the pipe overflow at the joint above the obstruction.
• Blockage below ground makes water back up out of the gully at the bottom while the pipe runs full.
• Tapping down the length of the pipe locates the blockage by sound, and it costs nothing.
• Repeated blockages usually mean something upstream, such as an overhanging tree or a roof shedding grit.

Three different faults with one name

People report a blocked downpipe as if it were a single condition, but there are three distinct versions and they need different responses. High blockage sits at the outlet where the gutter meets the pipe, usually leaves or a slipped tile fragment. Mid blockage sits at a bend, most often at the swan neck where the pipe steps out from the eaves to the wall, or at the offset around a projection. Low blockage sits at or below the gully at ground level, where the pipe discharges into the drainage system, and that one may not be the pipe's fault at all. Each shows itself differently in rain, which is what makes them easy to tell apart.

Reading the symptoms in rain

Go out during a decent shower and look at the whole assembly from the gutter to the ground. If the gutter is overflowing and the downpipe below is dry or barely running, the blockage is at the top, at the outlet. If the gutter is draining but water is escaping from a joint partway down the pipe, or from the back of a bend, the obstruction is just below that point. If the pipe is running full to the bottom and water is welling up out of the gully at ground level or pooling around its base, the blockage is in the underground drain rather than the pipe. Two minutes of watching separates them definitively.

The tap test

In dry weather, work down the length of the pipe tapping it gently with your knuckles or a wooden handle, listening to the change in sound. An empty pipe rings hollow. A section holding a blockage, or standing water above one, sounds dull and dead. The point where the sound changes from hollow to dull is the top of the obstruction. This takes a minute, needs no tools, requires no height access on the reachable lower sections, and tells whoever comes out exactly where to start rather than having them dismantle the whole run. It is one of the genuinely useful diagnostic things a homeowner can do without any risk at all.

Why the swan neck collects everything

On most houses the gutter sits further out than the wall, so the downpipe has to step inward just below the eaves through a pair of angled bends called a swan neck. That geometry means anything travelling down the pipe has to negotiate two changes of direction in quick succession, and leaves, moss and grit stop there first. It is by far the most common blockage location, and it is also usually the least accessible part of the run. If your downpipe blocks repeatedly and always seems to overflow at the same joint high up, the swan neck is almost certainly where it is happening.

The gully, which is not really the downpipe

At the bottom, the pipe usually discharges into a gully, which is a small chamber with a trap in it, connecting to the underground drainage. Gullies silt up with leaves, moss and grit washed down from the roof, and once the trap fills the water has nowhere to go. This shows as water pooling around the base of the pipe, or backing up out of the gully grating, while the pipe itself runs freely. Clearing a gully is usually straightforward and is one of the few parts of the system that can reasonably be dealt with at ground level. If clearing it makes no difference, the problem is further into the drain and belongs to a drainage specialist.

What keeps causing it

A downpipe that blocks once has been unlucky. One that blocks every autumn has a source. Overhanging trees are the most common: they drop leaves directly onto the roof, which wash down into the gutter and then the pipe. Moss on the roof is the second: it breaks down into a fibrous sludge that travels well and packs tightly at a bend. Ageing concrete tiles shed grit that behaves the same way. And birds nesting in the eaves push material into the gutter. Fixing the recurrence usually means addressing the source, whether that is cutting back a branch, dealing with roof moss or fitting guards, rather than clearing more often.

Gutter guards, honestly assessed

Mesh guards and brush inserts are widely sold as the permanent answer and they are a mixed proposition. They do keep large leaves out of the gutter, which helps. They also collect debris on top of themselves, where it is harder to remove than it would have been from inside the gutter, and fine material such as moss fragments and grit passes straight through them. On a house under a large deciduous tree they can be worth having. As a substitute for ever looking at your gutters again, they are not. Anyone selling them on that basis is overstating the case.

Cast iron, and why old pipes block differently

Many older properties still have original cast iron rainwater goods, and they behave unlike plastic in one important respect. The inside of a cast iron pipe corrodes over decades into a rough, pitted surface, and that roughness catches material that would pass straight through a smooth plastic pipe. So an old iron downpipe can block repeatedly with a quantity of debris that would never trouble a modern one, and clearing it does not change the surface that keeps catching. Cast iron is also heavy, brittle at the ears where it fixes to the wall, and expensive to replace like for like, which is why it is often worth repairing and keeping on a period property rather than swapping for plastic that will look wrong on the building.

Timing and who should do it

A blocked downpipe that is causing overflow onto the ground can wait for a planned visit, ideally combined with autumn gutter clearing so the whole system is dealt with in one go. Move it up if water is escaping at a high joint and running down the wall, because that soaks the same strip of brickwork repeatedly, or if water is pooling around the base of the wall, which over time affects the ground beside the foundations. Ground level gully clearing is reasonable to do yourself. Anything at the swan neck or higher needs proper access, and it is not worth a ladder for a blockage that somebody can clear safely in fifteen minutes.

Frequently Asked Questions

How do I know where my downpipe is blocked?

Watch it in rain. Gutter overflowing with a dry pipe means the blockage is at the top outlet. Water escaping from a joint partway down means it is just below that point. Pipe running full with water welling up at ground level means the obstruction is in the gully or the underground drain.

What is the tap test?

Tapping down the length of the pipe in dry weather and listening for the sound to change. Empty pipe rings hollow, a section holding a blockage sounds dull. The point where it changes is the top of the obstruction. It costs nothing and tells whoever attends exactly where to start.

Why does it always block in the same place?

Most likely the swan neck, the pair of bends just below the eaves where the pipe steps in towards the wall. Debris has to negotiate two direction changes there in quick succession and stops at the first one. It is the most common blockage point and among the least accessible.

Water is pooling at the bottom of the downpipe. Is the pipe blocked?

Probably not the pipe itself. If the pipe is running full to the bottom and water is welling up around the gully, the obstruction is in the gully trap or the underground drain. Gully clearing is a reasonable ground level job; if that changes nothing, it is a drainage matter.

Do gutter guards stop blockages?

Partly. They keep large leaves out and they collect debris on top of themselves, where it is harder to remove than from inside the gutter. Fine material such as moss fragments and roof grit passes straight through. Useful under a large tree, but not a reason to stop checking the gutters.

Can I clear a blocked downpipe myself?

The gully at ground level, reasonably. Anything at or above the swan neck needs proper access, and working at height on a ladder while pushing rods into a pipe is exactly the situation that causes falls. The work is quick for somebody with the right access and not worth a fall.