Blocked downpipe: the signs, the causes, and how it's cleared

Signs your downpipe is blocked

• The gutter spills over at or near the outlet, while the rest of the run stays clear.
• In heavy rain, only a trickle comes out at the bottom of the pipe.
• Water seeps from the joints between lengths of pipe, or overflows from the hopper at the top.
• There's a damp strip, rust staining, moss or green algae on the wall beside the pipe.
• Water pools at the foot of the pipe, because the drain below it is blocked too.
• After a hard frost, you find a split running down the pipe.

What blocks a downpipe

• Leaves and twigs, washed down from the gutter. They catch most easily where the pipe changes direction.
• Moss knocked off the roof by rain or birds, along with the sludge of grit and dirt that collects in gutters.
• Debris from building work. The Department for Communities in Northern Ireland, in its technical note on roof drainage, advises checking that nothing has blocked the rainwater system after any work on chimneys or roofs.
• Balls, nesting material and anything else that lands in a gutter.
• A blocked gully or drain at the bottom, which backs water up the pipe.

The swan neck: where blockages hide

On most houses, the gutter overhangs the wall, so the downpipe starts with a pair of bends that step it back in to the wall. This is the swan neck, and it's a common place for leaves to jam. It's also awkward to clear. In one DIYnot thread, a homeowner with a blocked swan neck asked how to clear it from the ground, and the suggestions ranged from a flexible drain rod, which one reply said would struggle to reach the top, to a leaf blower fixed to a length of waste pipe. Most swan neck blockages are simpler to clear from the top, which means working at height.

Why cracks at the back go unnoticed

The side of a downpipe facing the wall is the side you never see, and it's where faults usually start. The Northern Ireland note says fractures are most common on the face against the building, so damp inside is often the first sign. Cadw, the Welsh Government's historic environment service, agrees: "Holes at the back of downpipes can be difficult to spot, particularly with rectangular section pipes."

A blockage makes this worse. Cadw warns that "trapped water can freeze and split pipes". A partly blocked pipe full of water on a frosty night is how many splits happen.

Cast iron and plastic pipes

Cadw says silicone sealant can be used as a temporary repair on cast iron, but severely rusted or split sections should be replaced. On a listed building, swapping cast iron for plastic may need listed building consent, so read our listed roof guide first.

What you can check from the ground

• Watch the pipe in heavy rain. Plenty of water at the bottom means the pipe is clear.
• Look at the joints for drips and the wall beside the pipe for stains.
• If the pipe empties into an open gully, lift out any leaves and check the grid isn't blocked.
• The Northern Ireland note recommends gullies at ground level that give access for rodding, so the drain below can be cleared too.

Don't climb a ladder to look at the top. If you can see water spilling at the outlet, that's enough for us to know where to look.

How we clear a blocked downpipe

We test every downpipe when we clear a gutter, by running water through the outlet and checking it comes out freely at the bottom. If one's blocked, we find where, clear it, and test it again. If a section is cracked, split or badly rusted, we photograph it and quote to replace it. Clearing a badly blocked downpipe is extra work, so it's priced separately.

Written by Hawkes Roofing, roofers based in St Albans who clear gutters and downpipes by hand across Hertfordshire, Buckinghamshire and north-west London. Last reviewed 26 October 2026.