The short answer
Find where the water stops, then work from the ground. Clear the gully at the foot, check the pipe for a loose joint, and flush it from the top with a hose if you can reach safely. A drain rod can free a plug lower down. Anything that needs a ladder above the ground floor, or a pipe that is cracked or loose, is a job for a roofer.
A blocked downpipe fills the gutter above it, so the cure starts at the bottom and works up. Many of the safe checks need nothing more than solid ground underfoot.
Finding where it is blocked
The vertical pipe at the corner of the house carries the roof's water down to the drain. Rain tells you where it sticks. Watch the pipe during a shower: if water pours over the gutter near the outlet, the blockage is at the top. If the gutter drains but the pipe overflows at a joint halfway down, the plug is lower. If water backs up from the bottom and floods the gully, the trouble is in the drain at the foot.
How do I tell a pipe blockage from a drain blockage?
Pour a bucket of water into the gutter at the outlet. If it vanishes quickly and you hear it run, the pipe is open and the drain is the problem. If it sits, the pipe is blocked. If water appears at the foot of the pipe but spreads across the ground without draining, the gully or the drain below is the issue.
Feel the pipe too. A cold, heavy pipe with no sound of running water points to a full pipe, and a rattle suggests debris caught in a bend. Our answer on why gutters overflow helps you read the signs at the top.
Safe methods you can do from the ground
Start with the gully, the grating at the base that takes the water into the drain. Lift the grating, scoop out silt, leaves and moss by hand in rubber gloves, and flush the gully with a bucket of water. A stiff brush helps with caked silt on the sides.
What can I use to clear the pipe?
If the pipe is plastic and the blockage is low, a drain rod or a length of flexible garden hose pushed up from the bottom can break up a plug. A hose held at the top of the pipe, with the water on full, will flush loose debris down. Take care at the top: do not stand on a stepladder in the wet. If you cannot reach the outlet with both feet on the ground or a stable platform, stop.
Never poke a rigid rod hard into an old pipe. Cast iron pipes crack and plastic pipes can split at the joints, and a damaged pipe then leaks into the wall. Never put acid or chemical drain cleaners into a downpipe, because they damage the pipe and the drain and are dangerous to handle. Read the pack guidance and keep people and pets clear while you work.
When to call someone
Call a roofer when the blockage is above the ground floor, when the gutter is full to the brim and spilling, or when the downpipe is loose on its brackets. A slip from a ladder does far more harm than a wet wall, and the roof edge above a conservatory or glazed porch is no place for an amateur.
How does a roofer tackle a high blockage?
We reach the top with proper access, clear the gutter and the outlet by hand, and then flush the pipe from above while someone watches the foot. We check the brackets, the joints and the shoe, the angled piece at the bottom that guides water to the gully. If the pipe has cracked or come adrift, we photograph it and say whether it needs a new section.
A pipe that blocks again and again is a sign of something upstream: moss from the roof, a gutter with too little fall, or a sunken pipe run. Our answer on gutter fall explains the last of those, and why a slightly flat run keeps dropping silt at the outlet.
Keeping it from blocking again
Prevention comes down to what reaches the outlet. Clear the gutters before winter, trim branches that overhang the roof and deal with moss if the roof is shedding it. Our answer on how often gutters should be cleaned sets out the timing.
Do outlet guards help?
A small cage over the outlet stops leaves reaching the pipe, and for a gutter under a tree it can be worth fitting. It will need clearing itself, since leaves pile up against it. It is a help, not a cure, and it will not stop silt and moss. Our answer on whether gutter guards are worth it goes into the trade-offs.
When the pipe or the shoe is damaged, or a downpipe has been blocked for so long that the wall is wet, that belongs in a proper repair. Our gutters, soffits and fascias page lists the work we cover. Photograph the wet patch and the foot of the pipe before you call, and the survey can start from there.
More on clearing downpipes
Is a drain rod safe on an old downpipe?
Gently, yes, on plastic. Forcing a rod into old cast iron or brittle plastic can crack the pipe or push a joint apart, and the water then runs into the wall. A hose on full flow works first, and debris is better scooped out at the gully than flushed into the drain.
Why does my downpipe overflow at a joint?
Water is backing up behind a blockage below that joint, or the joint has come apart and is leaking under pressure. Look for a plug lower down, and check the pipe is straight and tightly seated in the socket. A joint that keeps leaking after clearing needs refitting or a new section.
The full picture: Gutters, soffits and fascias in Taunton ยท More short answers