Debris reaching the outlet
Most downpipe blockages start in the gutter. Leaves, moss and roof grit wash along to the outlet and pack together at the top of the pipe or at the first bend.
Bends, swan necks and leaf guards
Anywhere the pipe changes direction is a natural catching point. Swan-neck sections at the top of a downpipe are a common place for material to lodge, as are leaf guards that have never been cleared.
Nests and litter also account for a surprising number of blockages, particularly on open-topped pipes.
How to tell
During rain, listen and look at the bottom of the pipe. If the gutter above is overflowing but very little water is leaving the pipe, the restriction is in the downpipe rather than the gutter run.