Clearing the gutters is only half the job. Every litre a roof collects ends up at the bottom of a downpipe, and what happens to it there decides whether a heavy storm passes harmlessly or spends the night against the house.
Follow each downpipe to its end
Walk round in the rain and see where each pipe discharges. Some run into an underground drain through a gully, some feed a soakaway somewhere in the garden, and some simply stop over a splash block or a patch of gravel. That last arrangement leaves a whole roof's runoff a few inches from the brickwork.
A pipe that disappears into the ground needs a gully you can see and clear. Lift the grid, pull out the leaves and silt, then run a hose down the pipe and watch it go.
How a soakaway stops working
A soakaway is a buried pit, filled with rubble or a plastic crate and wrapped in membrane, that lets water seep away into the surrounding soil. Older ones were often a hole of broken brick with no wrapping at all, and over twenty or thirty years silt washes into the gaps until the pit holds water rather than shedding it. The symptom is a downpipe that backs up in a long downpour, with a soft patch of lawn nearby that never quite dries.
The soil matters as much as the pit. Clay drains slowly whatever you bury in it, and hard, dry ground can refuse water for the first part of a storm. Readers who have moved somewhere hotter describe the same thing at a far larger scale: in the Arizona desert a summer cloudburst runs straight off sun-baked earth, and householders there end up calling water restoration specialists in Mesa once the runoff has found its way under a door. A garden soakaway is the small domestic version of that problem, and it only works where the ground is willing to accept water.
Test before you dig
Before replacing a soakaway, test it. Fill the inspection chamber if there is one, or run a hose into the pipe for a measured ten minutes, and note how long the level takes to fall. A pit that empties within a day of heavy rain is doing its job. One still brimming the next morning has silted up or was always too small, and its replacement belongs at least five metres from any foundation.
Cheap measures that help
A water butt on each downpipe takes the edge off the opening minutes of a storm, and a diverter fitted to the pipe sends overflow back down once the butt fills, so it never spills against the wall. Where a pipe ends over a hard surface, an extension carrying the water a metre or two away, onto ground that falls from the house, costs very little and works well.
If water still collects at the base of a wall after every storm once all of this is done, the drainage itself wants a proper survey with a camera, because a cracked pipe underground gives no other warning until the damage shows.