Why French drains fail, and how to build one that does not
Most French drains that stop working failed for one of three reasons, and all three are decided before anyone digs.
Failure one: nowhere for the water to go
This is the big one. A French drain is not a soakaway. It intercepts water and carries it somewhere, and if that somewhere does not exist, the trench simply fills and stays full.
The discharge point is the first design decision, not the last. Options are a surface water drain, a soakaway where ground conditions genuinely allow it, or another suitable outfall. What is never an option is the foul sewer, which is both bad practice and prohibited.
Ask any contractor where the water will go before you ask what it costs. If the answer is vague, so is the drain.
Failure two: no membrane
Geotextile membrane wraps the aggregate and keeps fine soil out of the voids. It is the component most often left out to save money, and it is why so many trenches silt up within two seasons.
Without it, the stone slowly fills with the very material it was meant to exclude, and the drain quietly stops working long before anyone notices the garden is wet again.
Failure three: the wrong fill
Washed aggregate, not pit-run. Unwashed material carries fines that clog the voids from day one. It is cheaper by the tonne and considerably more expensive over five years.
Why London gardens flood in the first place
Clay subsoil across much of the south and west absorbs water slowly, so after prolonged rain the ground holds it. Add ground that falls towards the house rather than away, and a patio laid without drainage provision, and water arrives at the wall faster than it can disperse.
The consequence is slow rather than dramatic. Saturated ground against a wall shows up over a season or two as damp inside at low level, frequently mistaken for rising damp and treated at considerable expense.
We deal with this pattern most in Merton, Croydon and Dulwich, where clay and paved gardens meet.
When a French drain is not the answer
Sometimes the real fault is a broken gully, a downpipe discharging against the wall, or a blocked surface water drain, and fixing that costs a fraction of a trench.
We would rather say so. A contractor who reaches for the same solution regardless of what caused the water is not diagnosing, just selling. More on how we design and build French drains.