What a French drain is
A trench filled with washed aggregate, with a perforated pipe running through it and a geotextile membrane wrapped around the stone. Water moving through the ground finds the easier path through the aggregate, enters the pipe and is carried to a discharge point.
The principle is simple. The design is where these succeed or fail, and there are only two questions that really matter: where is the water coming from, and where can it legally and practically go.
Why gardens flood in London
Clay subsoil across much of south and west London absorbs water slowly, so after prolonged rain the ground simply holds it. Add ground that falls towards the house rather than away, and a patio or driveway laid without any drainage provision, and water arrives at the wall faster than it can disperse.
Standing water against a building is a slow problem rather than a dramatic one. It saturates the ground next to the wall, and over a season or two that shows up as damp inside at low level, often mistaken for rising damp and treated as such at considerable expense.
Where the water goes
This is the first design decision, not the last. A trench that fills and cannot discharge is a soakaway with extra steps, and the most common reason a French drain stops working within a couple of years.
Options are a surface water drain, a soakaway where ground conditions allow, or another suitable discharge point. What is not an option is the foul sewer: surface water should never be discharged into it, and doing so causes problems for the property and the network alike.
How we build them
- Survey the site, the levels and the subsoil to confirm where water is collecting
- Establish a viable discharge point before anything else is decided
- Excavate the trench to the depth and fall the design requires
- Lay geotextile membrane, perforated pipe and washed aggregate in the correct order
- Reinstate the surface and test the run under a controlled flow of water
The membrane is the component most often left out to save money, and it is the reason so many French drains silt up within two seasons. Washed aggregate matters for the same reason: pit-run material carries fines that clog the voids.
What it costs
French drain installations are quoted after a site visit, because the run length, the depth and the available discharge point all change the work substantially. We test the finished run with a controlled flow before reinstating, so you can see it discharging rather than take our word for it.
We are also straightforward when a French drain is not the answer. Sometimes the real problem 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.