When you need one
- Water pooling against the house after rain and taking days to clear
- A persistently soggy strip of lawn or paving that never dries
- Damp appearing along the base of a wall on the uphill side of a sloped site
- A retaining wall weeping or staining on its downhill face
- Water entering a garage or basement at floor level rather than through the wall
- A high water table — common through parts of the Southern Suburbs and the Cape Flats, where sandy soil sits over water close to the surface
How it is built
- Establish where the water is going to go. A drain that discharges nowhere is a soakaway that fills up. Municipal stormwater connection, a lower point on the property, or a sump and pump if there is no gravity fall available.
- Excavate to a consistent fall, typically 1:100 minimum, usually 600–900mm deep depending on the problem.
- Line the trench with geotextile, lapped generously up the sides.
- Bed 100mm or 110mm slotted pipe on washed stone, slots down.
- Backfill with 19mm or 25mm washed stone — never builder's rubble or unwashed material, which silts the void closed within a season.
- Wrap the geotextile over the top before the final layer so soil fines cannot migrate in.
- Finish with stone, a permeable cover, or topsoil and grass depending on where it runs.
The two failures we see most are no geotextile at all, and unwashed stone. Both silt up and the drain stops working within two or three winters — but only after the trench has been closed and grassed over.
Sumps and pumps
Where there is no gravity fall to discharge to — a basement, a garage below street level, a flat site — the drain runs to a sump chamber and a submersible pump lifts the water out. A sump needs a float switch, a non-return valve so lifted water cannot run back, and ideally a high-level alarm. On a property where a pump failure means a flooded basement, a second pump on a separate circuit is cheap insurance.
Below-ground rooms usually need both drainage and tanking or a cavity drain membrane.
Common questions
How much does a french drain cost in Cape Town?
About R900–R1,700 per linear metre installed. Depth, access for a machine versus hand excavation, rock, and existing services are what move the rate. A sump and pump installation adds roughly R8,000–R20,000 depending on the pump and chamber.
What is the difference between a french drain and an agricultural drain?
In South African usage they are used interchangeably for the same thing — a stone-filled trench with a slotted pipe. 'Agricultural drain' comes from field drainage; 'french drain' is named after Henry French, who wrote about the method in the 1850s.
How deep should a french drain be?
Deep enough to intercept the water and to maintain fall to the discharge point — usually 600–900mm for a domestic drainage problem. Alongside a foundation it should run below the level of the water you are intercepting, not merely below the surface.
Will a french drain fix damp in my wall?
If the damp is caused by ground water standing against the wall, yes, and it is the correct first move. If the wall has a failed DPC, the drain reduces the load but the wall still needs damp proofing. Diagnosis first.