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
French drain installation: 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.
Surface water, stormwater and where roof water actually goes
A french drain deals with water that is already in the ground. Half the time the more useful question is why so much of it is arriving there, and the answer is usually the roof — a single 100 m² roof sheds roughly 2,500 litres in a 25 mm downpour, and all of it leaves through two or three downpipes.
- Downpipes discharging at the wall. The commonest fault we see. A downpipe emptying onto a paved strip next to the house puts that entire roof volume against the foundation, several times a winter. It needs to be piped away — to a stormwater connection, a soakaway, or a tank.
- Paving that falls toward the house. Very common on additions and re-paving jobs. Check it with a hose, not by eye: a fall of a few millimetres per metre is invisible and decisive.
- Blocked gutters and roof outlets. Cheap to clear, and the cause of an outsized share of both roof leaks and wall damp. Clear them before the first winter rain, not after.
- Raised ground levels. Every layer of paving or garden fill added over the years brings the outside ground closer to, and eventually above, the damp-proof course. Above it the DPC is bridged, and no amount of drainage helps.
Weep holes — the drainage most people block up
Weep holes are the small open perpends left in the bottom course of a cavity wall, and in the face of a retaining wall. They are not a building defect and they are not a draught. They are the only route out for water that has got into the cavity, or built up behind the retaining structure.
Filling them — with mortar, with sealant, with a run of plaster across the bottom of the wall, with a flower bed banked against it — traps that water. In a cavity wall it appears inside as damp along the skirting line. Behind a retaining wall it builds hydrostatic pressure until the wall itself moves.
If you have damp at the base of an external wall and there are no visible weep holes, look for where they were. Painting or plastering across them is one of the most common unintentional causes of ground-floor damp in Cape Town, and reopening them costs almost nothing.
Behind a retaining wall the weep holes work with a drainage layer — stone, geotextile and a slotted pipe at the base, which is a french drain by another name. Where that wall also retains against a habitable space it needs waterproofing on the wet side as well: see basement and cellar waterproofing.
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.
Related services
- waterproofing systems
- basement and cellar waterproofing
- damp proofing in Cape Town
- choosing a waterproofing contractor
- a free inspection
- contact us
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.