The two systems that work
| Cementitious tanking | Cavity drain membrane | |
|---|---|---|
| Principle | Resist the water | Manage the water |
| Method | Multi-coat cement slurry bonded to the structure | Studded HDPE sheet, drainage channel, sump |
| Needs a sump | No | Usually yes |
| Tolerates movement | Poorly — cracks if the structure moves | Well |
| Reversible | No | Yes, and inspectable |
| Finish | Plaster directly onto it | Battens or a dry lining over it |
| Indicative | R600 – R950/m² | R750 – R1,200/m² |
On a structurally sound, non-moving wall with moderate water, tanking is cheaper and gives a usable plastered surface. Where the water table is genuinely high, where the structure moves, or where a failure would be expensive to discover, cavity drain is the more honest system — it does not depend on being perfect.
Which of the two you need, and how to tell
The choice is not a preference. It is decided by how much water arrives, how fast, and whether the structure moves.
- Tank it when the wall is sound, unmoving and the water is seepage rather than flow — a single cellar wall against a bank, a lift pit, a planter. Cementitious tanking is applied in two or three coats to a combined 3–4mm, brush-applied in alternating directions, with a 24-hour cure between coats and a fillet at every wall-to-floor junction.
- Drain it when the water table is high, the flow is continuous, or the structure has cracked before. A studded HDPE cavity drain membrane is plugged to the wall on roughly 400mm centres, the 8mm studs held off the substrate so water runs down the cavity into a perimeter channel and out to a sump.
- Do both where a slab is below the water table and the room has to be habitable — tanking on the structure, a drained cavity in front of it, and a twin-pump sump so a single failure does not flood the room.
Two readings settle it in an afternoon. A damp meter across the wall at 300mm, 900mm and 1.5m tells you whether the water is rising, lateral or general; a hole cut through the plaster tells you whether the substrate is sound enough to bond to. Nothing gets quoted before both are done.
Why retrofitted basement waterproofing fails
Below ground, almost every failure is one of four things, and three of them are decisions made before any product was opened.
- A coating used against pressure. Ordinary damp-seal paint on the inside face has no bond strength against hydrostatic pressure and blows off in sheets, usually inside the first wet winter.
- No relief for the water. Tanking a wall without dealing with what is loading it simply moves the problem sideways to the next weakest joint.
- The floor-wall junction skipped. More below-ground water comes in at that junction than through the whole wall above it. It needs a fillet and the membrane carried across, not a bead of sealant.
- A sump with one pump and no alarm. A drained cavity is only as good as the thing emptying it.
On a Cape Town site the wet season runs May to August and the water table moves with it, which is why a basement that was dry in February is not evidence of anything.
Getting the outside right first
Almost every basement problem is made much cheaper by dealing with the water before it reaches the wall. Where the site allows it, that means a perimeter french drain at footing level, surface water directed away from the building, and downpipes discharging into stormwater rather than against the foundation.
Internal tanking on a wall that has a downpipe emptying against it outside is money spent in the wrong order.
What a below-ground quote has to tell you
Below-ground work is the easiest place in this trade to be sold the wrong thing, because the wall looks the same before and after and the test is a wet winter eight months away. A quote worth comparing names six things.
- Which system, and why this one. Tanking or a drained cavity, with the reading or the observation that decided it.
- The product and the build-up. Number of coats and total thickness for tanking; stud height, fixing centres and channel detail for a cavity membrane.
- The floor-wall junction. Named explicitly, with a fillet and the membrane carried across. If it is not in the quote it will not be on the wall.
- What happens to the water. Where the channel discharges, whether a sump is needed, how many pumps and whether there is a high-level alarm.
- Preparation. How much plaster comes off, how the substrate is made good, and whether existing salt-contaminated render is being replaced or coated over.
- The guarantee term and what voids it. A guarantee on a system that has no drainage relief outside is worth what the exclusions leave of it.
Anything missing from that list is a number, not a specification — and the only way to arrive at it honestly is to inspect first. That is what a free inspection produces: the cause, the evidence and the recommended system in writing, before a price is attached to it.
Related services
- french drains and drainage
- waterproofing systems
- damp proofing in Cape Town
- choosing a waterproofing contractor
- a free inspection
- contact us
Common questions
How much does basement waterproofing cost in Cape Town?
About R600–R950 per m² for cementitious tanking and R750–R1,200 per m² for a cavity drain membrane system. A sump and pump installation adds roughly R8,000–R20,000.
Can I just paint the inside of my basement wall?
Not against hydrostatic pressure. Water pushing from the negative side will lift a surface coating off the wall — the coating has nothing but its own adhesion resisting the pressure. This is the most common failed basement 'repair' we are called to replace.
What is negative-side waterproofing?
Treating the inside face of a below-ground wall, i.e. the side opposite the water. It is what you do when you cannot excavate to reach the outside. It works, but it demands a system designed for it — cementitious tanking or a cavity drain — not a coating.