Water Under Pressure Behaves Differently
Above ground, waterproofing sheds water. Gravity is on your side, and the membrane job is to send water somewhere else.
Below ground, saturated soil around a structure creates hydrostatic pressure, pushing water at the wall continuously and from every direction. That pressure does not care about the membrane quality. It cares about which side of the wall the membrane is on.
Positive side. The membrane on the earth face, where the water arrives. The water is stopped before it enters the structure, and the pressure pushes the membrane against the wall. This is the right way to do it and requires excavation.
Negative side. The membrane on the inside face. The water enters the wall, saturates it, and is stopped at the last possible moment, with the pressure pushing the membrane away from the wall. It can be done, with the right products, but the wall stays wet and any weakness in the application will find it.
This is why the first question on any basement is whether the outside face can be reached. Where it can, that is the answer. Where it cannot, the specification has to be chosen for negative-side pressure specifically, not just a waterproof coating.
The Three Approaches
External tanking. Excavate, apply a waterproof barrier to the earth face, install a drainage layer and a subsoil drain at footing level, backfill. Most effective, most disruptive, and requires access for excavation.
Internal tanking with a cementitious system. A dense, waterproof render applied to the inside face, bonded to sound substrate. Suitable where pressure is moderate and the substrate is sound masonry or concrete. Requires excellent preparation and unbroken continuity, because water under pressure finds any gap.
Cavity drain membranes. Rather than trying to stop the water, this manages it. A studded membrane is fixed to the inside face, water that comes through the wall runs down behind it into a perimeter channel, and from there to a sump or a drain. The wall stays wet, which is accepted rather than fought, and the internal space stays dry. It is the most reliable approach where pressure is significant and excavation is impossible, and it is the one most homeowners have never heard of.
Which of the three suits your situation depends on access, pressure, substrate condition and what the space is used for.
What This Actually Means in Cape Town
Basements are uncommon in Cape Town housing, and the ones that exist tend to fall into a few groups.
Wine cellars and below-ground storage on the larger Constantia and Bishopscourt properties. These sit on the wetter eastern slopes, where orographic rainfall is considerably higher than in the City Bowl, and in soils that stay saturated for months through winter. They are also spaces where humidity control matters for the contents, which changes the specification.
Cut-in rooms on sloped sites. Very common in a city built on mountains. A garage, a store or a converted room with one or two walls buried into the slope. These behave like basements on the buried faces and like ordinary walls elsewhere.
Basement parking in newer apartment developments, particularly on the Atlantic Seaboard where sites are steep and space is tight.
Older structures with cellars in the City Bowl and the older southern suburbs, usually with no barrier at all on the buried face.
The seasonal pattern matters. Around 70% of the annual rain falls June to September, so the ground goes from dry to fully saturated and back every year. A basement that is dry for eight months is not a dry basement, it is an undiagnosed one.
