Sloped Sites Collect Water You Never See
On a flat site, rain falls, some soaks in, some runs off, and the ground around a house wets and dries roughly evenly.
On a slope, the ground is a catchment. Water falling anywhere uphill moves downhill through the soil, and it keeps moving until something stops it. A house built into a slope is exactly that something. The uphill wall of the building becomes a dam, and through a Cape Town winter it holds back saturated soil for months.
This is why so much of Cape Town has this problem. The city is built on and around mountains, and a very large share of its housing sits on ground that slopes. The eastern slopes above Kirstenbosch and through Constantia are wetter still, because orographic lift pushes air up the mountain and drops considerably more rain there than falls on the City Bowl or Camps Bay a few kilometres away.
The homeowner sees damp appearing at the base of an internal wall in July and assumes rising damp. What is actually happening is that the soil against the outside of that wall has been saturated for six weeks.
What Actually Fails
No barrier on the buried face. In a lot of older construction, the buried part of a wall was simply built, with no tanking or waterproof coating on the earth side. It works fine until the soil stays wet long enough.
Failed or absent surface drainage. Water arriving at a building should be intercepted before it gets there. A subsoil drain uphill of the structure, laid to fall and discharging somewhere sensible, does more than any coating. Many Cape Town properties have none, or have one that silted up decades ago.
Downpipes discharging at the base of a wall. Astonishingly common, and it saturates the foundation of one specific spot continuously through a whole winter.
Raised ground levels. Paving, patios and garden beds built up against a wall above the damp proof course. Water enters the wall above the barrier, and no amount of foundation work fixes it because the water is not coming from the foundation.
Blocked or absent weep holes in cavity construction.
Cracked or missing surface falls around the perimeter, so rain that lands next to the building soaks in rather than running away.
What We Do About It
Diagnosis first. Moisture readings, external ground levels, drainage layout, downpipe discharge points, and where the water is actually arriving from.
Drainage before coatings, wherever possible. Intercepting the water uphill of the structure is more effective and usually cheaper than trying to hold it back at the wall. A properly laid subsoil drain in a stone-filled trench with a geotextile wrap, discharging to a stormwater connection, solves a great many of these problems on its own.
Ground level correction. Where paving or beds have been built up above the DPC, lowering them or cutting a channel is a straightforward fix for what looked like a serious damp problem.
Waterproofing to the buried face, where excavation is practical and the situation warrants it.
Internal tanking, where excavation is not practical, holding water on the negative side. Less desirable than stopping it outside, but sometimes the only option.
Surface falls and perimeter drainage, so water lands and leaves rather than lands and soaks.
The Cape Town Winter Soil
The pattern here is different to a climate with year-round rainfall, and it changes what matters.
Roughly 70% of the annual rain falls June to September, in concentrated fronts. Soil that has been dry and shrunken through a long summer takes the first heavy front badly, because dry ground sheds water rather than absorbing it. As winter goes on, the ground becomes progressively more saturated, and by August the soil against a buried wall on a sloped site may have been fully wet for weeks continuously.
Cut-off low pressure systems complicate it further. They can form at any time of year, sit over the region for days, and produce sustained heavy rainfall well outside the normal winter window.
The practical result is that foundation problems in Cape Town are seasonal and severe rather than constant and mild. A wall that is bone dry in February can be visibly wet in August, which is exactly the pattern people mistake for rising damp.
