The Membrane Is Rarely the Problem
A concrete roof that keeps leaking after two or three waterproofing jobs usually does not have a waterproofing problem. It has a concrete problem.
Reinforced concrete relies on the alkalinity of the cement to protect the steel inside it. When that protection breaks down, the steel rusts. Rusting steel expands to several times its original volume, and that expansion cracks the concrete from the inside and pushes it off in flakes. This is spalling, and once it starts, anything bonded to the surface goes with it.
You can lay a perfect membrane on a spalling slab. It will lift within a few years, because what it is stuck to is coming away.
What Breaks Down the Protection
Carbonation. Carbon dioxide from the air reacts with the concrete over decades, lowering its alkalinity from the surface inward. When the carbonated zone reaches the steel, the protection is gone. This is a slow, universal process and it is why the problem shows up in older buildings.
Chloride attack. Far faster, and this is the coastal one. Chloride ions from salt-laden air penetrate the concrete and break down the protective layer directly, even where alkalinity is intact. Buildings within a few hundred metres of the Atlantic get this badly, which is why the flat concrete roofs and parapets of Sea Point apartment blocks are a textbook case.
Insufficient cover. If the steel was laid too close to the surface when the slab was poured, both processes reach it sooner. A lot of mid-century construction has thin cover.
Water. Everything above accelerates when the concrete stays wet, which is why a slab that ponds deteriorates faster than one that drains.
What Proper Repair Looks Like
Survey first. Delamination sounds hollow when tapped. Cover meter readings show where steel sits. Visible rust staining and cracking maps the affected areas.
Break out to sound concrete. All loose and delaminated material comes off, and the break-out goes back behind the steel, not just up to it. Leaving contaminated concrete against the bar means the rust continues under the repair.
Treat the steel. Cleaned back to bright metal and primed. Badly section-loss bar gets supplemented.
Reinstate with a repair mortar compatible with the parent concrete, then allow it to cure properly.
Then waterproof. And only then.
Parapets and copings deserve their own mention. They are exposed on both faces and on top, they take the full force of a north-westerly front, and they are usually the first thing to spall on a Cape Town coastal building.
Cape Town, Salt and Concrete
The Atlantic Seaboard and the False Bay coast put buildings in a chloride environment permanently. Salt is carried inland on the wind, deposited on surfaces, and driven into concrete by rain. A building three streets back from the sea is still in it.
Winter makes it worse in a specific way. North-westerly fronts drive salt water directly onto the north and west faces of a building at 50 to 65kph, so exposure is not even across a structure. The seaward parapet spalls while the sheltered one looks fine.
This is why we survey the whole roof rather than pricing the area you can see damage on. The damage you can see is usually the leading edge of a larger area that has not flaked off yet.
