Why Most People Get This Decision Wrong
Homeowners ring around for a price per square metre. Three contractors quote, the cheapest wins, and the same roof leaks again in four winters.
The problem is that a price per square metre only makes sense once the system is settled, and the system is settled by the roof. A flat concrete deck holding water for two days after rain has nothing in common with a tiled pitch shedding it in minutes. Applying the same product to both means one of them is wrong.
Before anything else, the roof needs looking at. What it is made of, where the water goes, what has already failed and why.
The System Depends on the Roof, Not the Price
Concrete decks and flat roofs. These hold water. They need a membrane that tolerates standing water and a substrate that falls to an outlet. Torch-on is the workhorse here. Liquid systems suit decks with many penetrations.
Tiled pitched roofs. These are drainage systems, not sealed ones. The waterproofing is the underlay beneath the tiles, plus sound bedding and pointing at ridges and valleys. Coating a tiled roof to waterproof it is the wrong instinct and usually traps moisture.
Metal roofs. Sheeting rarely fails first. The fixings do, and then the laps and flashings. A metal roof usually needs fixings addressed and a flexible coating that moves with thermal expansion.
Balconies, podiums and walkable decks. These need a membrane that survives foot traffic and tiling over the top, with falls to a drainage outlet.
Get the roof type right and the product choice follows. Get it wrong and no product saves you.
What Actually Fails
Almost never the middle of the sheet or the middle of the coating. Failures cluster at:
- Upstands and parapets, where the membrane meets a vertical and was terminated short
- Outlets, which carry the most water for the longest
- Laps and joints, where the bond was partial
- Penetrations, every pipe, vent, aerial mount and plant plinth
- The substrate, where a sound membrane sits on failing concrete
This is why we quote preparation separately. It is the part that determines whether the waterproofing lasts, and the part most quotes hide.
The Cape Town Rain Window
Roughly 70% of Cape Town’s annual rainfall falls between June and September, and it arrives on north-westerly cold fronts at gales of 50 to 65kph. That is rain moving sideways.
Sideways rain tests verticals. Parapets, upstands, flashings and terminations take water that vertical rain never reaches, which is why detailing on the vertical surfaces matters more here than the flat field does.
The summer south-easter is dry and does a different kind of damage. It lifts anything poorly bonded and abrades exposed edges. A partially bonded lap survives winter and peels in December.
Most waterproofing systems need a dry substrate, so March and April are the right months to book work, not the middle of a June storm.
