Materials and climate · 7 min read
Flat-roof waterproofing on new Windhoek builds: where it goes wrong
Updated
New Windhoek houses are increasingly flat-roofed behind parapets. It is a clean architectural language and it works well here — a dry climate is forgiving of flat roofs in a way that a wet one is not.
But a flat roof has no margin for error. There is no pitch shedding water away from a mediocre detail, and the details are few enough that each one carries real weight.
Falls decide everything else
Water has to move to an outlet under its own weight. If the screed is laid flat, or worse, falls slightly the wrong way, you get standing water within days of the first storm.
Ponding is not merely unsightly. Standing water finds pinholes that would never leak otherwise, accelerates membrane breakdown, and turns a small defect into an internal leak.
Upstands and terminations
Where the membrane turns up a parapet it must go high enough that a backed-up storm cannot get over it, and it must terminate mechanically: into a chase, under a capping, behind a trim. A membrane stuck to plaster and left with a bead of mastic on top is a countdown.
The tell for a bad upstand is a leak that appears along one wall only, always in the same place, only in heavy rain.
A flat roof does not leak in the middle. It leaks where it turns up, where it stops, and where somebody drilled through it later.
Outlets sized for a Windhoek storm
The design rainfall that matters here is not the annual average, it is the intensity of a single February cell. Outlets sized generously, plus an overflow that discharges visibly over the parapet, are cheap insurance.
A visible overflow is also a diagnostic: when you see it running, you know the main outlet is blocked, before the water finds another way out.
- At least one overflow per roof area, discharging where you can see it
- Outlets clear of the finished screed level, not proud of it
- Leaf and debris guards that can actually be cleaned
Penetrations and later additions
Every pipe, geyser feed, conduit and solar mounting foot is a hole in the waterproofing. Each needs a collar or a properly dressed detail.
The dangerous ones are added after the roof was finished. A solar installer drilling into a completed membrane without a roofer's detail is one of the most common causes of a leak on a two-year-old flat roof.
Protection and traffic
Exposed membrane needs UV protection at this altitude. Trafficked terraces need more: a protection layer, pavers on supports, or a system rated for foot traffic.
Planters and furniture standing directly on a membrane will eventually puncture it, and the leak will appear metres away from the pot.