Waterproofing your flat roof before the Cape winter

Cape Town gets most of its rain between May and August, and it usually arrives with wind. A flat roof or concrete deck that has baked through summer will show every weakness in the first proper cold front. Autumn is the right time to check it, while the weather still allows repairs to cure.

What to look for

Walk the roof on a dry day, or have someone do it for you, and look for:

Ponding. Water that sits for more than a day or two after rain means the falls are wrong or the outlets are blocked. Standing water shortens the life of any membrane.
Blocked outlets and gutters. Leaves, sand and old sealant are the most common cause of leaks we see. Clear them before anything else.
Cracked or blistered membrane. Blisters mean water or air is trapped underneath. Cracks usually appear where the roof meets a wall or a parapet.
Failed flashings. Check where the roof meets walls, around pipes, around skylights and at the top of parapets. These junctions move more than the open roof and are where most leaks start.
Damp patches inside. Stains on ceilings, bubbling paint and black mould at the top of walls are signs that water has been getting in for some time.

Common waterproofing systems

Torch-on bitumen membranes are the most common system on concrete roofs in South Africa. They are hard-wearing when properly installed, with good overlaps and a reflective or aluminium finish to protect them from UV. They do not like foot traffic unless protected.

Liquid-applied membranes, usually acrylic or polyurethane with a reinforcing fabric, work well on roofs with lots of details and penetrations, because they form a seamless layer. They need a sound, dry surface and the right thickness, which usually means several coats.

Cementitious waterproofing is common under tiles on balconies and in wet areas. It must be detailed properly at junctions and used with the right adhesive and tile system.

There is no single best system. The right choice depends on the roof structure, how much it will be walked on, what is on top of it and the condition of what is already there.

Repair or redo?

Small, local failures, such as a lifted flashing or a cracked joint at a parapet, can be repaired. But if the membrane is brittle across the whole roof, blistered in many places or patched several times already, patching again is usually wasted money. Stripping back and starting with correct falls and new outlets costs more up front but solves the problem.

A key point many people miss: waterproofing is only as good as the falls under it. If water cannot drain, it will eventually find a way in. Correcting falls with a screed before waterproofing is often the real fix.

Timing matters

Most membranes need dry weather to install and cure. Booking work in March, April or early May gives you a better chance of a clear window than waiting until the first leak in June, when every waterproofer in the city is busy.

Signs you need a professional

If water is coming through in more than one place, if the ceiling is sagging, or if you cannot see where the water is getting in, call someone who can investigate properly. Water travels along slabs and beams, so a stain is often metres away from the actual failure.

We do waterproofing as part of our renovations and new builds. If you are worried about a roof or balcony, let us know.