Were flat concrete roofs that do not conform to the minimum standards as laid out by SANS 10400-L: The National Building Regulations – Part L: Roofs as submitted to the South African Bureau of Standards (SABS) i.e. where roofs do not have a minimum fall of 1.50 ( 1.80 were no interruptions exist to the flow of water), we recommend that a standard 20mm sand/cement screed be laid on top of the slab prior to the waterproofing process. Screeds laid during the rainy season should have a polymer modifier added to the screed mix as per the manufacturer’s instruction.
It is important that pre-cast roofing panels and pre-cast roof structures be firmly jointed to avoid any subsequent movement of the concrete elements thus preventing screeds laid above from cracking.
Adhesion problems can be experienced where water proofing systems are laid directly onto concrete surfaces which is often the main cause of de-lamination, hence the need for a less dense substrate.
Clearly the number and size of gutters and downpipes must be calculated based on the catchment area of the roof. A minimum of 75mm diameter downpipes are stipulated on small roofs and 100mm diameter on larger roofs.
Downpipes are to be positioned at least 200mm away from parapet walls and located well away from expansion joints.
Concrete and screeded roofing surfaces must be smooth, sound and with an even surface with the correct fall without undulations and protrusions or contaminants. Ideally concrete surfaces should not be power floated resulting in a highly polished finishes.
Waterproofing should be applied to surfaces that are dry and that have a moisture content of under 7 %. Ideally there should be no penetrations through flat roofs