A render finish in place of the more traditional brick allows for creativity, so it is easy to see why this option is popular with self-builders. Celcon Blocks can easily be externally rendered as part of a cavity wall system or as a solid wall. Both traditional renders and third-party render systems can be applied to aircrete walls as long as the render is applied following the manufacturer’s guidelines.
Preparing the wall surface
Before applying any render to aircrete, it is essential to ensure the completed wall is free from dust, loose mortar and contamination. Mortar joints should be raked by 10-15 millimetres (mm) as the wall is constructed to provide a good key for the render.
If using thin-joint mortar, such as H+H’s Celfix Mortar, a surface treatment using a ‘stipple’ coat or ‘spatterdash’ should be applied to the completed wall before the render. This should consist of one part cement to two parts sharp sand with water and a bonding agent suitable for use outside (such as SBR, EVA or acrylic emulsion) mixed to form a thick creamy consistency.
Traditional sand:cement render mixes should be applied to the completed wall at DPC level and above. For exposed aircrete up to 150mm below DPC level, a bituminous paint suitable for outdoor use should be applied directly to the blocks. Any movement joints in the wall should be continued through the render finish.
For all but the most extremely exposed walls, two coats of render mix is sufficient. Each coat must be applied weaker than the last; easily achieved by just applying the next coat in a thinner layer. It is important that the sand used in this process is clean, sharp and well-graded. Sand normally used for brick and block laying is unsuitable for a render mix as it is too fine.
How to render a wall
The undercoat of the render needs to be around 12mm thick and either combed or scratched to give a good key for the following two (or three) coats. The scratches must be wavy in order to prevent cracking further down the process and the undercoat must be kept damp for up to 48 hours before being allowed to dry out for a further 2-3 days.
Subsequent coats should be around 6mm thick and finished off with a wooden float to achieve the desired texture.
This article is provided by H+H