Applied exterior finishes

Sand and cement render

Sand and cement rendering, whether used as a finish coat in its own right or as a background to tiling or other finishes, can be problematic for several reasons. The choice of mix will influence the performance of the coating and indeed the substrate to which it is attached.

For example, a dense cement rich render absorbs little water, but it is also inflexible. When the inevitable cracks occur, water may penetrate behind the render to promote deterioration by frost action, chemical attack or dampness. By contrast, a weaker, more flexible render (such as a mix of 1:1:6 cement:lime:sand) can absorb more moisture and will crack less. Such materials are more able to promote drying by evaporation so that damp penetration and resultant chemical or frost problems are less prevalent.

With a cement rich render, any moisture that does penetrate to the interface between the render and the background will take longer to dry out. Certain masonry, such as Fletton or common brickwork, contains high levels of soluble sulphates. In persistently damp conditions, the sulphates react with the alumina in Portland cement to form ettringite: a process which causes expansion, disintegration and loss of strength, with the render often developing a series of parallel cracks along bed joint lines.

Cement render shrinks on drying and imparts a force upon the background. The adhesion of the render to the background must be good enough to overcome the shrinkage force otherwise delamination will occur. When render is applied to smooth dense surfaces, such as concrete and certain types of well-burnt brick, it is essential to provide an additional mechanical key otherwise adhesion failure is likely.

This panel of tesserae was repaired about 10 years before the photograph was taken. Vacuum grouting was used to secure the background render to the concrete substrate. The repair appears to be performing satisfactorily.

Moderately porous surfaces such as clay and calcium silicate bricks usually provide a better key, although some wetting and possibly a spatter dash priming coat would be beneficial. For very soft or otherwise unsuitable backgrounds, stainless steel render carriers are essential. These can be configured so that the render coat functions in the same way as a rainscreen cladding. Some carriers have a building paper lining to facilitate this purpose.

To avoid unnecessarily high shrinkage stresses, rendering should not have a total thickness in excess of 20mm. Each coat should be no less than 8mm thick and no more than 16mm thick. (Wall and floor tiling. Design and installation of external ceramic and mosaic wall tiling in normal conditions. Code of practice, BS 5385–2:2006, BSI, 2006)

In the UK, cement based renders were often left smooth faced and painted, in contrast to European practice which often employed Tyrolean render to give a textured surface. Generally, textured surfaces absorb more water than smooth-faced surfaces and so help to protect walls against dampness. However, modern practice has drifted away from traditional render systems, and external insulation systems incorporating polymer-modified, thin coat renders have become more common in both commercial and residential construction.

Mineralite render

First developed in 1923, Mineralite render became a popular finish in the 1960s and 1970s. Although less popular now, it is still marketed. Intended to be the finish coat to sand and cement render, it was usually applied to a thickness of around 5mm by specialist applicators. Mineralite render is a cementitious-based product manufactured from white cement, graded sand and granite and glass aggregates. Various pigments can be added to create a variety of different appearances.

Mineralite was intended to be applied as a finishing coat to a 12mm thick layer of sand and cement render (1:0.5:4 cement:lime:sand or stronger), although it was commonly applied direct to visible concrete elements such as columns, beams and spandrel walls.

While the product is generally durable, non-combustible and of low permeability, it was susceptible to poor workmanship or delamination of the background render from the structure. Such problems could be due to lack of adequate key, laitance on the surface, mould oil, PVA adhesives and the like. Therefore, it is not unusual to see sheets of Mineralite peeling away from the surface and creating a hazard for passers by.

Brown painted Mineralite render applied to the columns and edge beams of this building. The material has been applied direct to the concrete structure without a backing render. Sheets of the material are delaminating.