Why Render Cracks

Why does a cement render coated with a conventional paint system crack?

The simple answer to this is, all cement renders irrespective of how they are formulated will eventually develop surface cracks (such is the nature of cement curing and hardening). Sand and Cement renders are highly alkaline, rigid and brittle surfaces that develop surface cracks through cement shrinkage and thermal expansion & contraction from daily temperature fluctuation.

Sand and Cement Render is conventionally painted, obviously for aesthetics. Decorative conventional paints are a low film build, this technology is not formulated to withstand the stresses caused by thermal expansion and contraction of cement based rigid substrates. It forces the paint film to crack due to insufficient film build, lack of flexibility (elongation) leading to poor crack bridging to withstand the substrate dynamics. These cracks over time will continue to expand & contract and become a gateway to water, corrosive and atmospheric pollutant attack potentially leading to delamination and/or 'drumminess'.

Traditionally "cement render" means site mixed sand and cement (bulk materials mixed on site) but increasingly these days includes various supplier pre-packed bagged cement renders. Pre-packaged bagged supplied cement renders offer some advantages over site mixed render in terms of easy of handling, projectability via machine, and product consistency of mix formula (ie. vs uncontrolled site mixed render), water repellancy, and being 'breathable' (ie. through coloured and therefore not painted).

Conventional site mixed render in not waterproof, and nor flexible in any shape or form and therefore prone to cracking over time. All sand cement renders once cured form part of the initial building substrate then a finishing coating system is applied for decorative and protective purposes.

The initial visual look of a painted sponge finish sand and cement render is very similar to the most widely specified and used Acrylic Texture "Sand Finish" style desired of today's contemporary architecture but that's where the similarities end with a significant performance gap between two types of systems. Traditional sand and cement render will crack over time, and the paint will peal and flake. This is unavoidable. The solution is to use technologically advanced, durable, modified premixed through-coloured renders or silicone or acrylic resin thin-coat render.

How do you select the correct render?

When selecting a finishing system for cement rendered substrates there are two factors that need to be considered.

1. Performance relevant to building location (environment).
2. The coating must satisfy aesthetically to deliver a finish that best suits owner and the building aesthetic type, accommodate the substrate's daily dynamic stresses caused by temperature and climate changes, protect from atmospheric pollutants, salt air, water ingress, dirt accumulation and carbon dioxide.

Builders and Architects have been known to make mistakes. Here we have a new-build which was constructed using lightweight thermalite blocks.These blocks were incorrectly specified and, therefore, so was the render system that was applied directly on top of them. And here is the result, literally two weeks later ....

Widespread cracking and delamination.  The delamination was repaired, and the cracks raked out and filled with a silicone compound.

And now for the solution! ...

High build, flexible acrylic texture coating systems (where the main binder is acrylic resin, not cement) and the aggregates / sands are of a much higher quality than those used in cement renders. We can apply in a range of aesthetic styles from trowel, spray and roller applied "sand finish" contemporary looks to feature texture finishes. Cement Renders are still used as the base-levelling coat and Acrylic Texture Systems then are applied as a finished coat for protection and style.

The substrate is fully hammer-tested to ensure that it is sound. Loose and cracked render is then repaired. Acrylic Resin Render is then applied, which Formulated specifically to give flexibility & elongation performance to accommodate substrate daily dynamic stresses and protect from atmospheric pollutants, salt air, water ingress, dirt accumulation & carbon dioxide substrate.

The result is a fantastic, uniform, flat and evenly textured surface ... a surface which is now resitant to cracks!