Traditional Sand and Cement Rendering

Do you want the good news or the bad news?  Okay, the bad news is that you won't find very much information here about traditional "site-mix" sand and cement rendering.  But the good news is that we're going to tell you exactly why!

CRACKS!, CRACKS! and more CRACKS!

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

There is a 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 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.