The short answer to this is predominantly ‘Thermal Expansion and Contraction’ caused by temperature fluctuation. Different building materials expand and contract at different rates; and yet others are more resistant to cyclic movement altogether.
It is an unfortunate fact that many Builders, Architects, and Developers today are still happy to trust that a traditional cement based render system will provide durable façade protection for many years. The simple truth is that it won’t. Why is this the case?
By very nature, cement based render systems are brittle and alkaline (calcium silicate) formulations. Because thermal expansion and contraction is an unavoidable phenomenon, standard mix designations were introduced to help reduce surface cracking and the resultant problems of water ingress and moisture encapsulation. Scratch-coats were applied and at 4:1 with a waterproofing additive (lime is absent because it is hygroscopic). The render topcoat is applied at a 6:1:1 mix designation with a plasticiser additive. Lime is present in its 1 part because it works as a plasticiser, and it will also hold in present moisture and aid workability. The presence of lime helps the render to cure at a slightly slower rate (through moisture retention) and therefore reduces shrinkage at the application stage.
From a technical point of view, if all traditional sand and cement renders are applied strictly as above, without variation, then this would provide (economically) adequate facade protection from the elements for many, many years. (You’d be surprised, by the way, at the sheer amount of professionals who do not know their correct mix designations or the reasons behind them. For example I’ve heard of renderers putting water proofer in a 4:1 topcoat, which is simply insane! Read on and all will become clear.)
Okay, so we know that the building will expand and contract. We also know that render with a correct mix designation will eventually form surface cracking. This sounds like a contradiction, but et me explain: Thermal movement is going to happen and no material or cement render is going to stop it. A less-hardened 6:1:1 render (and less-hardened being the optimum word here) has reduced liability of material fracture when subject to stresses. Upon movement - expansion - the render will undergo plastic deformation, but due to increased properties of plasticity these non-reversible changes of shape should instead manifest as fine crazing occurring homogeneously throughout the material as opposed to outright fracture and surface fissures. Upon contraction movement, the same properties of plasticity apply but in the other direction.
The calcium silicate in cement is primarily responsible for its strength, hardness and moisture resistance. The two-fold purpose of the stronger cement scratch-coat should now be quite clear: moisture and vapour can pass through the high-permeable render top coat (via capillary pores and micro plastic deformation); but moisture will be restricted from passing through the low-permeable scratch coat due to its higher calcium silicate content (and nowadays usually with further, specific water proofing additives).
Unfortunately, through the convention of aesthetics (“I want my house to be … white,” for example) enters a new complexity in to the equation: modern, low film build exterior paints. These paints on the most part have been engineered to be impervious and have properties of limited plasticity. These paint films tend to crack upon stresses of thermal expansion and contraction (plastic deformation fractures). Moisture can then freely pass into the render top coat but cannot pass back out at the same rate through the larger impervious paint film area. Moisture encapsulation ensues. From here on out (once moisture enters an external rendered surface and cannot pass out again freely as vapour) brick face damage, extensive cracking and delamination is as sure as the sun will rise tomorrow. It may take a year; it may take two or three or more. Mother Nature may take her time doing it (or not, with the next bitter, freezing winter) but that render is coming off the wall.
The solution to durability and efficacy of external rendering lay in observing two principles:
This of course opens up wide the doors to modern, technologically advanced render systems – of which there are many. There are Thin Coat Flexible Acrylic Render Systems, or Vapour Permeable Through Coloured Render Systems. You can confidently explore these further below with your Professional Nationwide External Rendering Company: DuraRend