Render or harling is a coating applied to the exterior of a home. There are two reasons for applying render:
There is now a range of render choices, from cement renders to new insulating render systems and more traditional lime and clay plasters.
This is the standard or traditional render used on external walls. Cement render is normally mixed on site and applied in two or three coats. The base coats (or scratch coats) are scored when they are still wet to give a key for the next layer. The top coat is applied more thinly and given a shiny finish, ready for painting. It tends to crack if the underlying structure moves. Cement renders require frequent repainting if they are to continue to look good.
This is the standard or traditional render used on external walls. Cement render is normally mixed on site and applied in two or three coats. The base coats (or scratch coats) are scored when they are still wet to give a key for the next layer. The top coat is applied and then stones are applied by throwing them at the wall. There is a large choice of coloured stones to choose from. It tends to crack if the underlying structure moves.
Usually sold in pre-mixed bags, polymer renders are based on either white cement or lime. They have polymers and other plastic-based products added to them to make the render less prone to cracking. Polymer renders are available in a variety of colours. They are often through-coloured, which does away with the need for painting.
Acrylic renders are usually applied as a thin finish coat to seal and enhance the appearance or the underlying coat. They also bring colour and texture. Fibres are added to prevent cracking and give a durable, lasting finish. Silicone is also used, promising even longer life and the capacity to be self-cleaning.
‘Monocouche’ (French for ‘single layer’ or ‘bed’ renders are a new development. A monocouche render is supplied in bag form ready for mixing with water, and it can be applied by hand trowel or sprayed on. It’s a practice that has spread here from Europe and many of the big names in this field – Sto, Knauf Marmorit, Weber – are German.
These renders use white cement and are pre-coloured (in any colour you want), meaning you will achieve a decorative finish as well as a weatherproofing layer. They can be applied in one coat (typically around 15mm thick), so they are much less labour intensive than traditional renders.
The downside is the material cost. However, monocouche render systems claw much of this added cost back through:
Monocouche renders have additives which make them more flexible and help to eliminate cracking. They can even be ‘self cleaning’.
There has been a re-emergence of interest in lime plaster_ a material not widely used for 60 years or more.
Lime can be used in place of cement render where flexibility and breathability are called for. However, lime tends to be harder to apply than regular cement an so is mainly used in conservation work.
It has some advantages over cement renders:
It is hand trowelled in the traditional way and is more expensive than standard renders.