What is Render?
Render is a coating applied to the exterior of a home. There are two reasons for applying render:
- To protect the walling material from the effects of weathering and rainwater penetration
- To make the walls look good — i.e. as a decorative effect
There is now a range of render choices, from cement renders to new insulating render systems and more traditional lime and clay plasters.
What Types of Render can you get?
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’s cheap on materials, expensive on labour and it has a tendency to crack if the underlying structure moves. Cement renders require frequent repainting if they are to continue to look good.
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. A 25kg bag costs between £8-£10 and only covers a m² of wall area. This compares with a materials cost on a traditional cement render of around £1/m². However, monocouche render systems claw much of this added cost back through:
- reduced labour
- no need for subsequent painting.
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 is called for. However, lime tends to be a little harder to apply than regular Portland cement and so it is mostly used for conservation work. Note that both Baumit and K Rend supply polymer-enhanced lime renders.
It has some advantages over cement renders:
- It is more flexible
- You are less likely to have problems with moisture getting trapped within the wall. This is a problem when cement renders are applied to old walls.
- Lime also tends to look very appealing, though it does require frequent coats of limewash.
Building limes are available in different formats, from traditional lime putties (which are bought wet, by the tub) to bagged hydraulic limes, which behave rather like a weak cement. They need to be mixed on site with sand, and are hand trowelled in the traditional way. Limes are a little more expensive than standard cement renders, but shouldn’t take any longer to apply.
There are even lime-based monocouche renders. Try K-Rend (K-Lime) and Baumit.
There is also an important difference between claddings which are self-supporting, such as brick and stone, and those which need to be attached to something in order to stay in place, such as render.
You can choose to place any cladding against any background. However, brick and stone tend to work best against a masonry background, and lightweight claddings tend to work best against timber frame.
In the UK, much of the insulation upgrading has been done via the cavity wall, but cavity construction is uncommon in other countries. There the preference has been to add insulation to the outside of the existing walls and then render over the insulation.
This has led to the development of proprietary external wall insulation (EWI) systems that comprise a build up of insulation layers, meshes and top coat renders. Many well-established businesses have their own take on how best to insulate solid walls. Names to look out for include Sto, Marmorit, Weber, Baumit, Dryvit, Wetherby, Permarock and K Rend.
External wall insulation has also found a niche in the world of home building. Here, it’s not so much the addition of insulation that sells it, but the promise of a modern, clean-looking finish. Thanks mostly to the success of external wall insulation, rendering itself has begun to go high-tech and we are seeing a wave of new techniques and materials offering exciting through-colours, which don’t require subsequent painting.
External wall insulation is a relatively complex procedure. It involves a build up of several layers and there are often many details to address, such as reconfiguring roof overhangs and realigning pipes. This means that suppliers feel uneasy about selling their system components directly to the public or the general building trade. They prefer to work through exclusive licensed contractors. However, you can source the components yourself.
While these new renders are more expensive than the traditional cement renders, they can often be applied in one coat. This greatly reduces labour costs.
They can also be applied directly to blockwork or some other substrate and this has proved popular with those seeking a clean, contemporary finish.