Modern Render Systems

Tim Doherty takes a closer look at the render design options for your external walls
by Tim Doherty
31st January 2018

The manufacturers of building materials are constantly coming up with new ways
of blending minerals and chemicals together to give us superior products to use on site – sometimes for easier application, but also with improved performance and lower maintenance characteristics.

Render is one such product; however, the technical terminology often used for modern systems leaves many self-builders and renovators feeling confused. Hopefully the following pages will help to shine a light on the choices and their main benefits.

Product options

Render is a coating applied to a building’s external walls – basically, the smooth, usually white surface on the outside of a house. Traditionally, a mix of lime, water and an aggregate (such as sand) created a flexible and breathable covering. This method hasn’t been particularly popular in the last few decades, but has recently experienced a bit of a resurgence; it’s certainly still used for period property conservation.

Conventional render is made from a cement, sand and aggregate blend, but many modern products are cement free. These newer options sometimes contain products to help waterproofing and resistance to algae growth – you can even get renders pre-mixed with a colour for a vibrant finish and ones that work with external insulation.

A Parex render system has been used on this coastal house to create a smooth, clean finish for the curved frontage
Saint-Gobain Weber’s monocouche render creates a stonework aesthetic on the gothic facade of this new build in West Sussex

Generally, modern render can be divided into three main types: mineral, acrylic and silicone – but there are other options, as I’ll present later. Normally these need only one coat of 1mm-4mm thickness, but this is highly influenced by the substrate material used under the render.

It’s really difficult to indicate prices for each option as they need different prep work and finishes; the cost is also dictated by the granular sizes of the filler selected, which can range between 1mm-3mm. If chippings are being added to the surface once the render is in place, then it may need to be at least 6mm thick.

A basecoat of 8mm-10mm will be needed to level out a poor brick surface, sometimes using a mesh membrane to reinforce. For lighter weight construction – timber frame or steel studs – a proprietary backing board can be applied to act as the substrate, which is then covered in a reinforcement mesh, much like the levelling coat over masonry.

One of the features making modern renders so popular is that they can be combined with external wall insulation (EWI) thanks to their light weights and thin coats. Normally there’s only need for one layer on the outside of the insulation, but still on top of a reinforcing mesh.

With houses built using timber frames or steel studs, a cavity might need to be specified to ensure no interstitial condensation beyond the insulation in the panel. Details vary job-to-job, depending on the construction method and any restrictions in place.

Here’s how the various render solutions work:

Mineral: This is a cement based one-coat top layer render, with lime and polymers in the mix. It comes as a dry powder, mixed with water on site, and is considered a breathable product, highly suitable for substrates with permeable mineral wool insulations. It’s quite absorbent, so you’ll probably need to paint it to protect against rain. But it’s generally the cheapest option.

Acrylic: As the name suggests, acrylic/polymer ingredients are used to bind this type of render. It tends to be pre-made, arriving on site in tubs – it can even be pre-coloured with near enough any pigment to produce a vibrant finish. It’s durable, but the waterproofing qualities aren’t always great and it can attract algae.

Silicone: This render option has the same flexibility and colour choice benefits as acrylic, but with an added level of waterproofing as it self-cleans through staying dry – sometimes with an alkaline content to discourage algae.

It’s more of a premium product so tends to come in fairly expensive, but is widely recognised as being easier to apply than the alternatives. As a rough guide, a silicone render could cost double the price of a conventional cement mix – coming in somewhere around £60 per m2.

Monocouche: This uses cement as a hardening agent and regardless of how many coats are applied, its overall thickness is designed to be 15mm. It arrives on site as a dry mix, ready to be blended with water. One layer can be sufficient for well-built, neatly finished blockwork, but two coats are recommended.

The first is a 4mm monobase: a polymer enriched, flexible render, including a mesh. The remaining 11mm monocouche is then applied. It comes in 19 standard through-colours and can be finished smooth, textured or rusticated to look like stone. Costs come in similar to silicone, at around £50-£60 per m2.

Lime: Although it hasn’t been widely used for over half a century, lime has experienced a recent resurgence thanks to its flexible and breathable advantages. People also like the characterful look it achieves. It’s a specialist product and should only be applied by those who understand how to work with it, especially as it needs specific ingredients and post-application washes.

If applied to blockwork, it’ll need three coats – two base and one top – each between 8mm-10mm thickness. The mix will be calculated individually to suit the specific property (based on its exposure, the substrate material etc) as will its application speed. It can be acceptably cured in 28 days but not completely set for up to a year. Because this technique isn’t widely known, it’s likely to be expensive.

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