Render Carrier Boards: The Ultimate Guide

Render carrier boards, sometimes referred to as sheathing boards, are typically made from cement boards, cement particle (CP) boards, magnesium oxide (MgO) boards or even gypsum.  

Render carrier boards provide a durable backing surface for thin coat render systems to be applied to. These boards are normally applied to a carrying substrate such as treated timber battens or a lightweight steel frame, and are common place in large residential and commercial projects.

For more traditional renders, i.e. a monocouche scratch finish or a lime render finish; a thicker board such as a Celenit wood wool board should be used. This is because of the extra weight a traditional render system has.

All of these boards have similar characteristics in terms of application, but all have subtle differences in terms of performance. Some boards are easier to cut, as they can be “scored and snapped”. Some offer class A1 non-combustibility, and some offer good impact resistance. They will all provide some weather protection before they are rendered, and in some cases can be left exposed for up to a year.

Here is a list of the most common boards for thin coat render systems and their features:

Render carrier boards provide a solid, durable surface that can withstand the extreme weathering effects of wind, rain and snow. They can provide an impact-resistant barrier, and most render carrier boards don’t rot or swell when they come into contact with water, so they survive a long time even when located on the exterior of the property.  

However, some boards are not as structurally stable as others and depending upon the applications manufacturers will sometimes advise leaving a space between joints – so always check the specification. At EWI-Pro, we recommend taping all joints.

Applying render to a render carrier board provides a fantastic looking finish, but also offers a further level of protection to the boards. So how do you go about it?

Applying the basecoat to the render carrier board

The first step in any thin coat render system is the basecoat layer. Applying the basecoat to the cement board is relatively easy – we recommend using the EWI-225 Premium Adhesive. The EWI-225 is a premium adhesive which comes as a dry mix and therefore must be mixed with 7 litres of clean, potable water prior to application. Please see our product data sheet for more information about how to prepare the adhesive for use. 

Once the adhesive has been mixed with water and the white putty has formed, it needs to be applied to the render carrier board with a notched trowel. We recommend using a 10mm notched trowel, this is the best sized trowel for applying a 6mm layer of adhesive to the render carrier board.

Once the adhesive has been applied it is time to embed the fibreglass mesh within it. The EWI-640 mesh gives the render system strength and flexibility and comes in 50 x 1m.  The mesh needs to be embedded in vertical strips within the adhesive. Each strip must overlap one another by 10cm at each of the edges and must be embedded within the adhesive layer – to embed the fibreglass mesh simply use the flat edge of the notched trowel and draw this up from the bottom of the wall which will pull the adhesive through the holes in the mesh.

Remember the render topcoat is incredibly thin so therefore the basecoat layer needs to be completely smooth before applying the render. If you have embedded the mesh and the finish isn’t completely smooth the easiest thing to do is to produce a very wet adhesive mix and apply this to the basecoat before the first layer goes off. This should allow you to create this smooth layer ready for application of the render.

Once the basecoat layer has completely gone off, the EWI-225 Premium Adhesive will be ready for the finishing coat. There is no need to prime the EWI-225 Premium Adhesive unless you are using a particularly dark colour, or if you want to help control the speed at which the render goes off.  

For each finishing coat there is a corresponding primer which can be tinted to match your render colour. Take for example EWI-333, our Silicone Silicate Primer which comes in either 7kg or 21kg buckets. This primer is applied to the basecoat layer using a paintbrush or roller, and is used in conjunction with EWI-040 Silicone Silicate Render. A 7kg bucket of EWI-333 primer covers approximately 20m2 and the 21kg bucket covers approximately 70m2 of wall.

Applying the finishing coat to the render carrier board

It is now time to apply the thin coat render to produce the finished façade! EWI-Pro have a wide range of thin coat renders, most of which are ready-mixed renders except for the EWI-060 Mineral Render. Our thin coat renders come in hundreds of different colours and can be tinted using our special colour tinting equipment to achieve a shade that you like best!

When applying thin coat renders, the thickness of the layer corresponds with the size of the granulate in the render. For example, EWI-040 Silicone Silicate render comes in 4 grain sizes, 1mm, 1.5mm, 2mm and 3mm. The layer of the render should therefore be either 1mm, 1.5mm, 2mm or 3mm depending on the chosen grain size – it should be no thicker!

This is really important to remember, and will ensure a perfect finish. We have heard several reports where people have been applying the render too thick and have not been able to achieve the required finish. With that in mind, apply the render to the render carrier board and basecoat with a trowel, and then spread or pull off all the excess with the trowel. Once you have taken off any excess, leave the render for 5 minutes and use a plastic float to work the render in a circular motion to achieve the final finish.

We have a whole database of approved installers who have undergone our training courses would be able to carry out this work for you, so give us a call if you wish to be put in touch with a trusted local installer!

Applying insulated render to sheathing boards

Many of the boards mentioned in the table above can also be used as sheathing boards. Some manufactures such as RCM have different boards for different applications, while others only offer a cement board for sheathing application.  

These differences occur because some boards are tested for racking strength, some are tested for the use of thin coat render systems and some are tested for both. Using insulation on sheathing boards means an insulation material such as Mineral Wool has been installed onto a suitable sheathing board.

These systems are normally found installed on non-traditional substrates like metal frame or wooden frame buildings. The cement board is installed onto the non-traditional structure using the appropriate fixing method. Once the boards are secured, the insulation system can be applied to this substrate. The insulation boards are securely fixed to the sheathing board (in keeping with the manufacturer’s fixing pattern) using mechanical fixings and adhesive.

EWI-Pro has ETAG (European Technical Approval Guidelines) Certification for this method of installation. Once the insulation is secured, the beading can be applied onto the insulation. This includes the corner beads, stop beads, reveal beads, drip beads and verge trims. Once it is secured using a basecoat mixture, you are then ready to install the reinforcement layer.

When the walls have been basecoated and primed if required, they can be rendered using our decorative renders. The EWI-040 Silicone Silicate and the EWI-075 Silicone Renders are our most popular through-coloured renders and would work very well on a nice and level basecoat.

There you have it – how to apply render and insulation to render carrier boards!

Cavities in render carrier systems

Historically, ventilation voids were mandatory to get NHBC approval and a guarantee. Although this has been counter intuitive on many projects, the practice is common place. Since the Grenfell tragedy, a lot of focus has now been placed on the roll of these voids and whether they are safe.  At EWI Pro, we advocate a breathable insulation fixed directly to the substrate through the sheathing board, which eliminates risk of moisture.

If a stone wool insulation such as Rockwool is used, it is class A1 fire rated and permits the passage of vapour to escape.  For a build up such as this, a pullout test would be required to determine whether any extra fixings per m2 are required.

Some systems also include a provision for ventilation and/or drainage in front of the render carrier board in addition to the ventilation in front of the sheathing board. Again, this is counter intuitive as all the ventilation and air takes away from the thermal performance of the insulation and increases the risk of mould growth.

It is always important to liaise with your EWI provider as early as possible when designing these types of systems. EWI Pro can offer full technical support and can also assist with the specification of your render carrier projects.

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