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Applying Render to Cement Board

Applying Render to Cement Board

Cement board provides a solid, durable surface that can withstand the extreme weathering effects of wind, rain and snow. The most common two types of cement board available on the market are Permabase (from National Gypsum) and Knauf’s Aquapanels. The boards tend to be made up of Portland cement, aggregate and fiberglass mesh. This combination provides an impact resistant barrier, but the real beauty of these cement boards is that they don’t rot or swell when they come in to contact with water, so they survive a long time even when located on the exterior of the property.

Cement boards can be finished in numerous different ways, but one of the most effective ways to finish them is to apply render to them. Applying render to cement boards provides a fantastic looking finish, but also offers a further level of protection to the boards. So how do you go about it?

Applying render to cement board

The first thing that you need to do is to prime the cement board. We recommend priming the cement board with EWI-310 Universal Primer, because the presence of silicate in the primer allows the basecoat layer to key into it – ensuring a really strong adhesion between the cement board and basecoat layer.

EWI-310 Universal Primer comes in 20kg buckets and each bucket can cover between 60 and 80m2 depending on absorption of the cement board. The primer is applied to the cement board by a brush or roller and is red in colour allowing you to see where on the wall it has been applied. We recommend allowing this to dry completely before applying the next layer.

Applying the basecoat to the cement board

Applying the basecoat to the cement board is relatively easy – we recommend using either EWI-220 or EWI-225. The EWI-225 is slightly stronger than the EWI-220 so it depends on how solid you need this façade to be. Both the EWI-220 Basecoat Adhesive and the EWI-225 Premium adhesive come as a dry mix and therefore need to be mixed with water prior to application. Once the adhesive has been mixed with water and the grey putty has formed, it needs to be applied to the cement board with a notched trowel. We recommend using a 10mm notched trowel, this is the best size of trowel to apply a 6mm layer of adhesive to the cement board.

Once the adhesive has been applied it is time to embed the fibreglass mesh within it; it is this mesh that gives our render systems such flexibility. EWI-640 Reinforcement mesh comes in 50m2 rolls (1mm wide), and this needs to be applied in vertical strips within the adhesive. The strips of mesh need to overlap one another by 10cm at edges and this is needs to be sunk within the layer of adhesive – this can be done by using the flat edge of the notched trowel and drawing this up from the bottom of the wall – this 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 produce a very wet adhesive mix and apply this to the basecoat for the first layer of adhesive 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, you are ready to prime the wall ready for the application of the render top coat. The primer depends on the render used, but provided you match the primer with the render, then the principle is exactly the same. 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, with the 7kg bucket covering approximately 20m2 and the 21kg bucket cover 70m2 of wall.

Applying render to the cement board

It is time to apply the render to produce the finished façade! All the EWI-Pro renders are thin coat renders, therefore the thickness of the layer is determined by the size of the granulate in the render. For example EWI-040 Silicone Silicate render comes in 4 thicknesses, 1mm, 1.5mm 2mm and 3mm. The layer of the render should therefore be either 1mm, 1.5mm, 2mm or 3mm depending on the render used – it should be no thicker!

This is really important to remember, and will ensure the finish is good. We have heard of several reports of people applying the render to thick and not being able to achieve the required finish. With that in mind, apply the render to the cement board with a trowel and then pull off all the excess with the trowel. Once you have taken off any excess, leave the render for 5 mins and then using a plastic float, work the render in a circular motion to give the final finish.

There you have it – how to apply render to cement boards!

A recap of the materials required to render cement board

EWi-310 Universal Primer

EWI-220 Basecoat Adhesive / EWI-225 Premium Adhesive

EWI-640 Reinforcement Mesh

EWI-333 Silicone Silicate Primer

EWI-040 Silicone Silicate Render

Re-rendered property vs old render property

Looking to re-render a property?

If you are looking to re-render a wall then EWI Store can provide all the materials. The process is relatively simple, but the finish should be long lasting and certainly improve the look of the existing façade.

Do you need to remove the existing render?

To re-render a wall, the first thing you must do is check the quality of the existing render. If in any doubt about its strength then it must be removed before applying the new render coat. This is a painful but necessary process, since the new render needs to be installed on a stable substrate. If it is not removed prior to installing the new render, then sometime down the line cracks may appear or worse, your new coat of render will fall away from the wall.

If the existing render is okay in patches, then you have a choice – you can either remove the existing render so you are left with the bare wall or you can using a wall levelling compound to where the old render has come away from the wall. The wall levelling compound we provide can fill gaps of up to 50mm, so this can be used to build up to the thickness of the good render to give you a stable surface on which to apply the new render.

Following either of the methods above will give you a suitable surface on which to re-render. Once you have a strong, stable surface, the next thing to do is install the reinforcement layer. Using a notched trowel, you need to add a 6mm layer of EWI-220 Adhesive to the wall. Each 25kg bag of EWI-220 (once mixed with water to form the adhesive putty) should cover approximately 5m2 when 6mm thick is applied.

Reinforcing the reinforcement layer (or basecoat!)

Once applied, you then need to embed the fibreglass reinforcement mesh (EWI-640). This comes in rolls – 50m long by 1m wide. We recommend doing this in vertical stripes, and each stripe needs to overlap the last by 100mm. The easiest way to embed the mesh is to first cut a strip the length of the wall top to bottom then, starting at the top, gently push the mesh into the adhesive, so it stays in place. Once the mesh has been gently pushed in by hand, turn the notched trowel around so you are using the flat edge and drive this down from the top – this will draw adhesive through the holes in the mesh and embed it within the adhesive layer.

The aim is to get the mesh embedded within the middle of the adhesive layer – the mesh is really important as it provides strength and flexibility to the render system, ensuring it won’t crack.

Once you have embedded the mesh, stand back and admire your handiwork. If it is not completely flat, we recommend using a wet adhesive mix to smooth out the surface. Do this before the underlying adhesive has fully gone off to ensure it adheres properly. It is very important the reinforcement layer is flat prior to applying the render because the render layer is very flat.

Once you are happy with the reinforcement layer it is time to prepare the wall for the final render topcoat. The key to a nice finish when rendering is applying the render very thinly – if the render is 1.5mm, then the render layer should be 1.5mm in total. Likewise if the render finish is 1mm (size of aggregate in the render) then the render layer should be just 1mm in total.

The first thing to do prior to applying the render is to prime the wall first. If you are using a colour render finish, then normally the primer will need to be tinted as well – check that this is the case before starting the render work.

One 21kg bucket of primer will cover approximately 70m2 of wall, while the small 7kg bucket will do just over 20m2. The primer can be applied with either a brush or a roller and this needs to go off before applying the final top coat of render. It is important to match the primer type (e.g. acrylic or silicone) to the final render you are using.

Picking the Render

We offer several different types of render including water based renders (acrylic, silicone silicate and silicone) as well as mineral render which is concrete based.

The water based renders come ready mixed and tinted as per the colour selection. The mineral render is a grey colour and needs to be painted once it has been applied to the wall. We have discussed the major differences between the renders previously and you can read more about that by clicking on the link below

>>> What is the difference between the renders? <<<

Applying the render

The render is applied using a trowel as usual; ensure you take off any excess though, as the render layer is incredibly thin. Once the render is on the wall, the final finish is achieved using a plastic float.

Screen Shot 2016-06-27 at 10.36.03

What does render look like up close?

We offer several different types of render (including mineral, acrylic and Silicone Silicate) and they all look pretty similiar up close. however, each render is available in different grain sizes – 1mm, 1.5mm, 2mm and even 3mm (Silicone Silicate render). We often get asked what the render looks like, so we got the camera out and took some photos of the different grain sizes.

The pictures were taken 30cm away from the render, so obviously the further you get away from the wall the smoother the top-coat looks, but hopefully this gives you some idea of what you will achieve. It is also worth mentioning that all of the renders we sell at EWI Store are available in hundreds of different colours. You can pick you favourite colour by clicking on the link below:

>>> See our render colour charts <<<

Without further ado – please take a look at the different top-coat finishes:

Silicone Silicate – 1.0mm

1mm Silcone Silicate Render up close

Silicone Silicate – 1.5mm

1.5mm Silcone Silicate Render up close

Silicone Silicate – 2.0mm

2mm Silcone Silicate Render up close