How one should go about applying render to OSB is a question that we get asked on a near-daily basis. The simple answer is that applying render directly onto the OSB is not an ideal situation - it’s important to remember that at the end of the day, OSB is wood. Nevertheless, there is a solution for rendering OSB, and we’re going to talk you through the best way you can do this!
Rendering OSB: Render Carrier Board
When rendering any substrate, especially when rendering OSB, consideration needs to be taken as to the suitability of the substrate for the materials that you are using. OSB is not completely waterproof and will inevitably absorb water from the basecoat, which can cause the wood to warp, eventually hindering structural integrity.To avoid this, the best solution is to use a render carrier board. However, the render carrier board needs to be attached to battens, rather than to the OSB directly. This is because the two separate fabrics (the RCB board and the OSB) need ventilation between them to allow moisture to escape, and to avoid a situation where water builds up between the two materials. An air gap between the two is therefore necessary for the reduction of condensation. The best course of action is to:
First, staple a breather membrane onto the OSB substrate - this will help prevent the build-up of condensation on the surface of the wood.
Next, install timber battens on top of the OSB (the positioning of the battens will depend upon the size of the render carrier board you are using).
Once these are in place, you can secure the render carrier boards onto the battens with wood screws.
The spacing of the boards will depend on the type of render carrier board you are using, but generally, you would leave 4-5mm between each board. After the boards are in place, the gaps between them need to be covered over by a render carrier board tape. We recommend a tape that is breathable with a self-adhesive membrane, so it will prevent water vapour from gathering in the gaps between boards.
Basecoating the Render Carrier Boards
Render carrier boards don’t require priming because they are specially designed for being rendered, so you can go straight into applying the basecoat with no problems. Although it is possible to use the 220 Basecoat Adhesive, we recommend using the 225 Premium Basecoat as a basecoat for extra strength and breathability. At this stage, the necessary beading required for the structure is sunk into the basecoat along with fibreglass mesh – remember to overlap each strip of fibreglass mesh by about 10-15cm.Allow the basecoat to set for a period of 24-48 hours, before priming the basecoat with a Render Primer. The type of primer you use will depend upon the type of render, for example, if you were to render using Silicone Render (we recommend this one for buildings that require a high level of breathability), you would need to use our EWI-333 Topcoat Primer. This can be painted on and then left to dry for 12 hours.
After all that essential preparation of the OSB and RCB boards, you are finally ready to render. Using our ready-to-use Silicone Render, apply it onto the substrate using a trowel. The render should go on at a thickness that matches the chosen grain size - for example, if you’ve chosen a 1.5mm grain size finish, then you should apply the render at 1.5mm thick.Once distributed across the surface, rub up the surface of the render with a plastic render float to bring out the texture and achieve a consistent finish, then leave to dry for 24-48 hours.
Struggling to chose a Silicone Render colour from our selection? Order a colour sample here.
Want to learn more? Come to one of the EWI Pro training centres! More information found in the video below.
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