The original render on the bottom of the house is cracked and i wanted to repair these defects. Ive been given some silicone render quotes and wondered if anyone has used silicone render on BISF house. What are the pros and cons? or should I just stick to the traditional cement render?
David B 30 November 2023 4:34 PM
@awie23 Hi awie23, I'm a little late replying to your comment and I'm not sure if you've had the work done already, but here goes.
Firstly, I'd need to see photos showing the extent of the cracking that you refer to, before commenting specifically on your case.
But for reference, remember that the rendered ground floor walls of a BISF house are self supporting and not supported by a brick backing, so care needs to be taken when repairing this type of wall.
Unlike render which is usually applied directly onto a brick wall, with BISF houses, the cement render was applied onto a thin bladed support mesh, which when cured and dried, formed a rendered wall of around 3"-4" thick. Over time, exterior cracks in the render can allow water ingress and corrosion to the steel mesh support, causing weakening of the wall structure. This tends to be more prevalent when multiple rows of thin horizontal cracks appear in the render.
If the wall is badly cracked, there is a risk of further damage if the installer attempts to chip away or remove any large areas of damaged render. So extreme caution is advised.
If I were simply attempting to patch parts of a BISF ground floor exterior rendered wall, I would use cement render.
If I were to have my ground floor walls completely removed and replaced, I would have an insulated render system installed along with silicone based render.
As for patching cement render with silicone, the answer tends to vary. Most builders and installers say yes, it's fine to use acrylic or silicone based render on top of existing cement render, providing the surface is sound and well prepared prior to use. WEBER (a major producer of silicone render), on the other hand, tend to discourage this.
What I will say though, is silicone and acrylic renders should not be installed during cold weather, and it can fail to cure correctly. Months to avoid are typically October through to March in the UK.
Regards for now
David