Render
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In the process of making this building I have used both clay and lime renders. Sadly, my experience with clay render has been perplexing. In this page I will try to deal with some of the issues raised. Throughout the build I have sought information on the processes I was about to employ from people with experience and from the web, but I have yet to find an exhaustive website on clay render. I would love this to be a definitive guide that makes all clear to the reader. In reality I think I will only achieve the opposite. Everything is as clear to me as muddy water. If anyone has answers to the issues raised, please do contact me.
Let's begin with Clay and move on to Lime. There are more references to these matters in the blog.
• Tyre walls
• Straw bale walls
• I-beam roof
• Living roof
• Clay/Lime render
• Insulation
• Earthbags
Mud, mud, glorious mud. We have a lot of it in our garden
and I intended to use it.
In fact, like much of Bristol, beneath a thin layer of topsoil there is clay of a very high quality. Just right, I supposed for rendering with.
The process is this:
Mix clay with water to make a smooth, sticky, clay slip. The slip is used as a base coat on the straw bale walls to act as a keying layer.
To work out what kind of clay you have you mix the saturated clay (that's left after the slip has been drained away) 50/50 with some fresh cow poo, then add plasterers sand in different ratios and make some test patches.
(Testers as applied and two weeks later)
Our best ratio was found to be 1 clay to 1 poo to 5 sand. It set very hard without cracks and appeared perfect for rendering straw bales with.
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| A clay rendered wall in a very dry climate. Photo by cer!se. |
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Mixing We started out mixing by foot. This was fun but strenuous and slow. We found the best mix was obtained to loud Tango music. Next session I hired a render mixer. It's like a giant dough mixer. And quite pricey. You can't use a conventional cement mixer as it will just make balls of clay in the sand. Apparently you can use a cement mixer if you make it vertical. I couldn't think of an easy and safe way to do this. Applying The process of applying the clay render is quite a pleasant one. There is a farmyard aroma, but it is a very natural product and very straight forward to apply. I prefer clay render to lime a lot. Lime is damaging to the skin and damaging to any surfaces it comes in contact with. Clay render just wipes off and it's good for the skin. Once the roof was on and the bales had been left to compress for a few weeks we began rendering the exterior. Just about anybody can master the art of applying render in just a few minutes. It's like playing mud-pies. Why I used clay on the exterior Oh woe is me. I knew there was a risk in rendering the exterior with clay. I have a strong streak of iconoclasm mixed with a habit of tilting at windmills and an urge to experiment. It's my building, I insisted like a petulant child, I'll try something counter intuitive if I want. It was a gamble and it didn't pay off. I can't complain that much, can I. I am in favour of finding low impact building techniques and clay is low impact building material. I only hope I don't put people off from using it. Others have done it successfully and if you plan to do it, find out what they did right. The problems (See blog for more details) When I decided to render the exterior with clay it was suggested I try something called active silicate paint. The idea is that the paint chemically reacts with the clay and sand to create a tough crystalline waterproof layer that is also breathable. So I tried it and for some reason the reaction didn't occur. The paint people, Keim, claim it's my render mix that is at fault and this seems likely to be true for two reasons. 1, both Keim and Barbara Jones say that other have done this successfully and 2, When I dig in to the render it is powdery under the hard surface. Anyway the long and the short of it is that the paint isn't waterproof and the clay render becomes saturated when rained upon. Why did this happen? I wish I knew. I wish I had time to research it. Part of me would like to be an esoteric building practices researcher full time, the other half of me (my wife) sees the need for me to earn a living. Keim were not able to tell me. They suggested that too much water in the render when it is painted means the reaction doesn't happen. We tested the level of moisture in the clay render both interior and exterior and his meter said the level was too high for Keim paint. This might indicate that Keim paint just wont work on clay render. Clay render is always going to have a high-ish moisture content as one of its properties is the regulation of humidity in a building. A clay rendered bathroom mirror is said to demist very quickly because the walls drink up the water vapour from the air. What I'd do next time |
It is very easy to get lots of very good and consistent information about Lime online. I have put some links below. There isn't much info specifically about rendering straw with Lime, but it seems to me the same principles as rendering lath apply.
To be completed...