A large part of the strawbale construction of our house is the natural plaster system that closes in the bales while still allowing a flow of air to pass through, supporting a breathable wall system that is totally weather proof. I am not going to pretend that I can write a whole how-to guide here on this post, but instead I hope to give a most complete account of what we did to our walls, and hope that it is helpful to anyone interested in taking on natural plastering, or it is interesting to anyone just curious. There are many books, websites, people of knowledge, and workshops out there that can give accurate (and sometimes conflicting) information, and we have taken these steps available to us for what makes sense for our situation.
We have been blessed to have with us in our community an experienced natural builder and good friend, Tracy Calvert. She has been integral in directing our experience, and helping out with the attaining of supplies and tools. She is especially experienced in the art of natural plaster and it’s many layers of applications. Our first layer was covering the straw bales with clay slip, which is essentially like splattering the straw with thick chocolate milk. Messy. We obtained the clay from a pottery studio on Mayne Island, it is the off cut pieces from the creations of the studio which were destined to be sent to the landfill. Some studios take the effort to reconstitute these wastes back into usable clay, but it is an arduous task to reproduce good quality clay. Many studios prefer to consider these bits unusable, and so having them picked up by natural plasterers is a positive move for everyone. After soaking the clay, we mixed it up with a heavy duty paint mixer mounted on a drill, until it was thick and smooth. My method for getting it on the walls was to splash it on with one hand, holding a bucket below to capture what didn’t stick while rubbing it into the fibre of the straw. Whatever technique is used to get the slip on the wall is a good one, there really isn’t any wrong way. I covered every visible straw bit of the house, inside and out, turning it a lovely shade of dark reddish-pink. It then spent a whole winter like this.
Layer number two is a cob mixture of clay, sand and straw mixed up in batches by foot and pushed on the walls by hand. We got local clay from a farm down the road who was excavating a pond, pit-run sand from a local quarry, and the shavings of straw from when we trimmed the bales to make the surface even. A good test for determining the quality of the clay is to do a jar test- put some clay in a jar with water and shake it up, then let it settle. This will show the ratio of silt to clay, and whether or not there may be too much silt in the clay. Ideally, the less silt, the better. We built a soaker pit from leftover bales draped with a solid tarp, and kept it filled with softened clay ready for using. After spraying the walls to get the slip wet, we pushed on the layer of plaster, spreading it about a half inch thick. We started in early spring with a birthday party for our 4 year old, inviting all his friends and their families- really, this layer isn’t very difficult to master and it’s lots of fun to do with friends. I had home school plaster parties, ladies’ night plaster parties, and afternoons with a few friends and their kids playing nearby. The inside and out was finally done by the fall.
I was able to get started on the third layer in April. Tracy came for a morning and went over the variation of the cob mix- this time the ratio of clay to sand was 1:2, with one and a half buckets of finely sifted straw. (I used a certain bucket for measuring, and went through a few mixes of experimenting with ratios before finally getting to the mix that cracked the least.) The sand we used was a finer, washed sand. This layer is applied with a trowel, and encompasses the task of sealing in the flaps of burlap that connect the bale walls to the timber frame. (Since clay doesn’t stick to wood very well and shrinks when it dries, a strip of burlap was stapled to the backs of the places where straw meets wood, and so the clay doesn’t move away from the wood when it dries.) The burlap has to be rightly stuck down, so we brushed it with more clay slip until it was nicely saturated, then we rubbed it in roughly with plaster before applying a smoothly troweled layer of plaster. We also took the time in this layer to shape those beautiful big curves of our windows, sometimes building up the flares with extra plaster until the shape of the curve was formed to our liking. This is also the layer into which any sculptural work can be done, which we all know is the best part of natural plaster and cob. Nana enjoyed contributing her artistic sculptural skills around the house.
I was much more careful about who I invited to help with this layer- I mostly took it on myself (although Taeven and Cedar both took on their own ways of plastering with the trowel so they could help out). I had the help of a few friends who are artists (good hand-eye co-ordination) and who helped with the previous layer, so they had an idea of what the result was to be and they came many times to help, building up their experience of working with the plaster. My own abilities changed so much with this layer as I worked around the house, that I have re-plastered much of the first area I started on, seeing flat spots on window curves and too much cracking in other places.
We also covered some of our interior walls with plaster. In the mud room, we built a stud frame wall, then stapled expanded metal lath to the sides, filling in the middle with straw. The plaster goes straight on the metal, in two layers to avoid any movement of the mesh beneath. The final layer of plaster is yet to be applied, and will undoubtedly require a whole new lesson in natural plaster. The outside will be a mix of lime, sand, and colour pigment, the inside coat uses finely powdered cayolin clay as its base. We fine-tune the quality of our trowels with Japanese trowels. There is more technique involved in applying the plaster to avoid lines showing up… I am looking forwards to working with Tracy again to learn this new skill level of natural plastering. The journey of using natural plaster on our walls has been long and enjoyable, I have massaged every inch of our house with friends, conversation, children, family, quiet musings, music, meditation, in the sun and the rain. I have mixed every patch of plaster with the energy of my feet and legs, and with the soles of friends.