Clay plaster sprayed on our straw bale house

 In Blog, Student Post

This week’s has been all about clay plaster, which involves a lot more than what you see. We started with the blank canvas of our straw bale walls (dressed and relatively even) which we then covered with a layer of slip – a mix of clay from a local excavation, sharp sand, and water to make a thick liquid. We used a compressor to spray the mix on which saved a lot of time but this could also be done by hand.

clay plaster sprayed onto the straw bales

It’s important that the clay mix gets well into the straw so once the walls were coated we went around jabbing into the bailes with our hands checking this had been done. At the same time we were checking for any holes or weak points in the bailes which we filled with more straw and a straw-clay mix. This straw clay mix is technically a plaster, the components/ ratios of which will change depending on the stage of the build, however the basic elements stay the same (in any plaster not just our version). You need a glue, a structural filler, water and fiber (additional additives are an option but not necessary.) For our mix these elements are clay, sharp sand (rather than rounded beach sand), water and straw.

working clay plaster into the straw bale

A Fibre Coat

Once all the gaps were filled we started the matrix coat which involved taking hand sized amounts of straw clay and working it into the walls, each overlapping the last. It’s important here that the first layer of clay mix is wet so we had to routinely wet down the walls with a fine hose. The matrix layer connects with the bale to create a strong mechanical fixing- the straw and a chemical fix with the clay. The matrix increases the bond between the straw bales and the wood fiber section of the wall and fills any gaps between the two materials. Due to the nature of the material the walls were still pretty uneven so the ‘daubing out’ stage involves using a slightly dryer straw clay mix to fill any dips, constantly checking with two long levels to ensure the wall is flat (But not smooth.)

clay plaster is worked into the straw bale

The walls that are partially exposed in the outside kitchen area will get a 20mm layer of lime rendered plaster followed by a lime wash. Similarly on the inside walls the next coat will be a 20mm finer version of the plaster followed by a 5mm top coat. 

Clay plaster recipe

clay plaster recipe

We began to do tests to check the ratio of the finer plaster, which depends on the natural materials available. In small quantities we mixed different ratios of clay: sand with water ranging from 1:1 to 1:9. The final results will be clearer when they are dry but we could see clearly that there were tiny air gaps that are a potential weakness. These gaps were due to a lack of finer aggregate in the mix. It’s important that the sand is well graded – meaning containing a range of particle sizes. To combat this we did some further tests adding controlled quantities of the sub soil from our foundations which contains high quantities of silt which would fill the gaps. We will check the results next week.

All of our clay has been purchased from HG Mathews – we really enjoy using their powdered clay!


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clay plaster in the strawbale house