Cob Mixing and Application Basics – “Cob Cottage Company” Natural Building Apprenticeship at Spirit Pine Sanctuary

The approximate area of the house we are constructing is 550 sq feet. We have 20 days and 15 not-so-experienced people to get the walls up and to put a roof on it (well, at least set up the ridge beam). Will we make it?

First few days are spent on getting familiar with the cob mixing process. In theory, it’s pretty simple.

  1. We place the dry materials (soil, clay and sand) on a 8 x 10 ft tarp and roll it by pulling on the corners
  2. Then we add water to the mixture and roll it again
  3. We stomp the mixture with our feet until it’s evenly moist and mushy. “Mud dancing”, we call it.
  4. Final step is to add straw, roll it again and dance on it until all straw is integrated

In practice it turns out to be pretty simple as well. If the mix comes out too wet, add a bit more soil and straw. If it’s crumbly, add clay. The material is very forgiving, and there really isn’t “the right” way to mix it. As long as all the materials are integrated into a malleable mix, it’s ready to be applied.

This is our process.

First we take soil samples to create some test cob bricks. Will they crack or be strong like a rock?
We start by taking soil samples to make test blocks. Will they be rock-solid or crack?

Mining for cob :) More earth samples from various areas to experiment with clay content.
Mining for cob 🙂 More soil samples from various areas within the property to experiment with clay content.
Earth that we use for building our home is also home to many creatures. This one is extra-cute!
Soil that we use for building our homes is also home to many creatures. This one is extra-cute!
Our cob recipe is 1 part local soil, 1 part sand and 1/4 part clay (high-clay soil). We place it all on the tarp and gradually add water.
We picked the soil source, now we mix a batch of cob. Our recipe is 1 part soil, 1 part sand and 1/4 part clay (high-clay soil). We place it all on the tarp and gradually add water.
Once soil, clay, sand and water are well mixed, we add straw to create tensile strength within the structure.
When the soil, sand and water are well mixed, we add straw to give the future structure tensile strength.
Sometimes we make cob balls for ease of transporting them onto the wall.
Hm…I think this batch is ready…
Finished cob is delivered to the wall in wheelbarrows.
Finished cob is delivered to the wall in wheelbarrows.
When we rest, we play fetch with Wednesday, she is the sweetest dog in the world.
Time for a break and a game of “fetch” with Wednesday, she is the sweetest dog in the world.
Before putting cob in the rocks, we pre-wet the surface with "slip" - a thin water and clay mix. It helps the cob stick to the stone.
Ok, back to work. Before putting cob in the rocks, we pre-wet the surface with “slip” – a thin mixture of water and clay. It helps cob to stick to stone.
Then we apply cob directly to the stone foundation to begin the wall!
Then cob is applied directly on top
First layer of cob is laid out on the foundation and we are ready to do the rest of the wall.
First layer of cob is laid out on the foundation and we are ready to do the rest of the wall.
Natural born cobbers.
Natural born cobbers.
To keep the walls straight (also known as 'plumb') we continuously check them with a level.
To keep the walls straight (also known as ‘plumb’) we continuously check the slant with a level.
Any excess cob is cut off with a saw or machette. The outside wall should lean in at a 5% angle, while the inside wall is kept plumb.
Any excess cob is cut off with a saw or a machette. The outside wall should lean in at a 5% angle, while the inside wall is kept plumb.
To help the wall dry on the inside and to integrate the layers of cob better, we create these spines and ribs prior to leaving the site for the day.
To help the wall dry on the inside and to integrate the layers of cob better, we create these “spines and ribs”.
Windows. Wall. Spines and ribs.
Windows.Buckets. Spines and ribs. Oh and some holes to help the air get inside the wall to dry it.
The wall grows about a foot per day.
The wall grows about a foot per day.
Our glorious structure... :)
Our glorious structure to be… 🙂

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