Cob is a natural material that consists of clay, sand, and straw. It’s sort of like building with play-dough. It is easy to work with while wet. When it dries it is similar to a concrete that can be broken down later with water. The recipe we have been using with our project is:

3 parts sand
2 parts clay
10-20 handfuls straw
Water until wet

These ingredients are mixed thoroughly. Everything on the wall to this date (Friday, May 16, 2008) has been mixed with our feet. Some people use a mortar mixer. Some people use a Bobcat uniloader or tractor to drive over the cob on the ground and mash it up. Cement mixers don’t work as well, because they don’t mash the cob, they just turn it over. I have, however, talked to a guy who is using one. He just has to mix it especially wet and then let it dry for a day or so.

Once the cob is mixed, the construction process is very simple. It is lifted to the wall and built directly onto the previous layer. Sometimes I use a pitchfork to lift the cob up. Sometimes I just grab a chunk. When the wall was lower, I was sometimes able to lift the mixing tarp directly onto the wall. Once the cob is on the wall, I use my hands to push it down and make what I think a wall should look like. Sometimes if it’s not too windy, I get up on the wall and use my feet to smooth out a newly placed cob mix. It’s a good idea to make the top surface uneven, so that the next layer has something to grip. It’s also a good idea to use your thumbs or a stick to make holes in the fresh cob. This also gives the next cob something to grip and allows more even drying.

Cob’s History

I don’t know as much about this as I should. Basically cob has been around for hundreds (thousands?) of years. It was popular in Europe several centuries ago. It has been built in many places internationally. There are cob buildings in the Middle East that are 6-8 stories tall that have been standing for centuries. If cob is given a good foundation and a good roof, there are few climates where it is not a suitable building method.

The word “cob” comes from a small unit of cob. A “cob” was a round ball about the size of a large softball. The “cob” was then easily thrown or carried to the wall.

What prevents the cob from getting wet?

Cob is amazingly resilient. Most people would be amazed what it can withstand. That said, prolonged exposure to moisture can be very harmful to cob. During construction, we use tarps to keep off rain and moisture. Once the project is done, we’ll use a lime based plaster (similar to stucco). The plaster will be applied to both sides of the wall, both for looks and to weatherproof the cob.

What type of roof and foundation will be used?

For the foundation, we dug a 3 ½ foot trench with shovels. At the bottom of this we placed perforated drain tile, which we covered with gravel. This is to prevent ground water from freezing under the wall and shifting the cob. On top of the tamped gravel trench, we mortared together chunks of concrete and stones to make a stem wall. We got this concrete rubble from a farmer down the road who was tearing up an old garage foundation. He gave it to us for free. The stem wall is about two feet high. Its purpose is to give a solid foundation for the cob above it. It is also important because it reduces the cob’s contact with groundwater.

There are many roofs that work with cob. We will probably use conventional wood rafters with tin for simplicity and cost. Some people have been more ambitious—using methods like living roofing (sod and prairie grass), thatch, or tile.

Why use cob and not conventional building techniques?

It is respectful to the environment. Our conventional methods of building are inefficient and can have negative effects on the environment. I have heard some alarming statistics regarding the construction industry. I don’t know what to believe sometimes, but at any rate, conventional construction significantly contributes to the dramatic rise in carbon dioxide we are experiencing. Most materials are transported large distances to the site of construction. Cob is very different in that it can be taken directly from the site of construction. Everything you see at this cob project has come from within 30 miles. Most of it was taken from the site itself.

It is very practical. Cob uses trends that are already at work around us. To explain—cob uses energy from the sun to heat the space inside. Its high thermal mass means that it will still have heat from the summer/day into the winter/night, and cold from the winter/night into the summer/day. It uses the properties of clay and sand, some of the most basic materials we have, to form a wall that can withstand earthquakes and fires. Cob allows a flowing form that is pleasing to the eye and creates a pleasant place to live.

There are other reasons to build with cob that I won’t go in to now for the sake of brevity.

What are some problems with cob?

Cob takes a lot of work. The foot mixing method that we have been using takes a lot of energy and time. It is a ton of fun when the weather is nice, but it is sometimes hard to stay focused over the course of the working season. One of the reasons why I have designed this site is to find inspiration and support in order to spend more time working. I am also in the process of finding a Bobcat uniloader to expedite the mixing. I have heard that this would speed up the process significantly (5-10 times faster).

It is a neglected practice. I cannot honestly understand why cob was abandoned in favor of wood. It was probably due to the aforementioned problem of time. We are at a time now where it is growing in popularity, but it is still a practice that few understand and fewer have perfected. This can be an obstacle to people interested in trying cob. Fortunately, cob is also very exciting for this very reason. It does not require a professional builder, and once people understand this fact there can be great progress and satisfaction.