How to Make Cob – The Tarp Method

There are many ways to make cob. One of the most common and regularly-practiced techniques in North America is called the “tarp method” of cob mixing.

This method can be done by yourself or with one other person. You will need a large tarp and 4 or 5 five gallon buckets.

1. Gather The Materials

Get all of your materials close to your building site. Sand, clay subsoil, straw, water, tarp, and buckets.

 

2. Place The Tarp

Lay out your tarp on a flat space. This is where you’ll be mixing. Remove any large sticks, rocks, or debris from underneath the tarp as these can hurt your feet to step on.

 

3. Measure Materials Onto The Tarp

Put your dry ingredients (clay subsoil and sand) in the middle of your tarp in a pile. Your ratio will always differ depending on where your materials come from. You will probably need to experiment with the first few batches to determine how much clay and sand are needed for a good mix.

Here are some common ratios:

Sand	Clay
2	1
2	2
3	1

 

4. Dry Mix The Materials

Now you want to mix the dry materials together on the tarp. Have each person grab two corners of the tarp and both people walk forward to the center of the mix, folding the tarp in half. The dry material should be together in the center of the tarp. Put the tarp back in its starting position and lay it out flat on the ground again. Go to the other end of the tarp and repeat the process of turning the dry materials over. Do this 3-4 times or until the dry materials are mixed thoroughly. Put the dry ingredients back to the center and lay the tarp flat again.

 

5. Add Some Water

Pile up your dry ingredients in the middle of your tarp and dig out a small crater. It will look like a tiny volcano. Add a little bit of water into the volcano crater. Its always best to add too little water than too much! It is a whole lot easier to add more water than to try and fix adding too much. You will have to first experiment to determine how much water you will need in your mix. There is not really a standard for how much water to add. You’ll figure out how much water to put in through experience. Remember not to add too much water though. It might be easier to mix, but it will not hold up as well when you start building and will slump sometimes. A wet mix of cob can also crack more as it dries. However, if you do add too much water you can either leave the batch to dry out in the sun, add more straw to soak up some excess water, or add more dry ingredients.

 

6. Mix With Your Feet

Use your feet to push the outsides of the dry material into the center of the crater filled with water. Once you cover the water, start stomping on the pile. You can do it by yourself or with others depending on how much room there is on the tarp. Twist your heels into the mixture for the best mixing. You can also jog or dance on top of it as a mixing technique. The goal is to make sure that all the dry materials are mixed together well and that all the clay and sand are smeared together thoroughly.

 

7. Flip And Repeat

Next, you will pull the corners of your tarp to fold the mix on top of itself again. Stomp the mix some more, and repeat this a few times until the mix flattens out like a pancake on the tarp. You can add some water to your mix if its hard to get mixed thoroughly together. Just add little bits at a time though. Eventually your whole mix should be forming into what some people call a “burrito” shape when you roll the tarp back and forth. Once it takes this shape you have a good indicator that your cob mix is almost done. At this point you are ready to add straw into the mixture.

 

8. Add The Straw, Mix, And Complete

Stomp on top of the burrito of cob mixture until its flattened again to prepare for adding the straw. Take some handfuls of straw and sprinkle it over the flattened mix. Again start to stomp the cob mixture until all of the straw has been covered and mashed with the cob. Use the tarp to gather the mix up and turn it over again. Stomp some more until flat. Add more straw and repeat the process. There is no exact amount of straw to use, but about 15-20% usually works well. Keep repeating the process until all of the straw is thoroughly distributed and mixed into the cob. You’ve just made cob!