How-To Build with Cob
Check out our How-To Make Cob Video
Check out our How-To Build with Cob Video
Cob is a natural building material made from clay, sand, straw, and water. Cob is an old English word meaning "lump," and it has been used for hundreds of years. To read more about cob, check out the Wikipedia site or visit the Cob Cottage Company.
Before you decide to use cob or another earthen based mortar to cover your bench, you need to make sure that you will be able to receive the expertise of a natural builder in completing a natural plaster. Every cob bench needs a good pair of boots (the foundation) and a good hat (the plaster and the roof). It is possible to build a cob bench without a roof, but you will need a very durable plaster that must be maintained. If you do not have access to a competent natural builder, if no one will be able to maintain the plaster (i.e. reapplying linseed oil every year or two), or you are not able to build a roof, it is HIGHLY recommended that you use a more conventional mortar such as cement in place of cob.
Cob is a mixture of clay, sand, straw, and water. Clay is the binder; sand gives the mixture strength; straw provides tensile strength (think "rebar"); and water helps activate the clay to hold the mixture together while building. Here are the basic steps to making cob. It is always recommended to have a competent natural builder on hand, but feel free to try it out! The good thing about a bench is that if you mess up it won't fall on your head :)
1) Sourcing your clay. Clay soil is abundant - typically if you dig a few inches below the topsoil you will reach clay in most places. Valleys and flatlands are best for clay soil because that is where water accumulates. You will have a harder time finding clay in a mountainous region. If you can't dig a hole close to the bench, then look up "fill dirt" on Craigslist. Typically people have huge amount of soil they need to get rid of - see if you can take a pick-up truck load off their hands. To ensure it is clay, add water to a handful of soil and see if you can roll it into a ball or twist it into a snake. If there is clay in the soil it should bind together. If your soil does not have much clay content, you can always add a couple bags of some clay mortar from the home improvement store.
2) Sourcing your sand. You want to use construction grade sand or river sand. You do NOT want to use fine beach sand - save the fine sand for the plaster. Rougher sand particles are better for making a strong cob mix.
3) Using the right ratios of clay to sand. It is very important to use the right ratio of clay to sand in your cob mix - if there is too much sand, it won't stick together and will dry flaky - if there is too much clay, it will shrink too much while drying and crack significantly. Every clay is different, and so its important to experiment before you start building the bench. For most of the clay I've used in California, we use 1.5 parts sand to 1 part clay. If the clay is very sticky, you may use up to 2 or 2.5 parts sand to 1 part clay. If the clay is not very sticky, a 1 to 1 ratio may be fine. Rarely will you add more clay then sand. Its best to use 5 gallon buckets to measure out the ratios (i.e. 1.5 buckets of sand to 1 bucket of clay). Make sure to use these ratios throughout your project.
3) Mixing your clay and sand together. Start off by dumping the clay on a 6'x8' tarp and sorting through it to pick out any large rocks, vegetation, or other things you don't want to step on. Throw those into an empty bucket or off to the side. Once the clay is sifted, add the sand. An easy way to mix cob is using a 6'x8' tarp and 4 volunteers per tarp. You can use the tarp, with two people on each side, to mix the material back and forth while adding water. Make sure volunteers walk backwards and pull on tarp rather than pushing. Don't add too much water at any one time - you want to make sure it gets evenly mixed. Volunteers can also jump in the cob and use their feet to "twist" the cob together. Once the cob is flattened, its time to tarp mix again. Remember, the end goal is to use the clay to bind as much of the sand together as possible.
4) After mixing and adding water the mixture should start to form a "burrito" when you flip it over on the tarp back and forth. Thats letting you know the mix is probably ready. Other tests you can do are the "ball test" - mix together a ball of cob and throw it up in the air. You should be able to catch the ball in your hand and have it maintain its shape. If it cracks a little, thats a good sign! That is letting you know that the mixture is binding well but there is as much sand as you can fit for the clay that you have.
5) The last step is to add straw. "Straw if for houses, hay is for horses." Make sure you are using yellow wheat straw - that is best. One straw bale will be more than adequate for the typical bench. Add a good handful of straw to the mixture and mix it up. You don't need that much straw for building a bench - a cob house would require more straw in the mixture.
6) Once the mix is ready, you can start adding it to your bench. Its nice to form cob blobs, small cob balls that workers can add to the bench. The cob mortar should be 1 to 2 inches throughout the entire bench. Ideally you have left room on your perimeter rocks to build up cob. You should also ideally leave a little room on the perimeter rocks to add plaster, too. While you are building up cob, make sure to push down on the cob to create a little foundation or ledge by which you can add more cob (don't just schmear it on to the bench - create that 1 to 2 inch thickness throughout). For the back of the bench it is especially crucial to create this little foundation so you can go all the way over. If you are having issues with covering the back of your bench with cob, make a mix that has more clay (1 to 1 sand:clay) or use a clay/straw mix, which is just straw covered with sloppy clay that has been soaking.
7) Don't worry about making your cob smooth - it should actually be rough or textured to help the plaster apply.
Check out our How-To Build with Cob Video
Cob is a natural building material made from clay, sand, straw, and water. Cob is an old English word meaning "lump," and it has been used for hundreds of years. To read more about cob, check out the Wikipedia site or visit the Cob Cottage Company.
Before you decide to use cob or another earthen based mortar to cover your bench, you need to make sure that you will be able to receive the expertise of a natural builder in completing a natural plaster. Every cob bench needs a good pair of boots (the foundation) and a good hat (the plaster and the roof). It is possible to build a cob bench without a roof, but you will need a very durable plaster that must be maintained. If you do not have access to a competent natural builder, if no one will be able to maintain the plaster (i.e. reapplying linseed oil every year or two), or you are not able to build a roof, it is HIGHLY recommended that you use a more conventional mortar such as cement in place of cob.
Cob is a mixture of clay, sand, straw, and water. Clay is the binder; sand gives the mixture strength; straw provides tensile strength (think "rebar"); and water helps activate the clay to hold the mixture together while building. Here are the basic steps to making cob. It is always recommended to have a competent natural builder on hand, but feel free to try it out! The good thing about a bench is that if you mess up it won't fall on your head :)
1) Sourcing your clay. Clay soil is abundant - typically if you dig a few inches below the topsoil you will reach clay in most places. Valleys and flatlands are best for clay soil because that is where water accumulates. You will have a harder time finding clay in a mountainous region. If you can't dig a hole close to the bench, then look up "fill dirt" on Craigslist. Typically people have huge amount of soil they need to get rid of - see if you can take a pick-up truck load off their hands. To ensure it is clay, add water to a handful of soil and see if you can roll it into a ball or twist it into a snake. If there is clay in the soil it should bind together. If your soil does not have much clay content, you can always add a couple bags of some clay mortar from the home improvement store.
2) Sourcing your sand. You want to use construction grade sand or river sand. You do NOT want to use fine beach sand - save the fine sand for the plaster. Rougher sand particles are better for making a strong cob mix.
3) Using the right ratios of clay to sand. It is very important to use the right ratio of clay to sand in your cob mix - if there is too much sand, it won't stick together and will dry flaky - if there is too much clay, it will shrink too much while drying and crack significantly. Every clay is different, and so its important to experiment before you start building the bench. For most of the clay I've used in California, we use 1.5 parts sand to 1 part clay. If the clay is very sticky, you may use up to 2 or 2.5 parts sand to 1 part clay. If the clay is not very sticky, a 1 to 1 ratio may be fine. Rarely will you add more clay then sand. Its best to use 5 gallon buckets to measure out the ratios (i.e. 1.5 buckets of sand to 1 bucket of clay). Make sure to use these ratios throughout your project.
3) Mixing your clay and sand together. Start off by dumping the clay on a 6'x8' tarp and sorting through it to pick out any large rocks, vegetation, or other things you don't want to step on. Throw those into an empty bucket or off to the side. Once the clay is sifted, add the sand. An easy way to mix cob is using a 6'x8' tarp and 4 volunteers per tarp. You can use the tarp, with two people on each side, to mix the material back and forth while adding water. Make sure volunteers walk backwards and pull on tarp rather than pushing. Don't add too much water at any one time - you want to make sure it gets evenly mixed. Volunteers can also jump in the cob and use their feet to "twist" the cob together. Once the cob is flattened, its time to tarp mix again. Remember, the end goal is to use the clay to bind as much of the sand together as possible.
4) After mixing and adding water the mixture should start to form a "burrito" when you flip it over on the tarp back and forth. Thats letting you know the mix is probably ready. Other tests you can do are the "ball test" - mix together a ball of cob and throw it up in the air. You should be able to catch the ball in your hand and have it maintain its shape. If it cracks a little, thats a good sign! That is letting you know that the mixture is binding well but there is as much sand as you can fit for the clay that you have.
5) The last step is to add straw. "Straw if for houses, hay is for horses." Make sure you are using yellow wheat straw - that is best. One straw bale will be more than adequate for the typical bench. Add a good handful of straw to the mixture and mix it up. You don't need that much straw for building a bench - a cob house would require more straw in the mixture.
6) Once the mix is ready, you can start adding it to your bench. Its nice to form cob blobs, small cob balls that workers can add to the bench. The cob mortar should be 1 to 2 inches throughout the entire bench. Ideally you have left room on your perimeter rocks to build up cob. You should also ideally leave a little room on the perimeter rocks to add plaster, too. While you are building up cob, make sure to push down on the cob to create a little foundation or ledge by which you can add more cob (don't just schmear it on to the bench - create that 1 to 2 inch thickness throughout). For the back of the bench it is especially crucial to create this little foundation so you can go all the way over. If you are having issues with covering the back of your bench with cob, make a mix that has more clay (1 to 1 sand:clay) or use a clay/straw mix, which is just straw covered with sloppy clay that has been soaking.
7) Don't worry about making your cob smooth - it should actually be rough or textured to help the plaster apply.
