COB is an alternative building material that has been around for centuries. "COB" is an English term meaning mud building. It was started in England around the 13th century from other types of earth building techniques like adobe, sod, rammed earth, straw-clay, and wattle-and-daub just to name a few. COB is composed of sand, straw, water and soil or earth containing high amounts of clay. These materials are mixed well, usually on a tarp using dancing people's feet to do the mixing. Then you grad a handful and slop it on. As the layers dry more is added. You can make almost anything with it: benches; houses, sheds, sculpture, only your imagination is the limit.
HOW DOES COB WORK?
Every component of COB serves a very important purpose to make it such a strong and durable building material. Sand has rough, sharp angles that want to stack on itself. Clay makes it stick together. Water mixes these together and gives the clay it's sticky-ness. Straw binds more sand and clay together. Straw is also used to bind layers of COB together.
WHO SHOULD USE COB?
People that want to work with their hands, include their friends and family, and have a lot of fun should use COB! People who care about the environment and don't want to make a huge impact when building something. People that want or have to save money on building materials and have the time to do it themselves; there is no expertise necessary.
WHY USE COB?
COB is FUN, inexpensive, and can be sculpted into any desired shape or structure. COB does not burn, or rot It is impervious to insects, and stable in earthquakes. COB is a thermal mass, which means it absorbs heat during the day, holds it, then during the night it releases the heat slowly and evenly to the interior . This will reduce your heating bill. You can have a COB party with music, dancing, food and of course drinks. Your family and friends can help with the labor, while having a great time, and make your structure more meaningful at the same time.
IS COB AN APPROPRIATE TECHNOLOGY?
The following are criteria for an Appropriate Technology (AT) according to Mike Manetas, a now retired HSU engineering lecturer :
1. Simple
2. Local Materials, local labor, etc.
3. Minimal Impact of environment, culture, etc.
4. Cost Effective. Affordable.
5. End Use. What is going to happen when you are done with it? Is it toxic to anything?
The following are reasons why COB is appropriate and correspond to the above criteria:
1. COB is simple. All you need is clay, sand, straw, water and people. No expertise is necessary.
2. All the materials should be easy to get locally and if you can’t then COB probably isn’t best for that area.
3. COB has a very low impact on the environment. On a small scale, sand harvested from a river bed or the beach will not make an impact. Clay in earth is just about everywhere and is not desired in high concentrations for growing most plants, thus is an unwanted material. That means there will many people who would like to get rid of it free of change. Straw is a waste material produced from farms. It's primary purpose is to line horse stalls. There is a great excess. In the past the excess used to be burned, which is very bad for the environment, but now, thanks to environmental regulation it goes to landfills, which is also bad for the environment. Drinkable, potable water is not necessary for cob construction. Dish water, bath water, and water from cooking can also be used.
4. COB is very cost effective and affordable. All the materials should be free or very inexpensive. The only cost is gas for transportation of materials and process materials like tarps, shovel, trowel, level, etc, which you may have already. Labor is another cost, but if you were to do it with friends and family, labor would be free as well.
5. The end use of COB is not detrimental to the environment. When COB is no longer wanted, it can be demolished with sledge hammers and hosed down into a sandy, clay earth. The straw will decompose. You would want to put this earth somewhere where you wouldn’t want to grow plants because those concentrations of clay and sand are not desirable for growing most plants.
PROBLEM/SOLUTION:
There are some problems that you might run into. But don't worry, there is always a solution.
Weather: COB needs to dry so if one is in a place that rains a lot, one should build a temporary or permanent overhang. Just laying a tarp over it will not allow it to dry. You could also build COB during the dry season.
Labor: Have a COB party and invite all of your friends and family. Provide food and choice drinks and I’m sure many will come. Have music and everyone can dance while mixing COB. This will make your COB more meaningful and personal.
Flooding: It would be wise to chose a site away from any know areas that flood during certain times of the year. Another solution would be to put the structure on raisers.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Dancing while mixing COB is fun for all!
Separating the cob from the tarp - getting sticky
Add Straw and mix well
Form into hand sized balls. The Cob at this point should have a dough-like consistency.
You want to make a foundation to raise the cob structure above ground so that it does not get wet.
Stack balls on top of each other to make a vertical wall.
Use a stick and drive it from the top layer of COB to the layer under it. This will bind the two layers together and make the structure stronger.
If you want to make a window in whatever you are making, then build a frame and cob all around it.
Or you can sculpt your cob around an opening to make a window.
For information about a finish that will make your cob wall or structure smooth and uniform, refer to the later part of this slide show document that shows the process of making a cob bench from start to finish.
Building with cob is very labor intensive and requires many hours and/or people. Nonetheless, building with cob is fun and a great way to make friends and make friendships stronger; a great bonding experience.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Benches: There is an example of a cob bench in two public locations here in Arcata, CA. One on the corner of F street and 4th street in a little community garden. This was a class project of mine for another Engineering course at Humboldt State University in Fall 2003.
Click here for a document that has a slide show of the process from start to finish of this cob bench above. This also talks about a finish you can put on your cob structure that will make it look smoother.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
There is also a cob bench at the Campus Center for Appropriate Technology (CCAT). CCAT is on the HSU campus located in the Buck House. It is open to the public during reasonable hours.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Here are some other benches for some ideas.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Tile adds a very unique touch.
You can make cob into just about anything. Your imagination is the limit.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
OTHER APPLICATIONS:
COB can be used to make many different things (benches, sculptures, structures, houses, etc) and the limit is your imagination. You can build a bench, shed, chicken coop, wall, cottage, house, sauna, just about anything. Here are some links to websites that have other applications.
-Old
St. Thomas Anglican Church in Shanty Bay, Ontario. Built with cob in 1839
-Very old
Ancient cob building in Devon, England. The date plate
reads 1539.
-Big Houses
-Small
Hot Cold
-Insides can be very creative.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Patrick McAuley (In fulfillment of Engineering 115: Intro to Environmental Engineering and Science)
DATE
last updated: November 2003
