Cob is a building material that is made from a mixture of sand, straw and clay. The materials are mixed wet, by foot or with a machine (tractor or mortar mixer). The word “Cob” comes from an old English word meaning “lump” or “loaf.” We often form our cob mixture into loaf-shaped blobs for ease of transport.
The wet cob mixture is used to build thick earth walls; the building technique is very similar to sculpting with modeling clay. Because cob building requires no forms, we can build our walls into any shape we choose. Curves, niches, arched windows and built-in furniture are common features in cob buildings.
Cob building requires no cement, no expensive tools or materials and is a “people friendly” way of building. Often we can find the bulk of the materials on or nearby the building site.
Once the material is dry, cob is incredibly strong. Although there is little conclusive research about how cob buildings perform in earthquakes, there is much empirical evidence that they would survive very well. Many cob buildings in New Zealand have survived major earthquakes. The country of Yemen in the seismically active Persian Gulf has many 10 story cob buildings that have survived for over 500 years.
One of the most commonly asked questions about cob is “What happens when it rains?” The cob tradition originates in England and Wales, no dry countries by any standard. These countries have whole villages of cob buildings, many of them hundreds of years old. The secret is the same as that of any building style: A good roof and a good foundation are the keys to survival in wet climates.
Cob works well in all but the coldest climates. If it gets too cold, it is easy to add extra insulation on the outside of a cob building. Cob houses benefit greatly from good passive solar design.
For all these reasons and more, cob is an almost ideal building material. It is easy to learn, inexpensive, beautiful, healthy, comfortable and the materials can be found almost anywhere. It is no wonder that about half the world population lives in an earthen house.
In the United States, cob is experiencing growing popularity. Besides the above mentioned advantages over conventional building methods, cob can offer people a way out of the crazy real-estate market, an unhealthy lifestyle and a job with no joy. Many people who never considered themselves builders have created a cob house for themselves. There are now hundreds of cob buildings in the USA, with a concentration in Oregon and California.
Cob is useful for much more than houses, too. We teach how to make cob ovens, fire places, earthen plasters and cob floors. Garden walls are also an extremely popular application of cob. Cob can easily be integrated with conventional houses and used in a “natural renovation”.