Cob is a building material generally made out of clay, earth and straw or hay. The straw within the cob prevents the mixture from falling apart as it dries and ages. Cob has many uses however is generally used for the building of houses and traditional wood fired ovens.
The term cob comes from the Old English word for “lump”. It is used with reference to mounds and lumps of soil rich in clay, which after extraction is mixed with straw and used to build walls and houses. Until coal and oil made transporting materials economical, and cement and bricks became the commonplace building materials, cob was the best choice for construction, since it was the only option available to people living in areas where timber was hard to get. However, the usage and history of cob dates back to prehistoric times. Some of the oldest structures known to man were made of soil acquired by scraping the topsoil and accessing the clay below. Ibn Khaldun, the Arab Historiographer and historian, described cob, and its related works, in detail in the fourteenth century. Today, the UK is probably the largest continuous area of land where cob buildings are still found and lived in. The concentration lies mostly in Devon and Cornwall in the West, Glamorgan and Gower Peninsula, South West Ireland, Finisterre in Brittany and a few other places. Africa, The Middle East, and even the United States have houses made of cob.
The traditional mix of cob had clay acquired after scraping away the subsoil, straw, and water. It was trampled under oxen feet, and used in building earthen walls. The mix, after preparation, was laid gently onto a foundation made of stone intermittently, a process defined as cobbing. The basic principle when it comes to building with cob is to let each layer dry in its own time. A new layer can only be caked on after the previous one is completely dry. The walls, or he respective structure being built, can then be trimmed, and openings for doors and windows can be made as the structure progresses.
The thermal performance of cob depends on the place where you are living. While it may be sufficient in some places to light a small fire in the morning to keep the place warm throughout the day, others that are chillier might need a little more effort, since the heat gains might be lost quickly. In order to keep cob houses warmer in colder areas, another additional wall may be built inside the house in order to retain the heat, whereas better insulation techniques may be used on the outside of the wall.
Cob is highly resistant to the brunt of the weather. It is porous, and can thus stand wind and rain. However, since it is basically made out of earthen materials, it is best not to take chances. Thus, cob materials should be protected by the "Boots and Cap" strategy.
Cob is commonly used as a building material for the construction of traditional ovens. Cob is readily available, cheap, and an easy material to teach inexperienced individuals to build with. These ovens are thus very common among DIY builders and within poorer communities.