Cob – The natural, recyclable building material that’s a “loan from the earth.”

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Cob house
18th century UK home made from cob.

The ideal building material would be ‘borrowed’ from the environment and replaced after use. There would be little or no processing of the raw material and all the energy inputs would be directly, or indirectly, from the sun. This ideal material would also be cheap and would perform well thermally and acoustically. If used carefully, mud or cob come close to this ideal.

Cobcobb or clom (in Wales) is a natural building material made from subsoil, water, some kind of fibrous organic material (typically straw in Europe, nuts in the US), and sometimes lime. Subsoil is the denser, darker, older, wetter materials consisting of, usually, sand, silt, and clay that lies beneath the surface. It has the ability to be sculpted when mixed with water and then dries hard, just like cement. It is used to make homes. It is very similar to adobe and is one of the most common building materials on the planet.

Cob is fireproof, resistant to seismic activity, and inexpensive. It can be used to create artistic, sculptural forms and has been revived in recent years by the natural building and sustainability movements.

In technical building and engineering documents such as the Uniform Building Code, cob may be referred to as an “unburned clay masonry” when used in a structural context. It might also be referred to as an “aggregate” in non-structural contexts, such as a “clay and sand aggregate” or more simply an “organic aggregate,” such as where the cob is an insulating filler between post and beam construction.

 Cob is an English term from around 1600 for an ancient building material that has been used for building since prehistoric times. The etymology of cob and cobbing is unclear but in several tenses means to beat or strike which is how cob material is applied to a wall.

Cob material is known by many names including adobe, lump clay, puddled clay, chalk mud, wichert, clay daubins, swish (African), torchis (French), bauge (French), bousille (French mud with moss), cat and clay.

 

Traditionally, English cob was made by mixing the clay-based subsoil with sand, straw and water using oxen to trample it. English soils contain varying amounts of chalk and cob made with significant amounts of chalk are called chalk cob or wychert. The earthen mixture was then ladled onto a stone foundation in courses and trodden onto the wall by workers in a process known as cobbing. The construction would progress according to the time required for the prior course to dry. After drying, the walls would be trimmed and the next course built, with lintels for later openings such as doors and windows being placed as the wall takes shape.

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A cob house in the Pacific Northwest.

The walls of a cob house are generally about 24 inches (61 cm) thick, and windows were correspondingly deep-set, giving the homes a characteristic internal appearance. The thick walls provided excellent thermal mass which was easy to keep warm in winter and cool in summer. Walls with a high thermal mass value act as a thermal buffer inside the home. The material has a long life-span even in rainy climates, provided a tall foundation and large roof overhang are present.

There is a great guide to building a cob home on this Australian website.