Cob is a building material consisting of clay, sand, straw, water, and earth, similar to adobe.
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.
Cob is an ancient building material, that may have been used for construction since prehistoric times. Some of the oldest man-made structures in Afghanistan are composed of rammed earth and cob. Cobwork was used in the Maghreb and al-Andalus in the 11th and 12th centuries and was described in detail by Ibn Khaldun in the 14th century.
Cob structures can be found in a variety of climates across the globe; In the UK it is most strongly associated with counties of Devon and Cornwall in the West Country; the Vale of Glamorgan and Gower Peninsula in Wales; Donegal Bay in Ulster and Munster, South-West Ireland; and Finisterre in Brittany where many homes have survived over 500 years and are still inhabited.
Many old cob buildings can be found in Africa, the Middle East, Wales, Devon, Cornwall, Brittany and some parts of the eastern United States. A number of cob cottages survive from mid-19th century New Zealand.
Traditionally, English cob was made by mixing the clay-based subsoil with sand, straw and water using oxen to trample it. 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.
The walls of a cob house were generally about 24 inches 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.