The cob technique of earth construction use a moderately wetted earth, which is built up in freestanding lifts without the use of formwork. In this form of construction the earth is moderately wetted, and may be mixed with chaff or straw. This is treaded and kneaded until it is a soft, cohesive, plastic mass. The mixed earth is passed on to the wall builder, thrown up in large balls, or using forks. This mixed earth is then either placed or forcefully thrown directly onto the wall.
Cob is the earth building technique that is most commonly identified in the southwest United Kingdom, (Cornwall, Devon, Somerset, Dorset and Hampshire), although historic buildings in the East Midlands, Solway Plain, Scotland and documentary evidence for non-load bearing walls on the Yorkshire Wolds attest to a much wider distribution prior to industrialisation.
Wall construction occurs in lifts, (often of a height equal to that between the hand and the elbow), and each lift pared down with a flat backed spade to form a straight face. Each lift is left to dry prior to commencing the next vertical lift. Techniques of construction comprising placed earth are evident both with and without surface renders and plasters, dependent on local practice and custom.
The various regional forms in the United Kingdom use a stone plinth for a foundation course and load-bearing and non-load bearing walls rely on a wide thatched roof to cast rainwater away from the wall body.
More information on Cob building:
DEBA – the Devon Earth Building Association