cob

Cob is a traditional earthen building material used in parts of Europe and the Americas. It is formed from subsoil with a substantial clay content, mixed with sand and straw. Water is added to produce a workable, sculptable material that can be shaped by hand. Walls built with cob are thick and self-supporting, often bearing a timber or stone frame.

The mix relies on natural components: clay provides plasticity and cohesion; sand acts as an inert aggregate; straw adds fibrous reinforcement and reduces cracking. A typical cob wall is laid in thick courses by hand and compacted to a unified mass. After shaping, the surface is finished with lime or clay plaster to protect against moisture and to create a durable exterior.

Origins date to medieval and earlier rural architecture in Western Europe. Cob reached prominence in the British Isles, especially Devon, Cornwall, and Wales, where climatic and material conditions favored its use. It has seen revival in modern sustainable construction, aided by improved plaster systems, moisture management, and prefabricated timber frames.

Advantages include excellent thermal mass, low embodied energy, local materials, fire resistance, and ease of repair. Cob walls absorb heat during the day and release it at night, contributing to energy-efficient heating. Limitations include labor-intensive construction, sensitivity to moisture in exposed locations, and the need for skilled workmanship to ensure long-term durability.

Other uses of the term cob include a sturdy, compact horse or pony known as a cob; in some contexts a male swan is also called a cob.