The making of and building in cob (also known as ‘cobb’, ‘clom’ or ‘clay lump’), a natural building material made from subsoil, water, organic fibrous material such as straw, and sometimes lime.
|Craft category||Walling and hedging; Building crafts|
|Historic area of significance||Global|
|Area currently practised||Devon and Cornwall predominantly|
Cob (or cobb) and cob based building techniques are an ancient form found all over the world with some of the oldest human made structures composed of it. In the UK the term cob goes back to the 1600s and is often referred to as clay lump in the East of England. The word cob possibly derives from the way in which the material is used and applied – beating and striking.
In the UK, cob buildings are most prominent in Cornwall, Devon, Cumbria and the East of England.
Cob is still being used in modern architecture and as a technique it gained a lot of attention especially in the light of the current sustainability drive, with the material being fully and purely recyclable and biodegradable as well as resistant to weather.
Traditionally, English cob was made by mixing the clay-based subsoil with sand, straw and water using oxes 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
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 and/or humid climates, provided a tall foundation and large roof overhang are present.
The basic ingredients are similar world-wide. However, there are regional names [of this medium] that vary.
Issues affecting the viability of the craft
- Lack of regulation
- Lack of proper training and opportunities to train
- There are many people who incorporate some cob building into their work, but only a few remaining experienced practitioners to pass on skills.
- Labour intensive and slow to build (each layer needs to dry before applying the next one which might take over a week and the whole structure can take months to dry)
- Cob can’t be laid in cold and wet – and so the construction is legally restricted to Summer months
- Mortgages institutions often do not support cob buildings
Craftspeople currently known
- The Cob Specialist
- Earth Blocks
- Heritage Cob and Lime
- Mike Wye Associates
- Kevin McCabe
- Matt Robinson
If experienced practitioners drop away altogether our historic built environment will be impacted upon