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Cob (material)

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.

History and usage

Cob is an ancient building material, that has possibly been used for construction since medieval times. Cobwork (tabya) first appeared in the Maghreb and al-Andalus in the 11th century and was first described in detail by Ibn Khaldun in the 14th century. Cobwork later spread to other parts of Europe from the 12th century onwards.

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, Ireland, Cornwall, Brittany and some parts of the eastern United States. Traditionally, English cob was made by mixing the clay-based subsoil with 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 wall height would progress according to how long it took for the last 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 deepset 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 temperature fly wheel inside the home. The material has a long life span even in rainy climates, provided a tall foundation and large roof overhang are present.

Modern cob buildings

When Kevin McCabe built a two-storey, four bedroom cob house in England in 1994, it was reputedly the first cob residence built in the country in 70 years. His methods remained very traditional; the only innovations he added were using a tractor to mix the cob itself, and adding sand or shillet (a gravel of crushed shale) to reduce the shrinkage.

From 2002 to 2004, sustainability enthusiast Rob Hopkins initiated the building of a cob house for his family, the first new one in Ireland in about one hundred years. It was undertaken as a community project, but destroyed by an unknown arsonist shortly before completion.

In 2006, a modern, four-bedroom cob house in Worcestershire, UK, designed by Associated Architects sold for £745 000. Cobtun House was built in 2001 and won the Royal Institute of British Architects' Sustainable Building of the Year award in 2005. The total construction cost was £300 000, but the metre-thick cob outer wall cost only £20 000.

In the Pacific Northwest of North America there has been a resurgence of cob building both as an alternative building practice and one desired for its form, function and cost effectiveness. There are more than ten cob houses in the Southern Gulf Islands of British Columbia built by Pat Hennebery, Tracy Calvert, Elke Cole and the Cobworks workshops.

In 2007, Ann and Gord Baird began constructing a two-storey cob house in Victoria, British Columbia for an estimated $210,000 CDN. The 2,150 sq. ft. home includes heated floors, solar panels and a southern exposure for passive solar heating.

The building process known as "Oregon Cob" is one which was refined by Welsh architect Ianto Evans and researcher Linda Smiley in the 1980s. Oregon Cob integrates the variation of wall layup technique which uses loaves of mud mixed with sand and straw with a rounded architectural stylism.

See also

Reference Works

  • Building With Cob, A Step by Step Guide by Adam Weismann and Katy Bryce. Published by Green Books ; 2006, ISBN 1-903998-727.
  • The Hand-Sculpted House: A Philosophical and Practical Guide to Building a Cob Cottage (The Real Goods Solar Living Book) by Ianto Evans, Michael G. Smith, Linda Smiley, Deanne Bednar (Illustrator), Chelsea Green Publishing Company; (June 2002), ISBN 1-890132-34-9.


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