What is Cob?
Cob is an earth-based building material used extensively across the south west of England, although not exclusively. It can be found in many corners of the country, especially where a ready supply of stone may have been lacking historically or where labour was readily available.
Cob, as we call it down ‘yer is essentially just a clay rich sub soil mixed with straw. The clay content hardens to provide a dense lump and the straw provides tensile stability to the mass. There are many forms of earth-based building material in the world and they all have slightly different characteristics with regards to their composition, manufacture technique or application methods. Rammed earth and Adobe to name two.
Traditionally, the earth used to build these walls was dugout locally which leads to a diverse range of ingredients, such as sand, stone, clay, silt etc, being present in old constructions in varying quantities. Subsequently, there is broad range of characteristics which lead to variations in the ability for Cob and Earth mortars to handle moisture and damp ingress and structural weaknesses such cracks and subsidence.
The generally low skill of the traditional builders combined with an apparent lack of a need for accuracy during the building process of cob walls, often leads to inconsistencies in the coherence of the materials within a historic earth structure.
Cob houses today have often evolved from lower grade buildings which may not have originally been living accommodation. Many houses today have incorporated structure which were animal sheds which may not have had windows or a basic workers house which may not have had a first-floor level and the standards to which these alterations were carried out are often dubious at best.
Click here to see the animation we have created which shows, generally, what is happening in a cob house with regards to water migration.