History of Cob

Cob is an old Devon word for a mud wall, and Devon probably has more cob than anywhere else in Britain, with cob being Devon's traditional construction material from the fourteenth century for over 500 years.

One reason for the popularity of cob in Devon may be the quality of the Devon clay sub-soils, many of which have a consistency and low shrinkage factor that makes them ideal for cob construction.

Traditionally, straw and sometimes dung, were added to the clay sub-soil to reduce cracking problems during drying. Often cattle would be used to tread the mixture, so the addition of dung was inevitable!

Once the mixture was ready to use, the walls were usually built up in layers, with each layer being given time to dry before the next one. Sometimes shuttering would be used - the exact technique varied according to the quality of the cob mixture, the weather and local tradition.

A good-quality cob can survive quite well without rendering, but normally it is coated with a render made from quicklime putty and course sand, followed by a lime wash. These traditional coatings have the important property of being porous, or "breathable", so that any moisture that does get into the cob, via penetrating rain or rising damp, can evaporate out through the render.

Cob walls are extremely durable so long as they are not allowed to accumulate moisture. Most modern coatings, such as cement render, gypsum plaster and vinyl paints, do not allow the cob to dry out after a spell of wet weather, so that it gradually accumulates moisture until it eventually starts to crumble. Many old cottages have probably collapsed in this way, sometimes quite dramatically, with the owners returning one afternoon to find the remains of their beloved cottage just a mound of rubble in the road!

Fortunately, the traditional skills, almost lost in the second half of the twentieth century, have been revived, and research carried out, so that now there are professional suppliers and builders who can ensure the safe repair of cob buildings. Cob is still being used for buildings, both in renovation of traditional buildings and for new houses. Because it shrinks as it dries, cob presents problems when used to repair or extend a building. This is where cob blocks are so useful, as each block is dried before use, so that when cob blocks are used to fill a damaged portion of cob wall, only a minimal amount of shrinkage occurs as the mortar dries.