Cob
Cob is a building material consisting of clay, sand, straw, water, and earth.
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.
The word '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.
Cob houses are made by building the cob walls up in layers.
You need to start with a foundation first, which is usually in the form of a stone plinth, which should be around 600mm high, and also should be approximatly 300mm wider than your cob wall will be.
The cob layers can then be laid. It is a good idea to make each layer about 450mm in depth, and then trod into place, which also helps to compact it.
Then this must be left to dry, before the next layer can be laid. Each layer should taper slightly as the building rises.
A good roof overhang is also needed to help protect the cob, and this should be about 200mm.
Cob needs to breathe, and hence modern materials, such as concrete render, gypsum plaster and vinyl paints should not be used. Instead, the use of lime mortar, and lime plaster should be used. These can be decorated by using lime wash.
Limewash is like paint, but with a more 'watery' consistency. It should be applied by
first dampening the area to be lime washed, and then brushed on. A typical lime rendered
wall will take about 3-4 coats of lime wash.
The above steps allow the cob walls to release moisture.
Modern materials trap water, and don't let the cob breathe which will result in damage to
the cob, which will gradually crumble.
It is not unheard of for complete walls to collapse due to products like cement render being used.