Earth buildings have been a traditional form of construction in many parts of England for centuries. Here in Wiltshire, Hampshire and parts of Dorset, chalk rather than clay forms the basis of cob. To this chalk, only straw and a little water are added to create the building material. All our cob wall repairs use only these traditional ingredients, with the addition of a lime render or limewash where appropriate.
Cob walls are said to need “a good hat and a good pair of boots.” The hat protects the cob from rain — traditionally this would have been thatch, although slate or tile roofs were also common. The boots refer to the plinth, usually made of brick, stone, or flint, which kept rising damp out of the cob, acting as a natural damp-proof course.
Before carrying out any cob repairs, it is essential to identify and address the causes of damage. These might include leaking roofs, failed plinths, raised ground levels that cover the base of the wall, tree roots, new extensions, altered openings, concrete floors or patios, leaking rainwater goods, or ground movement. Recently, we have also dealt with damage caused by masonry bees and woodpeckers.
We prefer to carry out our repairs using salvaged cob from the original wall where possible. If not we will usually find a source close too the wall as would have originally happened. The cob material can then be remixed with a little water and straw to make what we call ‘Mass Cob’. This is then carefully applied to the wall, and is fully mouldable to any shape allowing us to repair without having to remove any additional original material. Most importantly, this fits within the conservation principle of repairing like with like.
We prefer not to use cob blocks in our repairs for a number of reasons. Primarily its much simpler and cheaper to mix up salvaged material and relay it. We live in a chalk area with chalk cob walls, the vast majority of cob block suppliers are in clay areas. It certainly isn’t therefore a sympathetic repair to put a clay cob block in a chalk cob wall! In addition, the blocks are often bound with a lime mortar thus introducing a further deviation from the original wall.
As with all lime and cob work, careful control of moisture and suction is vital. Hessian and plastic sheeting are often used to protect the surface during curing. Above all, patience is the key to achieving a lasting and successful repair.
