Rebuilding Cob Walls

In the past, when a section of cob wall was to be rebuilt, it was usual to do this using stonework or brickwork.

In more recent times, insulating blocks have been used, on the assumption that their relative softness make them a similar material to cob. On the whole, they appear to have performed satisfactorily, but it is thought that the dissimilarity with the adjacent cob is likely to cause further problems.

There is always the risk that, at the junction between the two materials, different movement will occur as a result of temperature changes and, more particularly, changes in moisture content.

As a consequence, if stonework, brickwork or insulating blocks are used to repair cob, cracking is likely to occur at the junction between the two materials. This in turn leads to discontinuity within the wall and a loss of overall strength and stability of the building.

Serious failures in cob buildings may occur, often following periods of prolonged heavy rain. The advantage of cob blocks is that they can be used at any time of the year, as they are pre-dried, so that little or no shrinkage will occur. Of course, a small amount of shrinkage would be expected because the mortar would be damp.

It has long been considered impractical to use raw earth for repairs of cob buildings, especially in cold and damp conditions. However, recent experience of carrying out major repairs using cob blocks would suggest the problems normally associated with wet-placed cob, with its slow drying-out and subsequent shrinkage, can now be largely overcome by using cob blocks.

Cob blocks are made of the same materials as used for mass cob, but with the larger stones removed, so they can be easily cut. The blocks are made individually and are rammed by machine.