This June’s visit is primarily for us to focus on the task of re-pointing the existing Flint plinth, which together with the existing Cob walls of the property had been unwittingly nearly ruined by a reinforced cement render which had been applied about 20 yrs ago. For those in the know, cement and concrete are a notorious problem for introducing damp and mould into buildings and for all permeable (breathable) building materials such as Cob, Straw and Timber it is particularly bad news.
For Cob it means that the cement traps water within the wall, which over time can literally wash the cob wall and mortars away. Luckily Cob is pretty tough stuff and once we’d removed all of the cement our Cob wall was still intact if a little wet!
Unfortunately the mortar of the Flint plinth took the brunt of the water erosion and had in many cases nearly washed away completely.
Instead of replacing the original Cob mortar we chose to repair and re-point with a 3:1 sharp sand and Lime putty mortar. This was to add strength to the new flint joints (as with a repair and not a rebuild you are repairing holes in a wall rather than rebuilding where you can lay the stones more specifically on top of each other to take downward loads), whilst maintaining the permeability of the Cob.