LOOKING AFTER COB BUILDINGS


Properties built with sandy soil with little or no clay content have been known to fail at quite low moisture levels. It should be stressed however that as a material, as long as it’s kept dry, cob is very stable. Cob walls are normally at least 500mm thick and it is a fairly straight forward process to ‘stitch’ any cracks in the cob.


Having the correct render on a cob wall is imperative for its longevity. All too often, cement render can be seen on the exterior of a cob cottage, and not only is it inflexible, it’s not able to accommodate minor thermal and moisture movements in the wall, therefore opening up cracks in the render. Rain water penetrating the cracked surface is unable to evaporate and will eventually soak down through the cob to the foot of the wall. At this point the wall carries its maximum load of wet cob plus the weight of the roof and is at its most vulnerable.


There is a simple rule of thumb in keeping a cob house healthy and this is that it must be able to breathe. This entails using both a breathable render and an internal plaster of lime mortar, which allows any damp in the cob to evaporate. Allowing the building to breathe will help to eliminate damp and mildew issues.


Because cob walls behave much differently to those built of masonry, when faults develop a different approach is required to structural repair and reinstatement. In this instance, it is always advisable to seek the help of people who are have experience in working with cob.


64 Listed Heritage Magazine September/October 2018


Top left:


Cob wall on the point of failing


Top right:


Gable end is failing due to cement render


Above: Underpinning a cob wall


Showing how in the past the cob has been mistreated with cement render, wire mesh & bricks


Article written by Sue Rawle Heritage Cob & Lime www.heritagecobandlime.com


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