CARING FOR YOUR COB PROPERTY


Restored thatch and cob barn


Before restoration, and part way through the rebuild of the cob walls


resulting in a softened, weak and sometimes fluid cob which causes understandable structural failure.


There are three main causes of damp in traditional buildings:


1. Rising damp: where water travels from the ground and up into the walls. Rising damp isn’t normally an issue in cob walls as long as they are covered in appropriate lime renders and plasters which can regulate this moisture and allow it to pass freely into the internal or external environment. Tests have shown that vertical movement in an un-rendered cob wall will not normally exceed 150 mm to 250 mm. This is only a significant problem when largely impermeable renders and plasters (such as cement) have been applied, or the external ground level has risen above the stone plinth which causes the lower part of the wall to be permanently damp – thus weakening the wall and reducing its load bearing capacity at the point of greatest load.


2. Penetrating damp: usually caused when the porous external walls lose the ability to keep rain out. In some instances, north and east elevations of cob farmhouses and cottages were not rendered but were protected at the top by deep eaves and at


58 Listed Heritage Magazine September/October 2018


the bottom by the stone plinth – ‘a good hat and a strong pair of boots!’ The uncovered walls meant that they were subject to a certain amount of erosion caused by the exposure to rain water. However, the serious issues with penetrating damp and its effect on cob is exacerbated when the walls have been subjected to inappropriate coverings and repair works. Hard and brittle materials are prone to cracking under the natural, subtle movement of cob buildings which allows moisture to enter between the render and the wall causing the cob to break down. One recent study, carried out by Plymouth University, showed that 18 out of 22 (82%) cob buildings examined observed that failure could be partly or wholly attributed to the presence of a cement-based render limiting the breathability of the structure.


3. Condensation: occurs when warm air collides with cold surfaces, or in levels of high humidity and poor ventilation. This can be seen in cob homes where the interior has been finished with a non-breathable plaster and paint, and warm, moist air comes into contact with colder surfaces such as a window or wall. The water vapour in the air condenses on window and wall surfaces as tiny drops of water which often results in the familiar, unpleasant sight of black mould growth.


PREVENTION, SOLUTIONS AND BEST PRACTICE The Society for the Protection of Ancient Buildings (SPAB) advises that the philosophy of minimum intervention is one that is particularly relevant when seeking to repair cob buildings. Contrary to popular belief, cob is an inherently stable material, so long as it is kept relatively dry. Walls can range between 500-600mm thick which means that, as a whole, surface erosion is not a major issue. One of the many advantages of thicker walls is the ability to ‘stitch’ across cracks and fissures to achieve an efficient mechanical joint. This makes the reinstatement of the cavities and hollows resulting from more serious erosions, abrasions or minor damage a less complex task.


In cases where moisture is present in walls, you need to establish the root cause of the damp before seeking to rectify the issue. Chemical damp proof injections are known to be ineffective in most traditional buildings but particularly so with those built from cob. This is because of the large number of voids that often exist within the cob wall or the irregularity of the stone plinth that is almost impossible to completely cut off for an effective moisture barrier.


Continued >>


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