Cob & Lime Renders
There has been a considerable amount of harmful building work carried out to traditional vernacular buildings, not least of which has been the use of cement-based mortars and renders. Historically, rendered, solid walled masonry structures, including cob, remain dry on the principle that damp, such as rain soaking into the surface of the wall, will evaporate back out through the relatively porous coating of lime render. Cement renders are not as porous as lime renders and in the past cement mortars were applied to old buildings in the belief that they would stop the rain getting in; however, cement is a rigid material and during periods of atmospheric temperature variation cracks form which allow rainwater to pass into the wall. The dampness passing through the cracks in the cement render is inhibited from drying out on account of the low porosity of cement renders. Trapped moisture in cement rendered walls tends to migrate inwards and downards, and is often found to be the cause of damp at the base of external walls. If a cob wall becomes excessively damp it can, in some instances, lose its structural integrity and the wall can become prone to sudden and catastrophic collapse, such as is demonstrated in the example below.
Below is shown an example of a dramatic cob wall failure, the result of water ingress combined with a thick layer of cement render. Fortunately for the owner, we were able to warn of the imminent threat and they moved out of the living spaces and installed props below the floor joists, saving the building from a far more severe collapse.
