COB BUILDINGS: THE IMPORTANCE OF LIME RENDERS
In the past 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 in to 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 wet getting in; however, cement is a rigid material and during periods of atmospheric temperature variation cracks form which allow rainwater to get in to the wall. The dampness passing through the cracks in the cement render is then inhibited from drying out on account of the low porosity of cement renders. Trapped moisture in cement rendered walls tends to migrate downwards and inwards, and is often identified as the cause of damp at the base of internal 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.
Dramatic Cob Wall Failure
Dramatic Devon cob wall failure: a result of water ingress combined with a thick layer of cement render. Fortunately for the Client, we warned them of the imminent threat and they moved out of the living spaces four days before the collapse.
Contact Building Conservation Services if you need our help with a dramatic job such as the property in the images below, we'd love to help!