Would a damp-proof course need to be incorporated in a new earth wall? Are damp-proof courses appropriate in existing cob walls?
A. Traditionally, earth walls relied on the fact that, being of “breathable” construction, rising dampness was drawn out by evaporation through external wall faces. However, compliance with Approved Document C of the UK Building Regulations would probably require the installation of a dpc in a new masonry plinth (underpin course). Chemical injection could be appropriate in the very rare occasion where there is a solid brick plinth [although there is doubt about its efficacy even in these] under a cob wall but it is not appropriate for stone plinths in which voids make the injection application patchy and ineffective. Moreover specialist dpc installers also require the plinth to be rendered with a 3:1 sand:cement mix after they have carried out the injection and this drives the rising dampness of the failed areas into the base of the cob. Modern tanking systems can also cause damp to rise into the cob if the plinth is low.
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