It's not just said - it's an essential element in maintaining the structural integrity of mass earth walls - if the bottom gets wet it becomes mud, unable to support any weight and the wall subsides into a heap of earth. If the tops get wet they just erode away at a rapid rate. So mass earth walling is usually built off a stone base, high enough to act as a damp course and to prevent serious splashes from falling water hitting the base and roofed buildings often have a wide overhang at the eaves to throw water away from the wall. Cob and clunch/pise/etc walls will often have a wide coping to do the same (some thatched ones in the south of Emngland). Many were also originally lime mortar rendered and limewashed. Once this protective coating falls off the material will slowly weather back (or rapidly if the farmer's cows lick the base of the walls - one of my buildings projects has this problem!) and it's impossible to add cob material to a wall face to thicken it up (short of actually building a new cob wall against the original).
From: British archaeology discussion list [mailto:[log in to unmask]] On Behalf Of Patsy Jones
Sent: 23 October 2009 11:25
To: [log in to unmask]
Subject: Re: [BRITARCH] Heeley City Farm Mud Stomp
With cob it is said that the top and the bottom must be kept dry, hence the
old thatched walls in Blewbury
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