cob in patio wall

[Shannon Dealy]

On Sat, 31 Aug 1996, Janet Moore wrote:

[SNIP]
> 
> It would seem that cob has much potential for this freeform type of work. My
> question is, can a cob wall like this be adequately weatherproofed? There
> was a lot of talk in the SB list a while back about similar walls made of
> bales, and how difficult it would be to prevent moisture from getting into
> the top surface. Cob would be just as vulnerable, wouldn't it? 
> 
[SNIP]

It all depends on what you mean by weather proofing,  if you put a good
roof over the wall and foundation under it, the wall can last for hundreds
of years, on the other hand if you leave the top exposed to the weather
(no roof), a thick wall may still last decades, depending on your climate.
You can of course get better life out of either condition by plastering
or painting, providing that the plaster or paint are maintained.

Cob is not nearly as vulnerable as straw bales:

1 - Cob can't rot in the way that strawbale can, the straw in cob that is
    properly mixed ends up with every strand coated with earth which
    blocks the spread of decay.  In addition, the earth will help to
    wick excessive moisture away from the straw.

2 - Once moisture penetrates the outer shell around the strawbales,
    all of the bales in that area are at risk due to the generalized
    increase in moisture content, and the ease with which the rot
    can spead through the bales.  Cob has no outer shell to penetrate,
    and since cob is slow to absorb water (much like the ground is
    for the first rain after a long period of dry weather), most
    of the water will be shed by the wall.  When the rain stops,
    a cob wall will dry out and show no ill effects, whereas the
    moisture will be trapped in a strawbale wall and start to rot.

Cob can fail due to weathering, but it takes exposure well beyond what
bales can handle.  The cob can either erode over time, or the wall can 
absorb enough moisture to weaken it to the point of collapse.  In the
first case, the erosion can be easily seen from the outside of the wall
and patched or plastered before any real damage occurs.  In the second 
case, it generally requires that the bottom of the wall be in constant
contact with moisture (due to an inadequate foundation) in order for
sufficient amounts of moisture to be absorbed into the wall to cause 
a collapse.


Shannon Dealy
dealy at deatech.com