Cob Additions

[Shannon C. Dealy]

On Thu, 15 May 1997, Jacob Stevens wrote:

> Excuse my ignorance if this is a dumb question, but I'm still not too well
> versed in cob-know-how yet.  
> 
> How would cob buildings be for adding additions over time?  Are there issues
> of drying and settling that would make it especially difficult to do so?
> How would new cob do at sealing against 1-3 year old cob?  Any
> thoughts/ideas on this?

New cob can be attached to old cob at any time, but once the surface of 
the old cob has hardened (in really hot dry weather this can be just a
few days if the cob is not covered), it will never achieve a bond as good
as you get attaching to cob which has not yet dried.

To get a good bond between layers of cob (old or new), the layer to bond
to should be moist and there should be a mechanical 'keying' together of
the layers.  Whenever you stop work on a new cob wall that is incomplete,
the top (or whatever surface you intend to bond the next layer of cob to)
should be left covered in holes perhaps one to two inches in diameter and
a couple inches deep (thumbs or a 1" diameter tree branch work well to
make these holes) to provide a surface that the next layer can mechanically
bond to.  If you want to add on to an old cob wall that has dried, wet
down the attachment area repeatedly with a hose, cut holes in the surface
(to provide the mechanical bond), and fill the holes with water and let it
soak in some more, it might take several days of periodically soaking the
area to get it to stay moist (it will tend to run off or be wicked away by
the wall).  If the weather is dry, it would probably be a good idea to
cover the area with a tarp after each soaking to help keep the water from
evaporating.  The object here is NOT to make the wall soften significantly
(it won't), just to get it wet enough that the old and new layers will
bond and not dry to quickly.  If you are just trying to patch an area of
wall in a non-structural manner, you can wet the suface down a couple of
times, drive some old bent nails partially into the area to be patched and
then apply the new cob over the area.  The bent nails will provide the
necessary mechanical keying to hold the patch on the wall.

With regard to shrinkage/settling issues, cob stabilizes very quickly.  If
you keep your moisture and clay content fairly low in the fresh cob, there
should be very little shrinkage or settling of the cob once the surface
hardens (a few days in hot dry weather, a few weeks or more in cool wet
weather).

One important note, putting all of these holes in the cob surface causes
it to dry MUCH faster, so you may need to cover fresh cob once you have
put the holes in it if you are going to leave it for very long or if the
weather is very hot.

Shannon Dealy
dealy at deatech.com