Cob How-To: Applying Cob

[Shannon C. Dealy]

In my posting from several weeks ago, I gave a description of how to
make cob, here you learn what to do with it (or if you prefer you can
tell me what to do with it :-).

Applying the Cob:

You just happen to have a mass of cob lying around ready to use, and a
foundation (rock, concrete, whatever) waiting for a wall.  To apply the
initial layer of cob, just stick the cob on top of the foundation and
using your hands or feet, push the cob into any cracks and make any
adjustments to the width of the wall you need.  This first layer can be
made about 6" inches thick.  After the base layer has been applied, apply
succeeding layers in the following manner: take a piece of cob about the
size of a small loaf of bread, place it on the wall and push your thumbs
(or a blunt stick about 1" in diameter), through the cob "loaf" and about
1" to 2" into the cob wall below it.  Repeat this several times over the
whole surface of the loaf.  The purpose of this process is to tie the  
layers of cob together by pushing some of the straw fibers in the new cob
down into the previous layers.  Depending on the weather conditions and 
how wet your cob mixes are, you should be able to add 6" to 12" of cob to 
a wall in one day (possibly less if your walls are still soft from
previous work on the wall).  The limit on how high you can build your wall
in one day is due to the weight of the new layers of cob causing the
layers below to bulge out, thereby wasting cob.  Bulging will occur
gradually, so check the sides of the wall periodically while you are
working.  It is also a good idea to regularly check that the walls are
plumb or correctly tapered (depending on your design).

For building a wall that is all there is to it, except...

Trimming:

When your wall bulges out, is not plumb or is tapered incorrectly, it is
necessary to trim off some excess cob.  To do this, use a machette and
just hack off the excess.  I recommend that you get a machette just for
cob trimming unless you really enjoy sharpening knives.

Transport:

Moving the cob around your site will be easier if you take each batch of
cob and break it up into into lumps (cobs) the size of small bread loafs
and roll them into a kind of small football shape.  This makes it easier
to move the cob to the wall by tossing it from person to person.

Smoothing:

Don't spend to much time on smoothing the sides of the walls with your
hands, this will tend to create a kind of surface seal and slow the drying
of the wall.  It also is kind of pointless, since you will most likely
plaster over the wall anyway.  Also, NEVER smooth out the top of an
incomplete wall, in fact you should leave lots of holes in the top of the
wall (from the last layer of cob that was added).

Adding to a dry wall:

If an incomplete wall dries out, it is almost impossible to push a new
layer of cob down into the old layer, so the cob will be weaker where
the new layer is added to the old dried out layer.  Two things can be done
to improve the strength of the bond between these two layers.  First make
sure there are lots of holes left in the top of the wall before it is
allowed to dry out (hence the recommendation under 'Smoothing' above),
this ensures that there will be at least a reasonably strong mechanical
bond between the layers.  The second thing you can do is to soak the top
of the wall with water repeatedly over a period of a few days to try and
soften the cob.  While both of these will help, the wall will be stronger
if top of the wall is not allowed to dry out until it has been completed.

Faster drying:

If you are building when it is cool or wet, the wall will tend to dry more
slowly, one trick for speeding up the drying of the wall is to build up
the center of the wall about 4" - 6" above the rest of the wall with lots
of holes in it, then build up the outer edges of the wall to the same
height.  This allows a great deal of drying to occur in the center of the
wall in just the few hours it takes for you to get back to where you
started (because of the larger exposed surface area).

Slower drying:

Throw a tarp over it, wet it down periodically, or put in a requisition
for cooler, wetter weather.  This also works for saving a batch of cob
that you have mixed and don't have time to apply.


The above document has been brought to you by Fly-By-Night Industries and
comes with a money back garantee... if you send us money we will send it
back :-)

Tune in next time for the thrilling sequel: Doors of Doom, Windows on the
World, or How to get really attached to your walls!

Shannon Dealy
dealy at deatech.com