howto cobstruction

[Shannon Dealy]

On Thu, 5 Sep 1996, M J Epko wrote:

> Shannon Dealey wrote:
> 
> >... before starting work each day, check how firm the walls are.
> 
>         How firm should it be? Just firm enough so that it doesn't dent much
> with the heel of your hand (or foot)? Yet loose enough to gouge with a blunt
> one-inch thick stick?

These kind of things are always hard to describe, the main concern is 
that you be aware of how soft the wall is in a given area, so that you don't
build it up to fast and cause it to squish out (that was a real helpful 
description wasn't it).  A fairly simple test for short sections of the
wall is to stand on it and see how much of an impression you feet make
and how much the sides bulge out.  Generally, you can always add some
cob each day, as it doesn't matter how soft the cob is when adding to
it, the point is that the weight of the new cob will cause the cob below
it to bulge if it is to soft to support the weight of the layer you add,
so if you start to see it bulge out while adding a new layer, don't build
the new layer up any higher until it has had some time to dry.  The fact
that the cob bulged out doesn't hurt anything, it just means you have more
work to do trimming the bulge off the wall (quite easy), and mixing up
more cob than you would have had to if the wall hadn't bulged.  Every
pound of cob you trim from the wall is an extra pound of cob you didn't need
to make.

Basically what I'm trying to say is that if a section of the wall is soft
when you start the day, only add a couple inches of cob, if it's hard,
you may be able to add a foot.  How you determine how much cob to add in a
given day requires only a few days of experience and careful observation 
of the wall as you build it.

> 
> >You should also check how plumb the walls are before starting each day...
> 
>         The Backwoods article recommended taping an angled piece of wood or
> foam to a long level to match the exterior taper. That way the flat side can
> be used for the interior, which should be plumb(ish) and the angled side for
> the outside, which tapers toward the top.
> 
>         and later Shannon said:
> 
> >we put in lots of holes here in Western
> >Oregon, and leave it uncovered so it will dry enough to support the next
> >day's layer of cob.  It should be noted here that it is important that the
> >cob not dry to much between applying layers, since new cob does not bind
> >nearly as well to dry cob.  If the cob dries out, you can soak the top
> >of it several times over a period of a few days prior to adding the next
> >layer, this will improve the bond, but it will never be as good as it
> >would have been had the cob been added while the previous layer was still
> >a little soft.  For this reason, it is important to always leave the top
> >of an unfinished wall rough with lots of holes in it, since this will 
> >provide a mechanical bond between the layers regardless of how dried out
> >the wall gets before the next layer is added.
> 
>         Which causes me to again beg for additional clues on how to guage
> whether it's too one or the other.
> 
A cob wall that is to dry will basically feel like rock or the ground 
beneath your feet in dry weather.  A wall that is not to dry you will be
able to press your thumb all the way into, though it may require your 
full body weight if the wall has been drying a few days (or even one day
for those of you in hot dry climates).

[SNIP]
>         Sam wrote on the strawbale list:
> 
> >What about a cobblestone and cob mix?
> 
>         and Shannon replied:
>  
> >I'm not sure what you mean here, if you are talking about embedding stone 
> >in the Cob to reduce the Cob required, I don't think this would be a good 
> >idea, since this would tend to disrupt the matrix.  Think of cob as a 
> >really crude form of fiber glass, the straw is the fiber, and the 
> >clay/soil is the resin.
> 
>         And Ann Edminster wrote:
> 
> >Ensuring that there is enough straw (organic rebar) in the mix is crucial to
> >its crack-resistance.
> 
>         Should the soil used be screened for rocks? Why would a few rocks
> hurt things? Seems to me they might actually help (I also couldn't get a
> grip on that mass bit either, though.) Hmm.
>         If glass panes can safely be set directly into the cob (with the cob
> built into a bearing arch around the top of the window), wouldn't it stand
> to reason that kinda big rocks could also be used (allowing for substantial
> cob-mortar: like, foot-wide 'joints') with no ill effect? The cob would arch
> over the top of the stone, which would be stonger than the cob itself
> anyway... that would seem to be very stable and reduce the amount of cob needed.
>         Anybody care to speak to this (please)?
> 

You don't need to screen the soil for rocks, though it's probably a good idea
to remove any that are larger 1 inch if you mix bare foot (which I do).  With
regard to larger rocks to eliminate some of the need for cob, the thing to 
remember is that while it can be done, the cob encases the rock, but unlike  
conventional mortar, does not bind to it, so this area will be weaker since
its strength is based on the strength of the interconnection of the cob 
surrounding the stone.  From the perspective of compressive strength, 
I believe that you are correct, rocks would make it stronger, but from a
tensile strength perspective, it would be much weaker.  This would be 
fine if gravity was the  only concern, but earthquakes and other natural 
disasters tend to find  this kind of weakness.  With regard to windows 
set directly into the cob,  special techniques which take additional time 
are used to build the arch which encases them, and since there are generally
only a few windows, this would not be quite the same as loading up the
wall with rocks.

One additional note, since we have been discussing R-values in the SB list, 
putting lots of rock into your walls will also decrease its R-value.


My 2 bits for what it's worth.

Shannon Dealy
dealy at deatech.com