Cob floor (long)

[Eric D. Hart]

Shannon Dealy wrote:

>Cob houses tend to use cob floors, it's done in layers starting with a gravel
>base to prevent moisture wicking up from the ground.  Then a fairly thick
>layer of regular cob is applied, followed by a couple of layers of a finer
>cob mix used for finishing.  Finally the top is sealed with linseed oil and
>bees wax.  I'm doing this from memory, so I may have missed something, I'll
>dig up my notes and address this in a little more detail later.
        For our frost protected shallow foundations, we have been using
strawbales in the foundation and concrete.  Our architect said he would be
ok with using a clay/straw floor for the frost protected slabs we are doing.
Here's my idea and I welcome any input/ideas people might have.
        The way we did our last foundation was to pour a thin slab, let it
dry, and then stack strawbales with the infloor tubing on top of the bales.
We left gaps on the load bearing sides of the slab for a thickened edge that
would take the load of the posts.  On the outside part of that edge (and
around the whole slab) we had one row of strawbales that provide part of the
perimeter insulation.  When the top slab was poured, the thickened edge was
filled up with concrete and the interior strawbales and infloor tubing were
covered over with about 4 inches (give or take a few inches) of concrete.
So when you are done, the only thing left is the perimeter bales which the
bale infill wall gets stacked on.
        If you followed that, here's how I think we could use a clay floor.
You would still pour a thin slab of concrete over the whole area, then
instead of adding all the bales in the slab, you would only put the
perimeter bales in.  The form work you set up would be just wide enough to
pour a thickened edge of concrete and tie in the perimeter bales.   Then you
can put up the framework for the building, stack your bales, and get the
roof on.  Now you are ready to get the floor ready to pour.  The first step
would be to fill the floor space with bales (for those in a northern
climate, otherwise just start with the floor).  Then you would put down
plastic where the infloor tubing goes, put cribbing wire down, and attach
the infloor tubes to it.  At this point you are ready to put down a coat of
cob for the floor.  I would hope that you could put down enough on the first
coat to cover up the infloor heating tubes (to protect them from damage).
Maybe it would work that you could get the first floor coat on just before
you put the first coat of exterior stucco.  Get your friends together to
first mix and apply the first floor coat and then do the exterior stucco.  
        I'm thinking that since you are using standard concrete for all the
load bearing portions of the slab and there is a thin coat underneath, that
would keep the building inspectors happy.  Anybody know how code officials
view adobe floors in New Mexico and Arizona?  I know they are common there
but I don't know if people use these types of floors in areas with building
codes.    
        So if my scheme to use a clay/straw floor will work, what kind of
recipe should I use?  Steve of SSS said something about a ratio of clay to
sand.  How does that translate into a recipe?  I realize that it takes
experimentation but a recipe might help.  I'm also wondering if you need to
extract clay from the soil you have or you can just shovel in any clayey
soil you have on a site (and experiment with the amount of sand, etc. you
need).  I believe Robert LaPorte has a method for extracting clay from the
soil you have on site.  Would like to just shovel in soil and figure out a
recipe rather than separating the clay from the soil.     
        In case you are wondering why I have so many questions, we just got
done with a small cabin (hermitage, 350 square feet) 80 miles from here and
they want to build at least 3 more of them next year.  I'm thinking that if
I can figure out the cob floor idea that I can talk them into it and save on
the order of 5-10 cubic yards of concrete per cabin (about $400-800 here).
That would reduce the cost of this low cost foundation even more!



Eric D. Hart			*We do strawbale construction workshops!
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