[Cob] earthen floors

[Shannon Dealy]

On Fri, 16 Nov 2007 sly at riseup.net wrote:

> Hello everyone,
>
> I am about to do an earthen floor on a small cottage in spain.  Can I do
> it with just clay and sand and not use roadbase?  And do I have to do it
> in layers?  I am looking for the easiest way because the floor doesn't
> need to be fancy.

Road base is not required, it is used because it has high compressive 
strength and absorbs very little water.  The biggest problem with earthen 
floors is getting them to dry since they are at the coolest point in the 
house, so using a mix with a low water content and tamping it in will get 
the job done much faster when you consider the drying time.  As far as 
layers, you will need to do a minmum of two layers on top of the drain 
rock base, a coarse high sand cob type mix (possibly with roadbase in it), 
followed by a thin finish layer.  Make sure you get each layer as close to 
level as you can manage and it is MUCH more work to fix leveling errors 
in the next layer up.  For your earthen layers get each new layer as dried 
out as possible before adding the next one, it will take much longer to 
dry out the floor if the lower layers are still wet.  Use of a large fan 
can significantly improve drying times.

> I was hoping we could do sand and clay and then seal it with linseed?  And
> how toxic are the fumes gassing after of the linseed, would it be
> dangerous to sleep in the room for sometime after sealing it with linseed?

Things to know about linseed oil (a number of misconceptions have been 
posted on this list in the past):

    - Raw linseed oil is NOT a drying oil.  This does not mean it won't get
      drier, rather that its surface will not fully harden, remaining
      slightly oily/sticky and it may take quite a while to get to that
      point.

    - Boiled linseed oil is a drying oil, and while the oil itself is not
      toxic (though I wouldn't recommend eating it), most commercial
      preparations of it have chemical additives which often are, some
      contain small amounts of heavy metals and most contain thinners which
      may or may not be toxic.  These additives are used to make it dry
      faster (even with them it takes days for one coat to dry).

    - Material safety data sheet for the thinner in all of the different
      brands of conventional commercial preparations I have bought (I
      forget what thinner was used, possibly turpentine) indicated that
      breathing the thinner could cause "temporary" problems (I think brain
      and lungs).  I figure this is probably along the lines of the
      "temporary" effects of alcohol on the body i.e. people return to
      apparently normal functioning, but there are in fact minor levels of
      permanent and temporary damage (loss of brain cells, liver, etc.).

    - Alternative formulations of boiled linseed oil are available using
      "non-toxic" thinners (citrus based thinners are among the more common
      ones).  Keep in mind that just because something is from a natural
      "non-toxic" source doesn't mean that it is completely safe, even
      water will kill you in sufficient quantities.  Some people do have
      problems with breathing these types of vapors as well.  These
      alternative formulations are a fair bit more expensive.

    - In cool moist areas either boiled or raw linseed oil will support the
      growth of mold.

    - I am planning on finishing a floor in my house while I am living in
      it, so I have been looking into alternatives to what I have used in
      the past, tung oil looks particularly interesting (though it's about
      3x the price of linseed oil) because it is a drying oil, is readily
      available without additives, and it is claimed that it won't support
      mold growth the way linseed oil will.  As far as I can tell, it is
      superior to Linseed oil in every regard except price and drying time
      (recommendations I have seen say 2+ days between coats, and weeks for
      full cure which is comparable to my experience with Linseed oil).

NOTE!!! When applying linseed oil, cover the surface in linseed oil, 
possibly let it soak in for a few minutes, then move any puddled oil onto 
an area of the floor that hasn't yet been oiled if there are no fresh 
areas of the floor to cover, soak up the excess oil with a rag.  NEVER 
leave any puddles (no matter how small) of linseed oil on the floor. 
Puddled oil will dry on the surface of the puddle, trapping less dried oil 
inside the puddle which can literally take years to dry.  The result of 
this is that you can end up with a gummy mess when you walk on or set 
things on these semi-dried puddles.

ANOTHER NOTE!! Rags soaked in linseed oil will readily burst into flames 
(spontaneous combustion) as they are drying.  When you are done with the 
rags, spread them out flat in the sun in a non-flammable area and weight 
them down with rocks until they have dried, then place them in a tightly 
covered metal garbage can until they can be disposed of.

FWIW.

Shannon C. Dealy      |               DeaTech Research Inc.
dealy at deatech.com     |          - Custom Software Development -
                       |    Embedded Systems, Real-time, Device Drivers
Phone: (800) 467-5820 | Networking, Scientific & Engineering Applications
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