Rethink Your Life!
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The Work of Art and The Art of Work
Kiko Denzer on Art

[Cob] update on trench issues

Lance Collins collinsl at
Tue Aug 19 23:49:39 CDT 2008

I'm not an expert on concrete but I have used it on lots of small 
projects and I read a lot.

On lists like this we make general comments about clay, straw, 
concrete etc.  These things have a wide range of properties and it's 
easy say things which are not correct in all circumstances.

e.g. concrete wicks moisture.  Well yes and no.  Structural concrete 
usually does but with a higher proportion of cement it can be made 
waterproof (ever see a concrete water tank?)

>Can a cob wall be built on a concrete footing / stem wall or slab?


>Earlier assertions (paraphrasing) said "no" cuz the moisture wicks up
>through the concrete and can cause a failure at the cob/concrete
>junction.  Hope I got that right.
>Am wondering if concrete's tendency to wick could be moderated.

It can.

>Thoughts and questions ...
>     * How about a good vapor barrier between the concrete and the
>       ground?  Should moderate the wicking effect.

I'd expect to see a sheet of plastic before a concrete slab is 
poured.   Not only to stop wicking but to stop water draining out of 
the concrete before it's fully cured.

>     * How about a good drainage system under the concrete, much like
>       that prepared prior to a rubble trench?  That should also reduce
>       the wicking.

A good idea.   Can you be sure the plastic sheet won't get the odd 
hole in it before the concrete pour?

>     * How about extending the concrete vertically 12", or 18" above
>       grade.  That should contribute to the concrete's evaporative drying.
>Would a combination of these or other techniques make concrete an
>acceptable material as a cob wall footing?

Structurally acceptable: definitely.  Politically acceptable: maybe not.

>The last bullet point seems most relevant to Tim's thinking.  If a
>mortared urbanite stem wall can be used, I should think a concrete stem
>wall of similar dimensions could also be used, since urbanite is
>concrete chunks.  Is my thinking valid?  What am I missing?
>I understand the shortcomings of concrete on the environment and would
>prefer not to use it.  But ... in some situations it could be quite
>useful.  And if an urbanite wall has similar behaviors perhaps it is not
>an optimum solution?
>This has been a great conversation.  Caused me to think of several
>different topics.  Some good info has been provided.  I'd urge all
>participants to practice patience during the dialog.  There is sooooooo
>much information lost in printed communications.  We don't have all the
>visual and verbal clues and can easily misinterpret humor, sarcasm,
>irony, etc.  This list is a wonderful resource.  Thank you all.

I have a shed with a concrete floor.  Plastic sheet first,  poured 
earth (5% cement) to level off then 40 mm of concrete laid in metre 
wide strips.  It's been down seven years and no moisture problems 
(rainfall about a metre a year with most in winter).   The shed is 
divided into two parts (workshop/living).  The dividing wall is 250mm 
of poured earth (no additives of straw, cement etc.).

Warning: It may be just a fluke that my soil is excellent for cob 
without adding sand or clay.  YMMV

(in Aus)