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Kiko Denzer on Art

[Cob] update on trench issues

Henry Raduazo raduazo at
Wed Aug 20 10:34:57 CDT 2008

Cement foundation, fact or urban legend?:   In Ohio I helped to build  
a two story house at a place called Greenfire. on a concrete stem  
wall. I do not know if the house was finished. The owner's name was  
Hogan. The building stood for at least two years that I know of with  
no wicking problems or damage from water. I would think that if the  
concrete was dense good quality cement, and if you have a fairly  
decent drainage system around the wall, the cob wall will wick and  
evaporate water faster than the cement can transport it.
	Good quality cement is made with as little water as possible while  
still permitting the cement to flow and be leveled out and good  
concrete is made with lots of good quality nonporous rock and sand  
aggregate. I find it hard to believe that good quality cement made  
with good sand and stone aggregate will conduct enough water to cause  
a problem. Concrete is usually a problem because it does not have  
enough capillary action. Water is conducted through earth walls  
relatively quickly and condenses on concrete used as an exterior  
finish causing the earth under the finish to saturate and sluff off.
	With that said it is probably quite easy to make bad concrete. I  
teach a tufa pot class where peat moss, sand plastic fibers and  
cement are used to build a flower pot for outdoor use. The pot walls  
conduct water and breath like crazy because of the capillary action  
of the peat moss. Any porous aggregate like sand stone or certain  
types of shale would have the same effect, but a quarts aggregate and  
quarts sand would have the opposite effect.
	The place to look for a definitive answer would be out west where  
there are many adobe houses built on all sorts of foundations. Does  
any one know of an adobe house on a concrete foundation that had  
problems? Is there a code regarding adobe walls on concrete stem walls?
	I also built a cob wall on a cement block foundation in a problem  
location, and I had no problems from wicking water. The soil at the  
bottom of the foundation trench was so soft that you could poke a  
stick into it with just hand pressure, there was no way to drain the  
cite and the area was known to be 12 inches under water after heavy  
rain storms.
	We pounded roman bricks into the bottom of the foundation pit using  
an ugly stick, partially filled our pit with tamped rubble, and built  
up from the rubble with stone filled cement blocks. This is probably  
not a good test for a house wall because the cob wall was only 4 foot  
high, but there was no wicking problem.
	When you build a wall with urbanite the mortar joints and air spaces  
between the pieces of cement all act as breaks to capillary action.  
In my block wall above the air spaces between the stone in the cement  
block also breaks up capillary action.
	The problem with the cob and adobe industry is there is not enough  
definitive data and experience. I know of a couple places where  
concrete foundations caused no problems. What I need is to find a  
place where it did cause a problem so I can then go and analyze why  
the problem occurred and how it can be fixed.
	Vapor barriers are easy and cheap to construct. They do provide a  
potential shear line where a house can slide off a foundation during  
an earth quake, but you have the same problem with urbanite or stone  
foundations. In my block foundation, in the example above, I just  
filled the top course of blocks half way with stone and cob filled  
the other half of the blocks bonding them together and providing a  
keying action between the blocks and foundation. You could do the  
same thing by setting cement blocks or even stones in the wet  
concrete footing and partially filling the blocks with stone and part  
with cob. Thus you have a capillary break and a keying structure.
	Regarding bricks and blocks:
	Concrete blocks or cinder blocks are not as bad as many people  
think. They are mostly waste material and a little cement enclosing  
large air spaces. The cheap cost is a good indicator of how little  
embodied energy is represented by a cinder block.
  	Fired bricks on the other hand require a gallon of fuel to make  
eight bricks. I think of them as finding money when I see good bricks  
discarded along the highway.
	Good information helps us to make good decisions.  Most people do  
not analyze problems and report them when they occur. We need someone  
to collect data regarding problems. So we are not stuck speculating.

I know what I know, but I don't know what I don't know.

On Aug 19, 2008, at 10:54 PM, drub wrote:

> Hi all,
> Tim's email causes me to pose questionsthat began forming earlier in
> this conversation.
> 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.
> Thoughts and questions ...
>     * How about a good vapor barrier between the concrete and the
>       ground?  Should moderate the wicking effect.
>     * How about a good drainage system under the concrete, much like
>       that prepared prior to a rubble trench?  That should also reduce
>       the wicking.
>     * 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?
> 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.
> All the best!
> David
> Tim Nam wrote:
>> FYI:
>> I decided to not tie in to the existing drain line, opting instead  
>> to tunnel through the existing trench (of strawbale garden wall),  
>> to daylight, affording my trench an extra 3" of depth, well below  
>> the 12" frost line here in the Willamette Valley.
>> So the plan is: slope the bottom of the trench to daylight, put in  
>> some 3/4" minus on the bottom then the drain pipe with the 4"  
>> minus clean, river rock. I think I'll add some smaller concrete  
>> chunks and chunks of asphalt which were kindly left here by the  
>> previous owner (yes, hint of sarcasm) but I think a good use for  
>> it, as rubble that is.
>> So I'm still wondering, what do yall think about whether to line  
>> the trench with a poly lumber tarp? that is, to slow siltation of  
>> drain rock and pipe and deter root penetration/heave. My concern  
>> is the tarp though used, is still in good shape, and that it won't  
>> be porous enough...perhaps landscaping cloth will work better?
>> And about the issue of concrete and rising damp, I was planning on  
>> a mortared urbanite stemwall, do I need to have a vapor barrier  
>> between the urbanite-cob junction?
>> Thanks
>> Tim
>> ----- Original Message ----
>> From: Damon Howell <dhowell at>
>> To: coblist at
>> Sent: Tuesday, August 19, 2008 1:37:55 PM
>> Subject: [Cob] off the concrete subject
>> Ocean was right in saying it was a silly question about straw in
>> concrete. I didn't think of it causing such a stir either, but we may
>> talk about what we feel needs attention, and anything to do with
>> concrete gets cobbers going every time. But unfortunately, cob has
>> it's place, and on the ground isn't it. I have done quite a bit of
>> reading on Roman concrete, which is made from lime and volcanic ash,
>> mixed extremely dry, then packed into forms. The process was pretty
>> much like rammed-earth and it's still there 2,000 years later. Cob
>> doesn't really lend it's self to compaction. Just like in concrete,
>> cob is stronger the drier the mix. But I've noticed when I mix dry I
>> feel like the clay doesn't bond and will not form a solid structure.
>> If I pack it, it bonds, it just doesn't seem to stay together as
>> well. Can anyone enlighten me on this?
>> Damon
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