Rethink Your Life!
Finance, health, lifestyle, environment, philosophy
The Work of Art and The Art of Work
Kiko Denzer on Art

[Cob] update on trench issues

drub drub at
Tue Aug 19 21:54:52 CDT 2008

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!

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
> _______________________________________________
> Coblist mailing list
> Coblist at
> _______________________________________________
> Coblist mailing list
> Coblist at