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[Cob] Clay (Bill Christensen)

Bernhard Masterson bernhard_masterson at
Sun Nov 2 15:55:40 CST 2008

Hi Phil,
   As an experienced cob and bale cob builder I have a couple of questions for you.  First if you have not yet actually built anything with cob why are you considering such a large project for your first?   Depending on the size of your work crew the walls could easily be a two year project.  I would recommend starting with something much smaller, around a 120sqft.  Such a small structure can easily be built in a couple of months and would provide valuable experience, a warm dry spot to stay on site, and time to observe your property and plan. 
    Do you plan to taper your walls? Load bearing walls need to be a foot thick at the top so if your walls are eight feet tall they really only need to be about 18 inches thick at the base.  This will reduce your total cob volume.
    Also your concerns about building cob in a site that does not have good passive solar access is warranted.  I would recommend you consider straw bale as an alternative.  

- Bernhard 

____________________________________bernhard_masterson at

Natural building instruction and consultation

> Message: 1
> Date: Wed, 29 Oct 2008 23:00:40 -0700
> From: "philmoulton" <philmoulton at>
> Subject: [Cob] Clay
> To: <Cob_Building at>, <coblist at>,
> 	<coblist-bounces at>
> Message-ID: <E6D017CABB3941B3B9FD218C38967544 at phil>
> Content-Type: text/plain;	charset="US-ASCII"
> <
> d=464/stime=1225286169/nc1=4507179/nc2=3848640/nc3=4025338>   I know living
> in Portland this even to me sounds strange, as all dirt in Portland is
> mostly clay.
>  However we are moving,to Otis (just east of Lincoln City and  the ground on
> the property does not appear to be clay like.
> Is there a source for good consistent clay
> We are planning on building a cob home that  my probably incorrect math says
> I need a LOT of clay.
>  3 exterior walls 24 ft long  one interior wall and the entry way figure
> another 26 ft.
> figure also  8 ft high and 2 ft thick (may be able to go 18 inches tho...) 
>  That comes out to 1568 sq. ft. of cob, figure it rounded off to 1600sq ft.
> Although Cob is a mixture of sand and clay and straw it still it means I
> need a lot of clay.
> To be consistent I should have all my materials on-site before we start. 
> Getting a consistent mix is important, so getting it in buckets from here or
> there with varying content of clay is not sound thinking.
>  The lot we have will need some excavation prior to pouring a foundation but
> I do not feel there is near enough "dirt" to be worth while,As most of the
> foundation work will only be 1-2 ft down and the interior will be a adobe
> com[onsite floor of some sort so I may actually have to add material to get
> it all even.
> Also the lot we have is not conducive to Solar heat as there is a fairly
> large hill to our Southeast. There would be some solar gain during high sun
> but not much. the hill is just low enough to allow a dish to clear it but
> not by much so it could be as much as 30 degrees high. 
>  Being in the coast range should we reconsider Cob and think more Straw bail
> or a more clay/straw mixture for thermal insulation.
>  Btw We are still looking for someone who might be interested in "mentoring"
> us a bit who does have hands on experience building a Cob home.
> Phil and lilpony 
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