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[Cob] cob/straw bale sandwich wall

Bill Christensen billc_lists at
Mon Jul 21 17:23:16 CDT 2008

At 11:05 AM +0200 7/21/08, Robert Alcock wrote:

>We are looking at using a load-bearing hybrid straw-bale/cob wall
>consisting of straw bales laid on edge (35cm thick) with cob either side
>(10cm interior and 5cm exterior). We plan to pour a concrete bond beam
>at first floor and roof level of this two storey, 100m2 per floor, house
>and study centre.
>Does anyone on the group have experience with this "sandwich" type of
>wall construction?
>Is it better to put up the straw bale wall first, put in the bond beam
>and roof/floor beams, and then do the cob? The advantage of getting the
>roof on quickly is obvious, but then I'm concerned that the straw bales
>alone won't be strong enough to hold the roof.

Unless your bales are extremely loose they should hold up the roof 
just fine - assuming that everything is well braced against lateral 
movement.  I can think of at least one house that wasn't and the 
whole thing twisted and racked a fair bit, which isn't easy to 
repair.  In fact, you'll want the bales pre-compressed before you 
begin cobbing to prevent cracks from settling.  That weight will also 
stiffen the bale wall considerably, reducing any wobbliness and 
making it easier to cob against.

>If the cob and straw bale go up together in parallel,  the bales will be
>exposed to the weather for a lot longer (at least, the top of the straw
>bale wall will be) and we get rain regularly here, even in summer.
>In this case, on the other hand, we need to think about what will happen
>to the dry cob and straw bales when the load of the (green) roof and
>soil are applied - since dry cob is virtually incompressible it will be
>taking the whole load of the roof on its own, without the straw bales

The general theory is that it's the plaster that's carrying most of 
the weight in plastered strawbale construction.  See discussions on 
the SB-R-Us list at yahoo groups for more details.

I'd tend toward building the bales and roof first, then cobbing 
after.  I'd also suggest spraying a clay slip on the straw before 
cobbing to help adhere the two materials and to insure that your 
cobbing doesn't leave any air gaps with un-mudded straw.

>A compromise would be to build one storey of straw bale, pour the
>concrete bond beam, then apply the cob up to that height, relying on the
>weather protection offered by the concrete, and also the preloading that
>the concrete (+possibly roof/floor beams) provide.
>Any other guidance would be most welcome.
>Thanks a lot,
>Coblist mailing list
>Coblist at

Bill Christensen

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