[Cob] cob/straw bale sandwich wall

[Bill Christensen]

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
>helping.

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,
>
>Robert
>www.lesspress.com
>
>
>
>
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-- 
Bill Christensen
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