[Cob] Joints between cob and wood

[Jane Mondrup]

Hi Robert

We have a lot of wood-cob joints in our house, mostly wothout too much 
cracking problems.

When we build the structure, we put LOTS of nails and screws on 
differing sizes in the wood to connect it to the cob - I think that was 
an important factor.

But it is probably also wery important how the cob dries. If you build 
slowly (as we did most of the time), the cob dries cradually, with less 
visible shrinking (and cracking-away-from-things) as a result. If you 
build fast and effectively, the cob will dry "en bloc" and seem to 
shrink much more.

Of course the tempreature and the humidity is an important factor too, 
as are the amount (and kind) of clay in the cob. The more clay, the more 
shrinkage. We have tried to use as much straw as humanly possible.

And then there is the difficulty in determining when the cob is 
completely dry and ready for plastering. This actually takes much longer 
than you think - we may be talking months here, especially if you are 
building indoors in the winter (as we did in the end).

If the cob isn't completely dry when you plaster it, it will continue to 
shrink, making cracks in the plaster. We had absolutely no cracking when 
we plastered "old" walls that had dried for a whole year, but when we 
plastered the recently made walls, we had quite a lot of cracking.

So my suggestions are:

1: Use lots and lots of nails and screws on the wood where you are still 
building cob. Put a lot of straw in the cob.

2: Wait as long as possible with the plastering.

3: In the places where you have the nasty cracks, hack away some of the 
cob around the wood and add some screws or nails to the wood. Make sure 
that you leave the cob surface uneven, and maybe insert some screw/nails 
into the old cob also, to enforce the attachment of old an new cob. 
Repair the crack with new cob, which will have to be both strawy and 
sticky. Maybe use a little extra clay - you don't have to worry about 
shrinkage with small  amounts like this.

Hope it helps


Robert Alcock skrev:
> Hi everyone,
>
> Here at Abrazo House in northern Spain we have started building for 
> another season, hoping very much that our house will be habitable by 
> the winter... Anyone, especially experienced cobbers/plasterers or 
> people with experience of other finish work, is welcome to come and 
> help out in return for food and accommodation; if you think you might 
> be interested, please take a look at our web page, 
> http://www.abrazohouse.org.
>
> I have a question which relates to joints between cob and wood. There 
> always seems to be a nasty crack at the join between the two 
> materials, and we haven't managed to find any way of fixing that join 
> so that it won't be unsightly and dirty; plaster just cracks away 
> leaving the cob underneath exposed. Has anyone managed to find a good 
> solution for these spots?
>
> ¡Arriba la tierra!
>
>
> Robert
> www.abrazohouse.org
>
>
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