[Cob] Re: cob domes?

[Shannon C. Dealy]

On Sat, 14 May 2005 Raduazo at aol.com wrote:

[snip]
> My reason for asking this is that I have a theory regarding cob domes.  The
> problem with a cob dome is that at the peak of the dome the surface is  nearly
> horizontal. This means that water and snow will set on this mostly  horizontal
> surface for long periods of time and soak in, and when the dome  collapses it
> will be only the horizontal center that  collapses.

This is not the only issue, the strength to weight ratio of cob for an
application like this makes it ill advised if there is even the slightest
chance of an earthquake, and the larger the span, the greater the danger.
There are also other structural issues to be dealt with such as lateral
loads on the walls created by the roof.  These stresses can be massive,
potentially even greater than the vertical loads of the roof itself
depending on the design.  Unfortunately these are not easily described (at
least not by me) without visual aids.

[snip]
> Besides the shape of the dome we can also encourage water to move on down
> the trail by making the dome surface very smooth and treating it with a
> hydrophobic material like boiled linseed oil.

Every sealant I can think of for this purpose degrades seriously over
time when exposed to ultra-violet light (sun), and relying on adequate
maintenance for something like this is an invitation to disaster.

[snip]
> Hopefully a small shed will not be as heavy or as life threatening as a  full
> sized structure, and it will not be occupied during rain storms, but if
> smaller structures work out who knows. A zero-cost waterproof roof would be a
> nice thing if we could trust it.

If you really want to play with earth for a roof, why not try something
safer to start with, perhaps a wooden framework with lathe, then plastered
over with a thin layer of cob.  Try your sealants, and other ideas on this
first.  Keep in mind, even this will be a massively heavy roof, heavier
than a tile roof and I have seen serious structural deformation on
buildings roofed with tile that were not properly designed for the weight.

If you want to play with an existing earthen roofing technique, there is
an unfired clay tile approach I have been meaning to try (German I think),
I believe I saw it in the book "Shelter" possibly around page 70-80 (can't
be sure of that, it's been at least a year and the book isn't handy right
now).

FWIW.

Shannon C. Dealy      |               DeaTech Research Inc.
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