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The Work of Art and The Art of Work
Kiko Denzer on Art

[Cob] strength of cob arches

Predrag Cvetkovic predragcv at ptt.yu
Thu Jul 17 17:23:03 CDT 2008

Hi Dulane and All
Finally to answer your kind email with many useful suggestions and comments.
Thanks a lot again.

I think here in Serbia it is possible to build a small house in a yard but
the fee we should have to pay is too much high. So, we can not think about
that now. One reason more is that there are already two houses in my friends
yard. One is pretty old house, maybe older then 120 years (I think it is
rammed earth) that is under protection of law, and the other is a house
where they live, about 20 years old. There are also a shed, a garden,
flowers, trees...

Regarding the strenght of cob arches my friend suggest to put pillars to
avoid possible problems with this (big) number od arches. We would try to
fit beams into cob as well as possible.

Yes, you are right the pitch of the roof is too much but it is just on the
model - a real structure will have a roof with more gentle slope. We think
about not openable skylight in the middle of the roof because reciprotial
roof allows that.

The idea of thatched roof is really very good and when I showed your
previous message to my friend he started to think about that idea again. In
fact we already had that idea but because of many additional costs for
getting reed here we could not think about that more. Today while walking
through the town, we observed plants in gardens and among these some of
sedum family.

Thinking about waterproof roof doesn't make me happy  but I hope we will
find a solution. What about using tar or something similar to make it
waterproof if we punch holes?
The idea with fishnet and straw is very interesting too.

We could use perlit (available here but will see about vermiculite too) to
mix into the soil around chimney above the roof. I think we will not use a
stovepipe above but will find an other solution. And about the inner
diameter of the chimney - we think about 30cm (one foot) at least, but more
probably that will be 35-40cm.

Thanks for your good wishes. We will share the pictures with anyone

Dulane wrote:

> The design (minus an arch or two)looks plenty strong enough to me to
> your roof. I don't know what your building code calls for, but we use an
> international code here, and as long as you don't intend to have someone
> 'living' inside your dwelling (i.e. cooking and toilet) you can build a
> structure up to 6 meters or US 200 sq ft. Many folks would use such a room
> for an art studio or playhouse.
> You can use the structure (in my area) for a spare bedroom.
> Your roof design should definitely support 5cm living roof, but the pitch
> too much. You may have to find some fishnet and straw to weave in there to
> hold your organic matter. Makes me wonder about cellulose batting. (Which
> some companies make with seeds embedded, but you could try to create your
> own.)
> Look to find sedum mosses for the living roof. They can take the weather
> have short root systems. Or perhaps you could create a thatched roof. Tall
> reed grasses are used. Here, canary reed grass is considered invasive, and
> people would probably be glad if you asked them if you could harvest it.
> Your chimney could also be extended, and perhaps adapted for regular
> stovepipe, to keep it above your living roof. I poured a lot of
> into the soil around my stovepipe where it comes through the roof. I've
> checked the area when I had a roaring fire, and it stays cool.
> Make sure your roof is waterproof. Even heavy mil plastic sheeting can be
> used, as long as you can figure a way not to punch holes in it.
> Good luck, cute project.
> Dulane