[Cob] strength of cob arches

[Dulane]

The design (minus an arch or two)looks plenty strong enough to me to support
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 is
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 and
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 vermiculite
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

-----Original Message-----
From: coblist-bounces at deatech.com [mailto:coblist-bounces at deatech.com] On
Behalf Of Predrag Cvetkovic
Sent: Wednesday, July 16, 2008 1:12 AM
To: coblist at deatech.com
Subject: [Cob] strength of cob arches

My friend and me wanted to build a small round garden building about 3m in
diameter in his yard in our small town- primarily to gain some building
experience and then to have some space for sitting.
But later, to avoid possible problems with building codes, we decided to
modify the idea of building to be only with holes for doors and windows: we
will not build doors and windows in. Maybe during winter we could put some
protective covers on holes and because there will be a hearth, it could be
possible to sit inside.

A small model (1:10 ratio) pictures explain better then words, you can see
it at my friends link:
http://picasaweb.google.com/miodrag1963/Cob
We think of a stone foundation and stone arch for the front door. And a
gentle slope reciprotial live - green roof with a thin layer or earth (up to
5cm).

So, finally, our question is: are cob arches enough strong to bear the roof
in our garden house? Should we use wooden pillars as a support (from the
foundation to upper wooden beams)? We will put these upper beams on the
wall, to bear the roof, but in the case of round building shape it is more
difficult to do that then with rectangular one. Or maybe to put few wooden
posts inside the wall? We could build thick cob walls (about 30cm) but would
it be enough strong because in a big part of a building we will have only
cob arches and cob pillars instead of cob wall?
Hope that my question was clear enough. Any comment and suggestion are
welcome.
Predrag



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