Cob: "Me Tarzan...."

[Shannon C. Dealy]

On Fri, 12 May 2000, wgnmaker wrote:

> Can cob be used for a tree house (in this case will be almost 150 sq
> ft!)  What is the dead load for cob?  Thanks

My initial reaction is no-way, but it's better not to assume so . . .

I don't have any specific numbers handy for the weight of cob, and given
how much mixes and soil compositions vary, they probably wouldn't be more
than rough order of magnitude anyway, so I'll make some up :-)

According to my handy-dandy pocket reference guide, excavated dry loam is
about 78 lbs/cubic foot, and excavated wet earth is 100 lbs/cubic foot.
Assuming 10% straw and that the relative weight of straw is insignificant,
gives approximately 90 lbs/cubic foot for wet cob.  Assuming thin, short  
walls at 10 inches thick and 6 feet high, done in a circular shape,
enclosing a 150 square foot area gives about 217 cubic feet of cob,
assuming a couple of windows are in the wall, call it 200 cubic feet,
gives about 18,000 lbs (unless I messed up the calculations somewhere).
How strong is your tree? :-)  Of course you would have to construct a
platform to support all this cob, and it doesn't include a roof, or
anything inside the building, but once the cob dries, it will be something
like four thousand pounds lighter, which should make up for some of the
extra weight.

Frankly, I don't know much about the strength of different types of trees,
but this appears to me to be way off the deep end, even if you start
playing with thinner walls (dangerous in an exterior wall), bigger
windows, doors, etc. If you like the earthen look in a tree house, I would
suggest going with a wood platform with a wattle and daub structure -
basically sticks woven together to create the wall framework, and then
plastered with a cob like mixture. it's much thinner and therefore much
lighter.


Shannon C. Dealy      |               DeaTech Research Inc.
dealy at deatech.com     |          - Custom Software Development -
                      |    Embedded Systems, Real-time, Device Drivers
Phone: (800) 467-5820 | Networking, Scientific & Engineering Applications
   or: (541) 451-5177 |                  www.deatech.com