Cob: I'm new and adequately poor

[Shannon C. Dealy]

On Thu, 4 Oct 2001, Kate Kamper wrote:

> Some of my questions might best be answered by Canadians.

I can't answer your Canada specific questions, but as to the rest . . .

[snip]
> b) Is cob really load-bearing enough to hold up a roof?

Yes, properly designed and constructed cob walls can support any roof
including the heaviest ones you are likely to encounter such as tile
and living roofs.

> What are the beams used for? I saw a site that
> had rebar beams on the floor, and some with beams
> around doors and windows, but didn't see any that had
> beam framing.

Generally if you want to use conventional doors and windows, you must
embed some kind of framing into the cob in order to have something to
attach the doors and windows to.  Of course, there are unconventional
means that can be used as well which eliminate the need for this framing
around doors and windows.  Some people also like the decorative effect
that this framing around doors and windows provides.

> If you were to use beams for framing, would it decrease cobs walls
> load-bearing capacity, or mightn't it cause the wall pieces to fall out
> of the frame? (My idea was to have the frame's horizontal
> beams of 1'X 1' thickness above and below, with
> vertical beams (1'X 1')placed every four or five
> feet(eight feet tall), with the rectangular bits in
> between being cob. Could I make the beams less in
> width if I retained the 1' depth? If so, how much can
> I shear off?)

I am not sure I understand what you are suggesting here, but you want the
cob walls to be constructed as one continuous piece, if for some reason
you must use wood framing, I would build the complete cob walls inside or
outside of the framing, and if desired or necessary, make the wall thicker
where it passes each piece of framing in order to wrap around it.

[snip]
> d) Would the R-value of cob be 12 at 1' thinkness?
> Because that's not remotely acceptable (and I really
> don't like straw bale at all, so don't go there). 

R-value is still something of a debate for cob, I have heard number
running from 0.25 to 1 per inch.  In your area, you will need to wrap the
exterior of the building in insulation, some people are experimenting with
wrapping cob in strawbale, then plastering the outside.

> e) Why does it take four years to build a cob house?
> If you can build approx. 6" per day, wouldn't it only
> take a couple of months? (How long do you have to wait
> before you can move up the forms and add more cob?)

No forms are used, and it doesn't take four years, it can be done in a few
weeks (if you are building small enough or have a large enough labor
pool), though if you are doing it primarily by yourself, it will probably 
a few months.

> f) Is it advisable to build the roof before the walls,
> if the walls are not the (primary) load-bearing
> element?

Generally this is only done if there is a concern about weather problems
while building the cob walls.

> g) If you use beam supported cob for your basement
> walls, should they be thicker than the cob walls above
> the foundation? Would damp-proofing and extruded
> styrene foam at 2" be enough to to insulate and keep
> the basement dry? Is styrene foam considered a "bad
> thing" by yous guys?

> I know there was some discussion of straw sandwiched with cob
> (unacceptable because of hay rot),

This is simply a variation on standard strawbale building technique,
there is no reason to believe that rot is likely to occur providing that
the technique used for building the wall which the bales are embedded in
provides for relatively quick drying of the cob that the bales are
attached to.  Personally, when I do my sandwich walls, I will probably
build the interior cob wall first, allow it to dry, then wet the exterior
(like you do in preparation for plastering) and use a thin layer of cob to
mortar the bales to the exterior of the wall as they are stacked.  Then
plaster the exterior of the bales with a heavy cob plaster just like the
exterior of any other strawbale structure might be done.  Once the
exterior plaster has dried somewhat, I will probably apply additional
layers for better protection from the weather.  With this approach, the
interior wall will most likely draw the moisture out of the bale/cob
mortar layer faster than a normal interior plaster would dry, and the
exterior would dry at about the same rate as on a conventional strawbale
structure, so rot should not be anymore of an issue than it is with a
purely strawbale structure.

> but could you sandwich styrene foam in the
> walls?

Yes.

> What is the comparative R-value of concrete vs.
> cob for basement construction? I want a root cellar,
> so I don't mind a little basement cold and damp within
> reason, but I would prefer the cold not come through
> wood flooring of the first floor.

Generally, I wouldn't recommend using cob for a basement, since if any
flooding occurs or the soil is not properly drained you could end up with 
standing water in contact with the cob, which could collapse your whole
house.

> h) How do yous guys get away with building this stuff
> without permits? Won't somebody notice me building or
> ask questions when I try to sell my (totally
> imaginary) property? 
[snip]

In some places, there are no building codes or permit requirements,
in other places checking up on illegal construction is lax, and
in others, if they catch you, they can and will make you tear it down.
I don't recommend building without permits if they are required, but some
people apparently feel that it is worth the risk.


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