[Cob] RE: Coblistqueries

[Shannon C. Dealy]

On Sun, 1 May 2005, Bob & Lorraine wrote:

[snip]
> We finished up making our cobs fairly wet and patting them onto the
> wall.  I hope they don't shrink too much and cause cracking, but it's

I'm not sure what you mean by "patting" them, when applying cob, you
should be "knitting" them together by pushing your thumbs repeatedly
through the new cob and into the preceeding cob layer so that the straw
fibers from each new piece will be locked into the layer below.

> all part of the experiment.  Has anyone any idea how long a cob wall
> normally has to stand in reasonable weather to show up any imperfections
> in construction?

I'm not sure what you mean by imperfections, when properly applied, the
wall will look and behave as one contiguous piece, and the only
imperfection of any importance that I can conceive of showing up would be
a weakness in the bond between layers during an earthquake.

> I have a number of queries I'm hoping you can help with:
>
> 1:  If cobs are made the consistency of bread dough (I make my own
> bread, and bread dough is workable but much, much drier than the cobs we
> made), do you have to wet the outsides of each to make it adhere to the
> wall and its neighbors?  If not, how do you go about it?

It's been a long time since I last made bread, but I think the cob should
be a fair bit stiffer.  All you should need to do is knit the layers
together as specified above.  If you don't want to use your thumbs, you
could use a blunt stick roughly an inch in diameter to push the fresh cob
into the preceeding layers.

> 2:  Cob floors - how many of you have made them, how successful were
> they and what did you put under them?  Where we have been walking
> backwards and forwards over cob mix that didn't get used and dried up,
> it is as solid as can be, without any cracks.

Cob floors work fine, generally you will put several inches of drain rock
underneath to create a moisture break between the earth and the cob floor.
Do you have a copy of "The Hand Sculpted House"?  It should help you with
some of this information.

> 3:  Finishing off any kind of earth floor:  why boiled linseed oil?  Why
> not vegetable oils that are used for cooking, especially re-cycled
> cooking oil that has been used for hotel chip cooking? Has anyone tried
> it?  I'm wondering if there is some special reason why linseed oil is
> always mentioned (plus an addition sometimes of tung oil - whatever that
> is) and not other plant-based oils.  Or is a tradition that everyone has
> followed without experimenting with other oils?

You must use a "drying" oil.  Linseed oil will dry to form a solid
surface, cooking oils will not.  The linseed oil also provides a great
deal of binding strength when it dries, and is the traditional
basis/origin of linoleum (hence the name), which in it's traditional
form is a fiber matt that has been coated with layers of linseed oil

> 4:  Has anyone suggestions for inexpensive alternative roofing they have
> found successful and not TOO time consuming?

So far, the least expensive, least environmentally hostile roof I have
encountered seems to be the painted or enameled metal roofs.  They last
50 years, and are recyclable.  The more "natural" roofs like thatch,
slate, and shakes are generally much more time consuming and the materials
can be both difficult and expensive to come by.  In addition, for
permitted structures, thatch and in many areas, shakes can be difficult to
get accepted due to fire concerns.  Another good option is the
living roof, though the weight does require a significantly stronger
roof structure than a metal roof would.

> 5:  Has anyone tried laying a framework onto a level, plastic-sheet
> covered area and tried making a wall on the ground, drying it then
> standing it up and incorporating it into the building?  I'm thinking of
> doing an experiment of making cob panels (or perhaps it should be called
> rammed earth, using a cob mix) to make a bench or shelves to see how
> successful this method would be. I love cobbing, but in our situation of
> having to travel miles to do a layer of cob, repeated each day, it's
> proving to be not a very viable method of building.

I don't see this as being a viable approach:

 - You are likely to break the wall while trying to stand it up.
 - You will probably need a crane to stand it up.
 - For adequate structural strength and maximum stability, your building
   should have a single continuous wall, not wall pieces.  If the walls
   are not one piece, there is a significant risk of "panels" falling
   over and killing someone in the event of an earthquake.
 - Walls should be curved for maximum stability, which would be difficult
   to do in panels.

If you want to do panels, I would recommend going with leichtlehmbau (sp?)
also known as "light clay".  Which is a (non-structural) straw-clay panel,
however, this technique works best done on site, since it is primarily an
infill technique, and you would need some sort of structural building
framework to mount it in.

> 4:  Are any of you cobbers out there in Australia?

I will be for a couple months probably starting sometime in November, but
then you already know that :-)

Shannon C. Dealy      |               DeaTech Research Inc.
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