howto cobstruction

[Shannon Dealy]

On Fri, 30 Aug 1996, M J Epko wrote:

>         A distillation of the technique and general concerns outlined by
> Marna Meagaen in Backwoods Home Magazine #39, May / June 1996. Claims and
> statements given are those of the article's author. (My own comments are
> presented parenthetically.)
> 
>         Cob is a mixture of dirt, sand, water, and straw; it yields very
> solid, "remarkably" water-resistant walls - though generous roof overhangs
> should be given, and the stone / concrete foundation should rise six inches
> above ground level as a precautionary measure against the hazards of rain.
> (Same as sb; those things are just good building practice, far's I'm concerned.)
> 
>         The cob recipe should be 50 - 80% sand. Test the site soil with that
> old standby method: take an average handful of dirt & throw it in a mayo
> jar, then add water to almost fill it. Shake shake shake. Settle settle
> settle. After a while you'll have visible layers of stuff - from the bottom
> up: sand , silt, clay, water. If the bottom (sand) layer is 50 - 80% of the
> total volume of the three non-water layers, you're probably good to go.

It should be noted here that the clay layer should be at least 10% and
no more than 30% (20% would be better) for the soil used in the mix.  Since 
soils often don't meet this requirement, it may be necessary to find soil 
somewhere else (possibly elsewhere on the lot you are building on) which 
can be mixed with the soil at the site to get better proportions.  At Cob 
Cottage Company's site, they have a very high clay content and mix three 
parts soil to two parts sand.  On the other hand there is virtually no clay
in the soil where I am.

> 
>         If sand has to be added, get concrete sand, which is angular - *NOT*
> beach sand, which is rounded.
> 
>         Throw a bunch of dirt on a big tarp: 15 to 20 shovels-full. If sand
> has to be added, deal with it & mix everything thoroughly while dry. (You've
> already determined if & how much sand you need to add.) Hop in and add water
> *gradually*. Squoosh around for a few minutes 'til everything's evenly
> moistened & sticky. Then add straw... no more than a bale flake, the article
> says... still squishing around in it. Roll roll roll. Knead knead knead. A
> big nasty pancake appears, "slightly more crumbly than modeling clay."
> 
Before adding the straw, the wet mixture will have a look and consistency
very similar to freshly mixed cement.  When adding the straw, add it slowly,
sprinkling it lightly over the mix, work it in, then pull one end of the 
tarp toward the other in order to fold/roll the mixture up, add more 
straw and repeat until the mixture is very firm.  I usually gauge it by 
rolling up the mixture and then stand on the bare mix, when it's done,
your feet will only sink into the surface a little bit (1/4" - 1/2" ?).

>         The walls are built with handfulls of the glop, adding not more than
> a foot of height per day.

Unless you are in a hot dry climate, you probably won't be able to add this
much each day, before starting work each day, check how firm the walls are.
You should also check how plumb the walls are before starting each day, since
you don't want to be adding cob to a section of wall that is leaning, you
will just have to trim it off later.  Every pound of cob you have to trim
is a pound of cob you didn't really need to make.  Trimming is usually done
with a machete (get one just for trimming cob, it will become very dull and
there is no point in sharpening it).

> 
>         The article recommends (and I second the notion) that a few test
> runs be done to get a feel for it and check various mixes for strength. Make
> some blocks, it says, let 'em dry out, then toss 'em around. Jump on 'em.
> Whack 'em with a stick. Pick the strongest mix. (I have an adobe book from
> the late 40s that says that adobe test blocks should be plunged in water as
> an additional test. Of course, they recommended stabilizing the mix with
> asphalt emulsion, too...)

This is a must, since every soil combination is unique, the best mix will
vary quite a bit.  You will also want to check for signs of cracking in the
test bricks, this is usually an indication of either to much clay, or water
in the mix (it shrank to much as it dried), though this can also be caused
by drying to fast in a hot dry climate.  I am not sure what the water
test was supposed to prove.

> 
>         Wall dimensions are given as 15" at the foundation for 1-storey
> buildings, 20" for 2-storeys. (I'd go wider, personally; but that's a
> climate thing. Two feet minimum for me.) The interior of the wall should be
> plumb, it says, while the exterior should taper up at 2 in 36. It states
> that the taper increases the walls' strength, but it doesn't say how. (Seems
> to me that it wouldn't necessarily strenghten the wall, but rather wouldn't
> weaken it. Like removing the interior of a triangular chunk of wood doesn't
> make it less strong; it just removes unneeded bulk.)

The taper essentially creates a buttress as an integral part of the wall.

> 
>         The walls are hand-packed freestyle; no forms. Take a handfull and
> push in into the preceding layer using fingers and thumbs or a stick.
> "Massaging" is a word that's used. Don't smack it, it says - be firm but not
> mean.
> 
If you use a stick, make it a blunt one about an inch in diameter, the idea
is to push some of the cob (particularly the straw) from the new layer down 
into the preceeding layer so that it locks the two layers together.  I 
always use a stick, the skin on my thumbs cracks and bleeds otherwise
(though maybe the blood will help as a binder :-)

>         Again, a foot a day... or it might slump under its own wet weight.
> At the end of the building day, poke holes a couple inches deep, four to six
> inches apart, with an inch-thick stick, all over the top - the holes give
> something for the next day's work to grab onto / into. Cover the walls with
> some inches of straw & moisten it a bit; unless it's gonna rain - then cover
> it with a tarp. (I'm not clear on whether the tarp is instead of the straw,
> or in addition to it.)
> 
This person must build in a desert, we put in lots of holes here in Western
Oregon, and leave it uncovered so it will dry enough to support the next
day's layer of cob.  It should be noted here that it is important that the
cob not dry to much between applying layers, since new cob does not bind
nearly as well to dry cob.  If the cob dries out, you can soak the top
of it several times over a period of a few days prior to adding the next
layer, this will improve the bond, but it will never be as good as it
would have been had the cob been added while the previous layer was still
a little soft.  For this reason, it is important to always leave the top
of an unfinished wall rough with lots of holes in it, since this will 
provide a mechanical bond between the layers regardless of how dried out
the wall gets before the next layer is added.


>         (Like with sb,) the design needs to be well-considered so that
> attachment points for shelves, etc, can be taken into account during
> construction.
> 
>         Window & door frames can be set directly against / into the cob with
> nails pounded into the outsides of the frames extending into the cob as it's
> placed. (Follow that?) Doing it this way requires lintels. (I'd probably be
> tempted to run the frames full-height - foundation to roof - the horizontal
> member perhaps castoff 2x4s - with the cob fully encasing the 2x4s. The
> framing would attach to those verticals, and there shouldn't be any settling
> or de-squaring. But whaddo I know?)
> 
Window glass can be embedded directly into the cob without frames, lintels
or any other protection, it simply requires careful handling of the cob,
and arching of the cob above the window (so the glass doesn't have to
support any weight), though you have to build up the top slowly.  If the
window should break, you can simply knock the rest of the glass out, carve
out the cob on one side of the window, insert a new piece of glass and cob
it into place.

>         The article mentions a bale / cob hybrid possibility, using bales
> for the north wall & cob the rest of the way 'round. (Again, a climate
> thing.) It's suggested that the joining can be accomplished by pounding
> stakes horizontally into the bales & cobbing around the stakes. (I think I
> might like Frog's buttress idea more.)
> 
Cob Cottage has a couple of these types of buildings in progress, though
I can't remember how the bales are tied to the cob.

>         The author gives a plaster recipe of equal parts clay, sifted dirt,
> and horse poop (poop du jour deja vu for you sb list old-timers.) Different
> clays will give different colors; lime can be used to tint. Mix the
> ingredients, add some water. Throw it by the handful at the wall, or use a
> trowel. Or don't plaster at all, it says.

You don't have to plaster, though the wall will probably look better to most
people if it is plastered, and weather will erode the wall some if it isn't,
(let your grand-children build their own walls :-)

> 
>         The roof beams can be incorporated into the cob walls (as I imagine
> that vigas are incorporated into adobe.) Cob itself is too heavy for use as
> a roof.
> 
Pretty much any roofing system can be used by just embedding attachments for
the roof into the walls.



Anyway that's my two bits.

Shannon Dealy
dealy at deatech.com