howto cobstruction

[M J Epko]

        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.

        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."

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

        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...)

        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 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.

        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.)

        (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?)

        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.)

        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.

        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.