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[Cob] How to get a straight wall with cob?

Shannon C. Dealy dealy at deatech.com
Thu Mar 3 11:40:54 CST 2005


On Thu, 3 Mar 2005, Bob & Lorraine wrote:

[snip]
> What should a good mix look and feel like?
>
> Ours was patchy with some parts of it having a smooth, plasticiny feel,
> some lumpy/gritty although wet, and some being a bit dryish and rubbly.
> We tried various methods - forking whatever we came to onto the
> foundation and patting it into shape, pressing it into cobs with our
> hands then incorporating it onto the wall, and doing more mixing in the
> mud pile before forking it onto the foundation.

You should mix the sand clay and water until they have a uniform
consistency, only then should you add the straw.  This means getting
in there with your feet and breaking up all the clumps of clay,
rolling the mix over repeatedly, so you don't get blobs of dried
material on the bottom.  Many people will first dry mix the sand and clay
to get a fairly even distribution before even adding the water, though
whether or not this is just making extra work for little benefit probably
depends on your mixing technique, for me working alone doing mixing really
large (35 gallons of sand and clay) batches on a tarp, I find it to be
pointless.  Once the straw goes into the mix it is MUCH harder to break up
lumps and get the other ingredients to a uniform consistency.

As far as look and feel, when it's done, immediately after rolling the
batch (assuming you are using the tarp method here), other than the color,
it always reminds me of a side of beef.  If I stand on top of this
freshly rolled batch and it's the right consistency, my feet will
generally sink in about 1/2 inch (adult male, 190 lbs, size 12 feet).  Of
course there are times when you may want a wetter or dryer mix, so this
is only approximate.  If the mix isn't stiff enough, add more straw and
keep mixing, it helps alot if you learn to just use your heals when you
are treading the mix towards the end, as the concentrates your weight on a
smaller area allowing better mixing to occur.  I generally recommend that
people do NOT "stomp" the mix when starting out, since if there are rocks
in it you can seriously bruise or cut your feet particularly if you are
just starting out, and this will make it much harder to work until it has
fully healed.  Once you have been doing it for a while, your feet will be
tougher, and you will also develop a kind of reflex in your feet so that
when you encounter a rock, you immediately reduce the force you are
applying to that foot.

> How do you ensure that the wall is straight-sided, not rounded down at
> the edges, and that it does not gradually become narrower as you work
> upwards?

Basically, you don't.  What you do is make sure that the cob you are
putting on sticks out a little on each side, and then trim it periodically
(usually the start of the next day's cobbing is a good time).  The cob you
trim off can be added to batches of fresh cob so it's not going to waste.
Often one side of a wall will be tapered to act as a buttress, and you
achieve this by trimming also.  The basic rule is always go a little to
wide, because it is much easier to trim excess from the side of a wall
than it is to put more on the side (with wet cob, it's not to difficult to
get more to stick to the side, but generally it will be far inferior from
a structural perspective than the cob that went on from the top which
bonds much more deeply into the wall).

> How do you manage to walk on it without causing it to bulge at the
> bottom? (We didn't try)

There will be some bulging near the top where the freshest cob is, but
lower down, the cob should be dry enough to take your weight without
bulging, otherwise it is unlikely to be able to take the weight of more
cob without bulging either.  Usually it is a good idea to walk on the
first layer of cob you put on your foundation in order to make sure that
it completely connects without gaps or air pockets.

> How high can one go in a layer, and how long should one leave it before
> adding the next round/layer?

Generally one foot is considered the limit for a day, though most
projects don't have sufficient people and/or aren't sufficiently small to
allow for this much to be applied.  As far as how long, this depends on
your weather and humidity.  When it is hot and dry in summer, it's no
problem to add a foot every day, under other weather conditions it will
vary.  If you are crazy like me :-) you can build in wet rainy weather
(under some kind of roof or cover), but you may be limited to a foot every
couple weeks.  Mostly it's a matter of experience to guage how high you
can go in a day, the limitation is how much bulging the weight of the
additional cob causes in the cob below.  NOTE: there is nothing wrong with
bulging in the wall, you just have to trim it off, the main issue is that
the more you have to trim, the more work you are creating for yourself,
(since you had to mix and apply all that cob you trimmed off), so it's
mostly a question of diminishing returns on the amount of labor you put
into the wall.

> Do you have any suggestions to make the job better, safer, quicker, etc?

Nothing immediately leaps to mind, but I have made alot of them over the
years, so if you read the archives you will probably find some useful
tidbits.  Many people miss the fact that there are two sets of archives,
one for the old list (before I made a major software change) and one for
the new (after the change), one of these days I will figure out how to
merge the two:

     http://www.deatech.com/natural/coblist/coblist-web/

     http://www.deatech.com/pipermail/coblist/


Shannon C. Dealy      |               DeaTech Research Inc.
dealy at deatech.com     |          - Custom Software Development -
                      |    Embedded Systems, Real-time, Device Drivers
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