Cob fibrous cement

[Gordon & Laura Solberg]

>To: jmcniece at juno.com
>From: earth at zianet.com (Gordon & Laura Solberg)
>Subject: fibrous cement
>
>        I'm the editor of Earth Quarterly and the source of all those
exclamation points about fibrous cement.  I am presently building a small
structure (16x16 feet) out of f.c.  I have infinitely more hands-on
experience with the stuff than I did when I wrote my first article, so you
can expect the f.c. coverage in the next issue of EQ to be far more nuanced.
>        First, I need to point out that our front cover said, "How to build
a paper house for 75 cents a square foot."  People have asked me, "Is that a
typo?"  Others think I'm conning them.  Others flat-out don't believe me.
However, there are many people, including myself, who are honestly intrigued
by this amazingly low cost figure.  (It should be pointed out that we are
talking about materials only, and the cost of the mixer isn't factored in.)
>        I visited Mike McCain and Sean Sands three times, spoke to them at
great length, watched them at work, helped them a little, and I had, and
still have, no reason to disbelieve the low per-foot cost they were quoting.
But face it -- what do you get for 75 cents a square foot?  No foundation,
no windows (Sean uses glass bottles), an exterior coating (on the house of
Sean's that appeared on the EQ front cover) that may or may not work.  I
would love to visit Sean's house the morning after a 3", all night rainfall
(yes, such rains do happen every few years in southern New Mexico).  If the
interior of his house is still dry, then this would be a significant fact
worth reporting.  If the interior is soaking wet, then some redesigning work
would be called for.
>        Sean Sands' work in particular is very innovative.  He has reported
a cost for his domes of 39 cents a square foot.  Like everyone on the
cutting edge, he is going to make mistakes.  But he and Mike McCain are
pioneering building methods that potentially have revolutionary
implications.  There are millions of people in the US (and billions
worldwide) who would appreciate the opportunity to have a snug shelter that
costs next to nothing.  Not everybody requires a house that is up to
middle-class standards.  If it's snug, dry, and keeps out the elements, what
else do you really need?
>        And I really enjoy the science fiction quality of people living in
structures made from the detritus of a decadent and decaying civilization.
>        A dry fibrous cement block will absorb water literally like a
sponge.  This is a limitation that can be worked with, however.  Eric
Patterson of Silver City, NM (he's the granddaddy of the present f.c.
movement) has done a lot of experimenting, and has found something that
really works:  He paints on a layer of Home Star silicone coating, lets it
dry for a couple of days, then covers it with Elastomeric coating.  (This
will of course increase your per foot cost, as will nuances such as a
concrete foundation, windows, etc.  My motivation for promoting 75c a square
foot was to say, "Hey, this is POSSIBLE.  There are people who are REALLY
DOING THIS.  Such a primitive house might not be your style, but you can
modify it with more expensive components, and it's STILL gonna be cheap.") 
>        A saturated f.c. block will retain its shape, even under load, but
you can pick it apart with your fingers if you work at it.  Eric Patterson's
dome was left uncoated for the first year before he finally coated it.  It
got completely saturated with water from heavy rains -- you could press on
the lower course of blocks and water would ooze out.  His dome went through
repeated saturation/drying cycles.  He says that he didn't notice any
structural problems.  But nobody would recommend leaving a f.c. strucuture
open to the elements year after year.
>        I would recommend building a f.c. structure on a concrete
foundation with a moisture barrier to keep water from wicking up into the
f.c.  Galvanized roof edge between the f.c. and concrete is advisable --
Eric has had problems getting caulk to seal the crack between these two
dissimilar substances.
>        F.c. is a remarkable substance.  Its limitations can be anticipated
and compensated for.  However, it's not for everybody, and it's not for
every climate.  It dries VERY slowly, and I think that in a cool, wet
climate with high humidity, newly-made blocks might take forver to dry, even
if they were covered from the rain.
>        Then there's the issue of the mixer.  Many people, especially
"sustainable types," don't particularly like machines.  With f.c., first
you've got to build your mixer, then you've got to spend many hours having a
close relationsip with it.  A mixer of my design (which will be described at
great length in the next issue of EQ, out in Sept.) costs approx. $350 using
all new components.  If you're going to build only one house, you've got to
add this $350 onto the cost of your house.  You can buy a lot of straw bales
for $350.  I think that ultimately, f.c. blocks will be manufactured in huge
numbers by factories that can take advantage of the economics of scale.
Perhaps a cost-effective and environmentally benign waterproofing agent can
be found that can be added directly to the slurry.  At present we have a
hardy band of f.c. pioneers, inventing their own mixers and mixing their own
slurry.  This is the equivalent of making your own straw bales by hand
before you build a straw bale house.  It CAN be done this way, but
ultimately there will be far more efficient ways of doing it.  (Eric
Patterson has recommended to one would-be f.c. factory that they make their
blocks slightly oversized, then mill them to exactly the same size.  That
way, they could be glued together rather than mortared with f.c. mortar,
eliminating the need for equipment altogether.)
>        I think Mike McCain has the right idea -- buy a large stock tank
(used if possible), use a car rear axle/differential (available for next to
nothing), and power it with a vehicle.  This gives you a very cheap, very
powerful mixer that lets you do some serious production.  But most people
I've talked to prefer a smaller, electric-powered design that costs a lot more.
>        Laura and I have time to mix only one batch a day (if that), so we
are making slow (but steady) progress on our building.  After the initial
start-up frustrations and mistakes, we have a pleasant production routine
worked out.  F.c. is an intriguing material, to be sure.  And we do get a
special thrill whenever we throw our junk mail into the mixer.
>        I'd be happy to answer any questions anybody might have about f.c.
>
>                                        Happy building,
>
>                                        Gordon Solberg
>
>
>EARTH QUARTERLY
>Box 23-J
>Radium Springs, NM 88054
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