Ken Kern in "The Owner Built Home", described a $1901 house with outside
composting toilet, whose most interesting feature was the use of
cement-asbestos sewer tiles cut in half and laid up in interlocking
lip-up and lip-down configuration.
Nobody, of course is ever again going to use cement asbestos overhead
anything in a house, but the design principle is unexcelled in a
low-cost, very strong, easy-to-attach roof. Make your own
roof-components by making an earth-mold on the ground---cut out a
plywood screed so when it is dragged across the earth-mold on two
parallel wood runners, it will produce in the earth or wet sand, the
profile of a lip-down channel of whatever length is desirable. The depth
of the lip determines loadbearing capacity---and if more is needed, you
can run small-diameter rebar in the floor of the channel---or rather in
the corners of the precast channel floor. Another way to screed a
profile is to place an accurately spaced 2 X 6 on each side of the
earth or sand pile, drag the screed along that---the outer surfaces of
the channel would be more accurate if the concrete mix used the 2 X 6
surface as a mold. I think I would do this in preference to trying to
get accurate cavities in damp sand alone.
At each end of the mold, insert a plain plywood stop.
So you don't have to make a new mold every time you strip a cured
channel out of it, I would stabilize the surface with a Bondwall mix as
follows: Cement 19.5 lbs
Lime 3.75 lbs
Calcium Stearate 0.25 lb.
Calcium Chloride 0.50 lb
Glass fiber 1.0 lb (could use more easily available
Water1.5 gallons polypropylene fiber)
I would then spray or paint the mold surface with a
silicone or wax release compound
Lay down chickenwire reinforcement on channel and lips, and insert waxed
wooden pegs through the lips of the channel (accurate spacing) and
trowel the concrete mix on the mold. After the mix sets up, withdraw
the waxed wooden pegs. I would also use polypropylene fiber in the
concrete mix---probably overkill, but would discourage formation of tiny
cracks in flat, horizontal surfaces.
Strip the concrete channel from the mold, and invert it, install with
lips up on the roof concrete bond-beam, into which is imbedded metal
tie-downs. The concrete channel lips will be bolted together, including
one or two bolts through the sheet metal tiedowns.
Probably a good idea to lay a strip of EPDM between the channel lips
too---as a better seal against air leaks. Forgot, you should install a
cap over the channel lips---and put your through-bolts through the cap
as well as the lips.
You will now note that your roof resembles a standing-seam metal roof,
except it is practically bomb-proof and will even outlast an excellent
standing-seam metal roof.
If you have a different configuration of house than
rectangular---hexagonal, for instance, you could make triangular
concrete pans with reinforcing cast purlins produced by indents in the
mold. Ok, so it's more work, but I like hexhouses. Make more molds for
tip-up hex-panels---match the lip angles and fasten walls the same way
as the roof.
You probably won't buy or use one, but the Paul Sarnstrom Tirolessa
Hopper-Sprayer is interesting for small-scale spraying of ferroconcrete
in conjunction with a Hole-Hawg powered Onebone Mixer.
Some concrete additives for water-resistance, plasticisers to reduce
water for more strength, and aiding mixing.
http://www.moxie-intl.com/tutorial.htm
(concrete tutorial, note you want to get rid of capillaries, make
concrete vapor-proof, not just water-resistant)