Bill Bolle wrote in message ...
Before you start building with PVC take a look at this hoophouse:

http://www2.moment.net/~wingnut/hoophouse.htm

Bill

some more considerations that will probably interest only backyard
hobbyists:

1) the link above uses cattle panels (basically very strong rebar),
and (this is important) they bend them quite a bit, so the hoophouse
is taller than a semicylinder. The rebar then provides some tension to
push up against the snow, in fact they easily hang pots onto the
rebar without having the house sag. But, if you want to use the soil,
you have less area underneath. I suppose the tradeoff between tension
and surface is inevitable. Incidentally, my 11 feet PVC hoops don't do
that. After two years at most the take a permanent hoop shape. They
are still strong enough to shed snow without collapse. Rebar also
provides excellent support to the poly, effectively preventing pooling
of water or accumulation of snow.

2) when using the rebar, they have two problems. The first is that all
sorts of of rough spots will scratch and break the plastic. IMHO, this
is the perfect place where to use that old hose you have in your
garage. Slit it lengthwise and fit it over the ends. The second is
that they have trouble clamping onto the rebar. the PVC clamps are
great (amongst other things, they are capable of holding the plastic
even when the wind is blowing from the inside, such as may happen when
one leaves an opening after harvest) but alas, only come in PVC sizes
(3/4 and 1). Probably just a matter of clamping a PVC pipe to the
rebar on the inside, then clamp the poly on the pipe (that will be a
pain when one removes the poly for the summer, but safe until then.

Various other issues include anchoring the whole thing (probably
enough to bury a few cinder blocks around the perimeter, with hooks at
the surface for strapping), double layering (do the second layer
inside, it will sag, do it outside, it will sag and touch the first),
chicken wiring (when I take off the poly, I still want the rabbits out
of my garden), and how to do the ends. Nice clean design overall.