Some ideas on how to build a polyethylene (poly) greenhouse.
Ok you build a cheap greenhouse, it was fun. Or you are a person that
wants to start a nursery and make a fortune and is planning on building
a bunch of polyethylene covered greenhouses with the money you made on ebay.
Whatever, you are thinking you need to buy a greenhouse frame or kit
with metal hoops. This page is intended to help you make the buying and
construction decisions.
First understand what the package will have. Usually all they send are
some pipe and bolts, sometimes the nuts to go with them. Directions
seem to be lacking and you'll usually need to order your poly covering
and shade cloth separately.
Before you order anything, figure out if the thing will fit where you
think you want to put it. They have to be a rectangle and
squared. This sounds simple, but usually there's a tree, house, fence
or something else inside of the rectangle. Air flow is vital. Aeration
works best if one end is pointed into the wind. Do not put the
greenhouse in a hole, nor on a hill that gets 200 MPH winds. We've had a
couple of cheap greenhouses blow away like the flying nun. One made it
a hundred feet.
The feet of the greenhouse have to go into the ground between 18-24
inches. This doesn't work in rock.(look at the fence page
for the water hole tool). You'll need at least a couple of
feet around all sides to attach the poly coverings.
So
before you spend anything, get a square (a sheet of plywood
works great) and measure off where 
the greenhouse is supposed to
go. Heavy snow,
four foot spacing between hoops on sides, no snow six foot spacing. (I
tried seven feet but the polyethylene sags.) So a 16 foot wide
greenhouse needs about 20 feet in width and 4+multiples of 6. So
a 16x24 greenhouse would actually need at least 20X28 feet of
room.
Do you need power? Where's the water?
Does the site drain? Sometimes getting the water to the
greenhouse is more work than the greenhouse.
Are there building codes that will make you remove the greenhouse the
day after you built it? Go online and see if you can find the actual
building code for your community about greenhouses, then call the city,
county, whatever and confirm it. Hope you do not live in one of those
Human Oaf Associations, the oaf in the golfcart will never
allow it.
Ok, basic structure stuff,
they should sell you the hoops, feet, and top runners to hold the hoops
at the right spacing. You'll need to buy the plastic, shade cloth(if
you need it), vents, side runner(s).
Here's different pictures of
poly greenhouse structures.
I hate the aluminum side clips, screwed wood is easier. Untreated fir lasts about as
long as
the plastic, 7 years if you get the good uv stablized stuff. Use a counter sink if
you screw the lath on. And use deck screws. Most everything else rusts
off, including galvanized. You can nail the clips but they do not stay on unless you use sheet rock nails and those you cannot remove. Stapling the plastic down tight and then screwing down the cleats seems to work best.
