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 give ideas 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. One of our friends had a homemade greenhouse he made out of 2X6's, the wind got under it and broke the 2X6's and lifted his concrete footings.

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 of space 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 are 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.

Side walls of the greenhouse. This greenhouse has metal hoops, poly and shade covering it. - grid24_12

Metal greenhouses commonly use clamps like this to hold the runners together. You can buy these separately, but they are hard to find. I've used the clamps that hold on mufflers when I can't find these. The old greenhouse hoops that you can find used are heaver and may have a bolt through them to connect to the two runners. The new greenhouse hoops are very thin and need to be clamped. - grid24_12

A center brace to keep the stupid greenhouse from blowing over. The wind still managed to break one of these. When we rebuilt this greenhouse we used four side braces running to the center. That seems to have stabilized it. - grid24_12

The poly needs to pretty tight over the greenhouse ribs or the greenhouse top will load up with water and collapse the greenhouse. One layer of poly if you are in a mild climate, three layers, bracing, and some internal support if you are in real winters. Each layer of poly adds a great deal of strength and moves your greenhouse one USDA zone.

If your poly covering is not tight enough rain, snow and ice will build up on to the top of the greenhouse. An elephant setting on your greenhouse roof is bad. - grid24_12

The small used cold frames we made into greenhouse had aluminum runners. I hated them. They worked great but were a awful pain to use. - grid24_12

Greenhouse sides can be of pvc, polycarbonate, fiberglass or poly. We've found the best was polycarbonate for the first couple of feet. The rest of the greenhouse can be poly. Notice the polycarbonate vent. Greenhouses have to be vented. - grid24_12

We usually staple the polyethylene to the wooden runners and screw a cleat over the staples. But make sure your stretch the poly first or your greenhouse will fail. - grid24_12

Wooden side runners of greenhouse. This is how you attached the plastic. Even better is when you run two runners with polycarbonate between them and attach the poly to the top runner. - grid24_12

Inside of large poly greenhouse - grid24_12

Looking through a little greenhouse. You can see the wooden side braces and the door leading into greenhouse. - grid24_12

the sides of a polygreenhouse. - grid24_12

side runners clipped on to vertical posts - grid24_12

two greenhouses put together so you can walk between them - grid24_12

We've learned over the years to overbuild the greenhouses. It's a aeolian process. - grid24_12

To see more how to build stuff go to our
How to build garden stuff
or
Simple solar vents for greenhouses.
How to build a cheap and easy greenhouse
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