I learned to build these structures originally not for horticultural purposes but for the boat business. We often need indoor workable space for boats that need extensive work done to them during the winter months. These structures provided my mechanics with a dry, wind free and relatively warm area to execute their duties. At the marina we were exposed to high winds off the bay and heavy wet snow loads. These structures handled all very, very well.
The below structure is about as inexpensive as it comes when you want a fairly permanent structure, about 5 years before skin replacement. Made of 10ft sections of inch and a half PVC pipe, 90 degree PVC right angle fittings, pipe clamps, anchor posts, a covering and wood for a kick board.
This one I made is 40 feet long by 12 feet wide and will house trees for winter storage. Of coarse you can make one much shorter!
For this thread I left the front off so you could have a clean view of the assembly. The design is a bit different than normal hoop house you see as its a cathedral shape, or a heresy kiss shape as my daughters refer affectionately to it. This design concept comes from previous trials that succumbed to wind and snow loads. With a fast tapering peak the skin is allowed to mold around each bow. Also with the fast taper it has the ability to shed water, ice and snow quickly. This ability allows you to from having to do additional supporting during winter months.
You start out by laying out the outside dimension of your structure. I would suggest spacing of anchor posts be no more than 3 feet apart...unless your structure is greater than 21 feet. You would then want to space closer for center sections if your in a geographical location that receives high snow loads.
When your anchor posts are laid out and driven into the ground, slip the 10 ft sections of PVC over them. I suggest you drive the anchors in at a very slightly inward angle so you don't fight assembling the two pipes together. Use the 90 degree angle fitting to join the two opposing pipes together with PVC cement. Once you complete all the sections you'll want to put a ridge pipe up to keep each hoop vertically in place and prevent swaying.
Secure this ridge pipe with U-shaped pipe clamps and tighten the snot out of them. You'll notice when you do that the flat bar will form to your ridge pipe providing a secure molded union. The final structural thing you'll want to do is is put two anchors in mid-way on the interior of the structure. Then use sway lines (can be rope or wire) to tie of the ends of the structure at the ridge pole height and then brought to the center anchors center. This will prevent the whole frame from rocking in strong winds.
At this point the structure is complete and its time for installing the skin. Before I begin a word on skins.
I use commercial shrink wrap when covering my structures. This type of shrink wrap is used for covering boats in the winter and is UV resistant. It comes in two thicknesses 5mil and 7 mil, I would suggest 7mil as its much more forgiving and longer lasting. The covering has a 4 year warranty and comes in clear, white, and blue. Blue makes for an excellent heat conductor when the structure is being used for propagation and will keep your heating costs lower. Other important benefits from shrink wrap is that after heat is applied to it the skin shrinks and when cooled forms a rigid shell which is excellent for keeping the peak of your hoops in place , for dealing with wind, and shedding snow. If you notice in the picture the skin as pulled around each peak and that peak will stay in place and be impossible to move with out cutting the skin. Shrink wrap's other advantages during construction and/or maintenance is that it will make a permanent bond with itself when heated eliminating the need for bonding agents or other structural bindings. If ripped it can be easily patched with a scrap, this is done by heating it and patting it in place over the tear. If your using your structure as a greenhouse you'll need to apply two layers. The first is heated the second is laid over the first unheated. This will form a air pocket for your fan to blow warm air in between the two layers for insulation. Shrink wrap can be purchased in a wide variety of sizes on rolls. If you live near a boat dealer they can supply you with exact dimensions you require. Or you can order it from U-line products on rolls. If worst comes to worst feel free to drop me a note as I always have it on hand for a variety of applications.
Other skins can be used and its limited only to your imagination. I would suggest if any other skin is used that you install a rigid pole at the peak since it will not form a hard skin like shrink wrap.
Back to installing the skin......To start you'll need to install a baseboard around the perimeter to secure your skin to. You can use most anything but a 6 inch width is probably the minimum you'd want to go. Secure the base boards with pipe clamps once again and at the butt ends scab a scrap piece of wood over the joint.
Now pull your skin over the frame and square it off. Use 2 x 1 strips to anchor the skin to the base board and screw these strips in after you have pulled the skin as tight as possible. Pleat your corners and temporarily secure the pleat with duct tape, later you'll come back and heat the pleat binding the plastic to itself. Now is the time for heat to be applied to pull the shrink tight. Any number of heat sources can be used. They make propane guns specifically for shrink wrap use however a heat gun can be effective also. If you assemble the structure early summer by seasons end the sun will do the same as any instant heat source....though you may need to touch op the peaks for a really secure fit. If you choose to use an instant heat source do not shrink the plastic on a windy day...the heat will soften the plastic and the wind will blow it into a bubble gum type balloon...making for a loose fit when it cools.
So that's it pretty much in a nut shell. A easy to build structure at a very inexpensive price, that will last the better part of 5 years without the need to replace the skin. The 40 x 10 building cost me 275.00 bucks to build (and a lot of shrink left for my next structure) and it took me about a day and a half by myself to erect. If I had a helper this would have taken easily less than a day.
Hope this helps anyone that wants to build a structure but is financially handicapped to do so.
