We began with a basic frame, and pounded 4-foot rebar into the ground on the inside of the frame at 2-foot intervals. The dimensions of the greenhouse are 32 feet by 12 feet. We used pressure treated wood, as we intend to build beds or use containers inside the greenhouse and are not concerned with contamination in our vegetables.
We did a few things different than the one posted on the previously mentioned blog, one being that we made it a lot heavier and more (hopefully) structurally sound. The hoops are made of PVC electrical conduit, two 10-foot pieces spanned this distance perfectly. To beef them up a little, we first cut 16-foot pieces of rebar to fit inside the hoops, the ends butting against the rebar supports that are in the ground.
Secondly, we used a different method of supporting them, drilling holes through 2x2's and threading the pvc through them. The result of this, however, was that we now had 2 large sections that had to be lifted into place with a picker truck, thankfully we have access to one!
One concern we have with the modifications we made is that the pvc has a pronounced bend in it where the ground supported rebar meets the rebar in the hoop. We are hoping this won't become a weak point and fail, time will tell.
We've already had a heavy wet snow since putting the plastic on and saw some flattening of the arches, but with a few 2x8's on the inside holding up the center purlin, it withstood the storm quite nicely. If we were to do it again, we would likely make the center purlin even more sturdy, either a 2x4 or 2x6 and put a few vertical supports on the inside of the greenhouse.
The ends of the PVC are placed over the rebar stakes and fastened to the inside of the wood frame.
We fastened the hoops on the inside of the frame so we could use the polyfastener from Northern Greenhouse Sales. We wanted to have a nice tight seal and a method for attaching that wouldn't punch holes in the plastic in case we remove the plastic for the winter and wish to re-use it next year.
The rounded ends of the greenhouse were finished with strips of 3/8" plywood, bent around the arch and screwed into place. The polyfastener was attached to this strip and we left enough plastic to wrap around to the ends. We stretched the plastic over the hoops and zipped the inside strip of the polyfastener into place. It worked really quickly, but it did slice through the plastic in a few places where we were stretching it a bit too tightly. Our original intent was to apply 2 layers of poly and have a fan blowing between the layers, but we didn't have quite enough so we may do that at another time. We will just have to open the polyfastener and add a layer.
We used plastic lath, also from Northern Greenhouse sales to attach the plastic to the doors.
We protected the plastic from the rather rough wood on the purlins with a double layer of plastic, stapled in place.
We have made a few more changes since this photo was taken. We built a window into the door and installed the heat-activated window opener from Lee Valley Tools. The door on the other side is yet to have one installed, so when I see that the window is wide open, I know it's time to go prop open the door on the other side! We installed doors on both ends of the greenhouse for access and ventilation. We hope to keep the temperature and humidity inside the greenhouse moderate both day and night with effective ventilation and using a couple of black barrels of water inside the greenhouse as heat reservoirs.
We're ready for the growing season; melons, pumpkins, zucchini, eggplant, flowers and tomatoes are at home in our new greenhouse, growing like weeds already!
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