A big offseason project in anticipation of next year has been this brand new greenhouse. The most important purpose for this area is to start seedlings for transplants, but the structure is sized large enough to do some season extension as well, which I'm really looking forward to trying.
I had hoped to slowly work on this as a late fall project, but an early cold snap froze the ground and I was worried that it wouldn't happen for another year. Fortunately, a brief thaw gave us just enough time to quickly lay out and set the ground posts for this structure. It was a tough couple of days getting it laid out (thanks for the help, Dad!) but we got the job done.
I managed to get all of the hoops bent and bolted in place a couple of days later:
The metal hoops are made from chain link fence top-rail. It's a widely available, reasonably economical way of getting tubing. I made a bending jig (basically some curved blocks of wood screwed to a big sheet of plywood) to shape the hoops. The only tricky part was accounting for the spring back of the metal - after a bit of trial and error, I managed to get the hoops to relatively repeatably bend to a finished 6 foot radius.
One of the many items lost in our 2011 house fire and forgotten about was apparently my plumb bob, so I made do caveman-style:
In this photo you can see all the main structural elements coming together. The purlin on the top is centered and the two wires on either side are suspended from every hoop, giving me three rows to do overhead trellising after the bulk of the seedlings are out in the spring. It would have been much harder to put these wires in after the plastic was on so I put in the time and effort up front to lay them out the way I think I want them - I hope they get used! I'm also considering ways I can use the wires to hold a layer of row cover fabric for extra insulation in the coldest weather. You can also see the hip boards that will form the upper point for the roll up sides (for warm weather ventilation).
Here is the end wall finished with a door and plastic. I've just rolled up the excess so I can adjust the tightness when the top plastic goes on - it gets trimmed at the end.
The plastic is held to the hip boards and the end bows with this "poly lock" system - an aluminum channel with a springy wire that just barely fits in. It is a bit expensive, but it makes it much, much easier to get the plastic on tightly compared to squeezing it down with a wood furring strip (as I've done in the past).
Finally, the plastic goes on over the hoops. We went out as a three person team early in the morning (usually the least windy part of the day) and got it on without incident. With such a large sheet of plastic even the slightest breeze would pull it fairly hard but once we were over the peak it went pretty well.
The last step was to trim the sides and set up the roll up sides. The bottom edges of the plastic are rolled around some metal tubing (half inch metal electrical conduit) and secured with clips. Then, parachute cord lacing goes on to hold the rolled plastic against the hoops. To roll up the sides, you rotate the end of the tubing (with a t-fitting) and the plastic rolls around it as it goes up to the hip board. I was a little worried it would be hard to do alone but the process is very smooth - important because rolling the sides will probably be a twice daily routine for six to eight weeks in the spring.
Now I just need to build some seedling benches, figure out an emergency heat source, and run some irrigation lines - easy!