Keep checking here, as I will update this page with photos of the polytunnel project from start to finish. Find the full size photos here.
Day one:
The test hoop sits on the unprepared site. The location slopes towards the middle, and is uneven but is located between the garden and the chicken yard. Too tight to practically get a tractor in and other corrective measures being too costly we will correct the sites deficincies over time.
After clearing some brambles the perimeter is staked out. The tunnel’s size will be 12 feet wide by 42 feet long with ribs space 3 feet apart. 18″ long 1/2″ rebar stakes were driven into the ground every three feet down both sides, leaving 6″ poking out of the ground. 10 foot 3/4″ Schedule 40 PVC pipes are placed on the stakes, flexed to the middle and connected. At first I wanted to do this without glue, but found that the coupling often didn’t hold. Because we want to be able to take this down eventually and keep what ever materials are practical I used duct tape to secure the joints.
The end hoops are joined at the top with a tee. All the other hoops are joined with a cross connector to allow for ridge pipes to connect everything. Working from the back end towards the front everything is pieced together. At the mid-point and again before the final tee is assembled it is a good idea to slip a 1/2″ piece of lightweight metal electrical conduit into the ridge cavity for added support–something I didn’t think of until after I had passed these stages. I will be going back and adding the conduit the next chance I get. . . . This will also help keep the ridge, ie the center of the tops of the hoops, in a straight line.
All of the ribs are connected in this photo. I didn’t allow for the ridge to run all the way through originally, but I soon saw the error of my ways. I’ll be correcting this as soon as I get some more cross connectors. I started around 9am and this was the condition of the project by 1pm. Not bad for a first days effort. I left the project to source some lumber needed to secure the bottom of the ribs, to build door ways and to anchor the plastic sheeting to the ridge.
More will follow on day two.
Stage II
I have revised the ridge detail by connecting it straight through. Also I ran a 1/2″ piece of conduit down the entire length of the ridge. This allows for greater stability and helped hold the ridge a bit more true. This modification to my original plan should also allow me to forgoe the boards along the ridge which would only add unnecessary weight to a strong, but delicate structure.
The baseboards along both outside lengths helps anchor the ribs and allows someplace for the sheeting to be anchored. Many polytunnels keep this space open to allow the sides to roll up for ventilation, but in our Maritime Pacific Northwest climate where August days often don’t begin until the fog has lifted after lunch I don’t think this will be a problem. If need be, a couple of vent windows can be cut into the sides and of course the doors on both ends can be left open.
On the inside, the ribs are anchored to the baseboard with 3/4″ conduit straps at every rib–both sides. This should keep everything tied together and strong at very little additional cost.
The end wall framing for the doorways is tied to the baseboard with a modified framing anchor.
The head of the endwall at the doorway is held in place with plummers tape.
This detail is used on both sides of the doorway. There is no screw into the pvc piping as that might cause a weakness, so it relies on friction to hold everything in place. A bit of flex in a stucture of this sort probably isn’t a bad thing as buffets from the wind will likely do less damage–up to a point–than if the stucture were ridgid.
The doorway framing is held ridgid by a diagonal stud running from door top to bottom edge of the baseplate. This is blocked in the middle with a horizontal board. This both stiffens the doorway and end wall, but also will allow a further place to anchor the endwall plastic.
I have run rope down the length of the tunnel on each side, midway on the arch, to hopefully keep the plastic sheeting from sagging in between ribs. This, again, was done for simplicity and cost concearns. Other methods could be employed, including using a similar detail as the ridge down the mid-point of both sides. In actual fact, that would probably only raise the cost of the structure by another $50 and lend greatly to its strength.
I didn’t want to use glue
in case we decided the polytunnel couldn’t remain where it was year-round. Instead, at every joint we used duct tape to secure the joints from coming apart during the abuse of construction. This seemed to work very well and will allow for modifications and quick disassembly. Once the plastic is stretched over the top the tape won’t really need to be there. It was more of a security–whew I don’t need to worry about ribs springing open while I wrestle in the end framing–sort of thing.
The final stage this week, while the weather permitted was to begin to prepare the soil inside. Using a combination of a Mantis Tiller, pitchfork, shovel and pick-axe I broke up the grassy dirt clods on both sides of the tunnel, including the path down the center. This allowed me the opportunity to stir around and mix the three different existing soil types–clay, sand and rich loam.
While it would be easier to do this when it is warm and dry inside the finished tunnel, for the initial heavy work it made more sense to work without the limitation of the plastic.
Once the soil is fully loosened–nothing has happened on this site for years–10 years ago we had some blueberries and strawberries planted here, but we lost them due to the poor nature of the soil–we will mix in as much chicken manure as the coop holds, as much compost as we can mine from the heap in the garden, and as much bought in mushroom compost as we can afford. We will probably till all this in at least twice more.
Each time ground around here is tilled, more and more rocks rise to the surface. So there is also the process of raking and scaping to be done. Most of this should be able to occur once the plastic is in place. Which is where we will pick up the story next time.
Stage III
Before continuing, it was time to “fill in the gaps” as it were. I began by blocking the joints in the outer rim board where they met. This should lend strength and keep things from shifting over time. I also finished tying the ropes along the mid-run of the tunnel for supporting the plastic and I added a wire running the length of the tunnel down both sides about mid-way between the rope and the ridge to allow for tying vertical strings and attaching mesh for climbing plants.
The next stage was to cover both ends with plastic. This plastic is in addition to the 100 x 10 ft roll I purchased for the top covering. You can see here that I wrapped the top of the plastic back over the PVC and stapled it to the inside framework to help hold it in place. It is also firmly stapled to the outside of the frame and the doorway.
I have moved the brace, which keeps the tunnel from flexing too much, from the inside of the tunnel to the outside, where I think it will stay. I still have to decide how I am going to work the doors around it though.
The next stage was to prepare the main fabric. I used a roll of 100 x 10 ft 6 mil visqueen. This meant that I had to cut the roll in half at the 50 ft mark and join the two halves somehow. I remembered a mis-spent youth making various hot-air ballons out of dry-cleaner plastic bags and how we’d join the gores of the ballons together with a hot clothes iron–as with ballons, so with polytunnels. Now I am not sure how this will work in the long run, but initial tests proved quite durable in a tug of war test. So set your iron almost on high, find a large area–like and outside deck–and seam the two halves with an iron. I used a 3 inch overlap which left me plenty of width to attach the plastic evenly at the base. In the picture you will see that I am using a piece of silicon parchment paper–for baking–to protect the plastic from the sole of the iron. Watch out because a few times I melted a small hole in the plastic when the edge of the iron sole came into contact. The parchment allows for heat transfer and a smooth ironing experience without sticking and burning. You’ll have to practice on some scrap to see what works with your iron.
Once the plastic is joined carefully roll it up so that you will be able to unfurl it up and over the sides and top of the tunnel. It will not work to pull it over the ends–too much risk of snags and tearing.
A mild breeze is helpful, but anything more is a nusance. You should also have several helpers–big or small. My 5 daughters turned out to help hold corners and add weight as the wind tried to lift the plastic into orbit at one point. Full sized helpers with tools would help secure the plastic in record time but I was able to manage mostly by myself, with the help of the live weights, in about 2 hours.
I initially used staples to help secure the plastic to the bottom rim board and the ends. Start in the middle of one side and work along the bottom outward towards each end. On the opposite side pull tight and staple, again from the middle outward. On the ends pull out and down and staple to the endwall framing. Make sure before you begin that there is equal amount of plastic hanging over each end. This tunnel is 42 feet long and the plastic was 50 ft. The extra is neccesary for secure attachment.
Once the plastic is stapled everywhere and tight, use battons to secure the plastic over the ends. Attach them to the endwall framing and trim off the excess plastic.
Along both bottom outside edges down the length of the tunnel, use 1×4 or some such to attach the plastic. This should help secure against breezes and keep things tight as the sun heats the plastic. Nail it well as this is really about the only thing holding 80% of the plastic to the tunnel.
Now that the soil is covered and warming in the early spring sun it’s time to start planning what’s going in. Once the soil is drier a final grading can take place. But more of that later.
Project Update
After a week of hard work in the tunnel–a delight when the weather outside was 40 and inside was in the upper 60’s–the first seeds have gone in.
I still have to finish the project by building some doors, but I couldn’t resist getting some lettuces growing, as the soil temp is around 60 degrees F–it’s only February 19th!
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