Building walk-in tunnels on the cheap usually limits the width of the tunnel, because the hoop materials you use can't handle the forces involved in larger structures. Johnny's Seeds sells a pipe bender that converts chainlink fence toprail to hoops, but the resulting tunnels are only 12' wide, or were; they now sell a bender that can get you to 14' wide. I've built the former. I've also adapted the model: same dimensions using PVC pipes.
Both models have served me well, but damn if the 12' width isn't frustratingly limiting. Tunnels of this size are hard to move around in, aren't very efficient moderators of temperature (they heat up and cool down very quickly), and the polyfilm cost per square foot of growing space is high.
I've wanted a bigger model, but my problem is that I'm reluctant to build a big, bad pre-fabricated monster on my short-term lease. What I love about the much cheaper DIY models is the much cheaper part, and the ability to erect and break them down very quickly.
This season I took a chance on an adaptation of 12' wide model that doubled the width of the structure, using oh-so-affordable PVC pipe. The result: a 24' wide by 81' long (length can be whatever you like) by 7.5' tall tunnel that is now up and planted. Before I describe how I built it, some very important notes:
1. These PVC hoop tunnels, whether 12' wide or 24' wide models, will not hold up to snow loads. I erect them in late Winter and take them down in late Fall.
2. This new, 24' model hasn't been tested yet. I just built the thing (March 2016). I'll provide an update later if it collapses in bad weather. That said, I'm pretty sure it's going to hold just fine.We'll see!
Ok, so: each hoop is made from two 20' lengths of 1" PVC pipe. I bought the thinner-grade pipe because it's cheaper. I've never tried the thicker pipe. I bought a gallon of UV resistant, white paint, and painted the pipes so that they're better protected against the sun. PVC turns brittle over time, you see.
Each pair of pipe is fit together (they have male and female ends), and then, from the center of the resulting 40' length of pipe, I measured 15'8 in both directions. You need to cut off a piece at either end so that the male-female junction becomes the center of your hoop. I'm left with a piece of pipe that is 31'4. This will be my hoop.
My hoop spacing is 3'. So, an 81' hoophouse requires 28 hoops, or 56 pieces of 20' pipe. I bought 60 pieces so that I could make a purlin out of the last four pieces. In the photo above, the purlin is on the right. I didn't use one on the left side so I can deterimine how much the purlin contributes to rigidty of the tunnel.
I also bought a one and a quarter inch 'T' joiner for each hoop, so that I could use the off-cuts from my hoop to form a pillar at each hoop apex. This represents the small tradeoff that, I think, allowed me increase the width of the structure--I'm left with a central row of pillars. See photo. Small price to pay, as far as I'm concerned.
I used 3' rebar for each stake. I recommend going to a hardware store and buying in 10 or 20' lengths, and then paying them a cutting fee to cut them down for you. Cutting rebar sucks. Don't go less than 3'. You want at least 2' in the ground, and at least a foot sticking out to receive the hoops. I had a bunch of 2' pieces of rebar lying around, and used them to receive the pillars along the center of the tunnel. In the photo below, notice that I used clamps to attach some of the hoops to the lumber.
You can anchor the the plastic in various ways. I prefer building simple end walls, and then placing some 2x6s at various points along the sides. See photos. I really like channel lock/wiggle wire for securing poly. I used it wherever I had lumber to attach to.
I built this thing for around $1500 CAD. It gave me six 75' beds.
One problem so far: the pillars keep coming loose when the structure moves in the wind. I may have to glue them into the 'T' junctions. Again, see photos.
In late June, I'll take the poly off and replace it with shade cloth. The poly will go back on for a Fall crop in September.
I'll write an update in a few months. Leave me a request in the comments if you think I've forgotten. Thanks for reading.
I experienced some problems with wind and made some changes that have helped a lot. They're explained in the below slide show.