High tunnel construction – Part 1

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The high tunnel project officially got underway on the 22nd with (friend) Duncan not only lending me his truck but also his time. We started at Windsor Plywood and picked up about $600 of materials, including 1.25″ Sched 40 PVC, .75″ Sched 40 galvanized water pipe, and a stack of lumber.

We began by setting up a single guideline close and parallel to the seasonal creek. If, at that point, I knew how wide the tunnel was going to be, we could have run another parallel line at that distance. In this case, I wanted to use up some existing 24′ wide polyethylene. But the 24′ is what gets pulled over the structure, not the interior width. Furthermore, some of the poly width is consumed because it has to be wrapped around 1″X2″ battens along each sides’ edges Accounting for that fact, I figured on only being able to use 23′. Mathematically, one could then approximate the width using the equation Circumference = PI X Diameter and assuming that the high tunnel is essentially a half circle; an upside down half-pipe for all you boarders out there. So, doubling the 23′ to account for the full circle that the equation is based on and re-arranging it to solve for Diameter (width)we have:

Diameter = Circumference / PI = 46’/3.14 = 14.6′ or 14′ 7″

In practise, I mumbled something about PIthagoras, both of us briefly tried and failed to find a pencil and we began cutting the water pipe into 4′ sections. I then post-pounded in all the posts on the marked side, leaving 2′ in the ground and 2′ above. Then we slipped one end of a 23′ coupled piece of PVC over one of the posts and bent it over in an arc to see what length it naturally gravitated to. Turns out, right around 14′.

The .75″ water pipe, BTW, went into the sopping wet soil really easily, so easily that I was concerned that they might also come out really easily. But when I tried to pull them out, I nearly pulled my back out first. Which probably means that I’ll need some sort of pipe jack to extricate them if/when we leave the property, particularly if the soil is dry at the time. In any case, I’m confident that they will not come out due to structural lift on a windy day.

With the width now known, we ran another guideline square and parallel to the first line (oddly, I didn’t mention Pythagoras even though I used his 3,4,5 rule for finding square) and pounded in the other sides’ posts before slipping each 23′ PVC rib on to each of its designated posts.

The area we marked out is as flat as unlandscaped ground gets around here, which means there’s a bit of variation in elevation from post to post. In addition, the posts I pounded in were 14′ apart plus or minus maybe 3″, plus their degree of plumbness varied by a few degrees. All this to say that, once all the PVC “ribs” were mounted on posts, we had to adjust some of them up a little so that, when viewed from the top of the structure, the tops were even.

One of the critical components in the building of this type of high tunnel is the top purlin which goes on top of the highest point of the ribs, running perpendicular to said ribs, from end to end. In this case, the top purlin is also 1.25″ PVC which we carriage-bolted to each of the ribs. Having the top purlin on top of the ribs prevents the polyethylene cover from sagging between ribs at the top where water is at most risk of ponding. Once a volume of water has accumulated at the top, the weight stretches the poly and allows even more water to accumulate which then threatens to collapse the entire structure.

To finish off work day number one of this project, we ran a guideline on the inside of the ribs at about 5′ above ground and used it to screw 1″X2″ wood purlins on either side. While the purlins were 12′ long, they did not meet up nicely at the mid-point of the 3rd rib spaced at 4′. There was simply too much variation in the installation of the ribs for that to happen. As a result, we had to overlap the purlins, above or below each other to ensure all ribs were effectively tied to one another. This ended up looking quite bush-league and, as you will see in a later post, was re-done quite skookumly.

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2 responses to “High tunnel construction – Part 1

  1. Leonie Muldoon

    Very cool!

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