ClearSpan Pro “High Tunnel 1” Construction Summary: April 2014 – November 2014
By JT

Type: 30’ x 100’ ClearSpan Pro Solar Star High Tunnel www.clearspan.com
Full Photo series: www.flickr.com/northslopefarm/hightunnel1

In the 2014 season, NSF purchased two 30’x100’ high tunnels to expand production and season longevity. In this summary, I’m going to lay out our construction methods, talk in layman’s terms about the construction phases, and look into the true cost of the high tunnel including the equipment rentals, labor, and materials.

Looking back at the notes, it was evident that the construction was broken down into six phases. Throughout this summary, I’ll delve into each specific phase in more detail to both provide a record NSF can share with others and for my own learning reinforcement as I oversee the construction of HT1’s twin, HT2.

Phase 1: Map out HT1, confirm dimensions, excavate ground post holes, set ground post holes
The first phase of the HT1 involved what any build calls for: site and design. Siting HT1 was done by MR using his understanding of NSF land contours, solar access, and accessibility to irrigation. After the general location was decided upon, the crew set out to confirm the dimensions of the greenhouse using the 3:4:5 triangle method. This method is as such: any triangle whose sides are in the ratio of 3:4:5 is a right triangle. To properly use this method of measurement, we measured from our first corner post along one edge 3 feet. We marked that measurement. Again from the corner post, we measured along the adjacent edge 4 feet. Finally we measured diagonally across. If the measurement was 5 ft then the triangle is a 3:4:5 right triangle and the corner is ultimately square. We repeated this step for all four corners. Next, we strung the batter boards 3 feet from the construction to maintain permanent line attachments where we could continually confirm, square, and level our measurements. It was critical to have accurate measurements early on as the rest of construction was supported by these measurements.

Here at North Slope Farm, we deal with a subsurface sheet of shale that is impenetrable by hand. In order to excavate the 34 total ground post holes, we first rented a hand hydrologic auger as seen in the picture below.

First attempt at digging post holes

The auger pictured above simply lacked the power to dig out the holes efficiently, so we eventually rented a skid steer with an hydrologic auger attachment and aimed to dig the holes to 3’. Most holes were dug to three feet, but a few holes hit bedrock before the three foot mark. We lived with the results and moved on. After the holes were dug, we filled the base of the holes with concrete and set the ground posts. To make sure each post was plumb, level, and the correct height, we confirmed each of those variables with two East to West string lines and used a level to assure posts were plumb. Once all variables were in check, we added a bit more concrete then back filled the holes with soil.

TH and MR with the rented Skid Steer and auger

Phase 2: Rafter, Rafter Struts, and Purlin Assembly
After setting the 34 posts, the next step is to mount the rafters to the posts. Before we could mount the rafters, we had to first construct them on the ground. All rafter assemblies consisted of four rafter sections joined by a single chevron at the peak.

Rafters assembled & aligned with corresponding posts

Once we put together the rafters on the ground, we then moved on to attach them to the posts with the help of the John Deere bucket.

Mounting a rafter. Note MR adjusting tension of the chevron at ridge.

After lifting a rafter up to the corresponding posts, we aligned the mounting holes and attached the rafter to the post with a bolt into each mounting hole. This is being demonstrated by AM and RM in the photo below.

AM and RM placing bolt to fasten rafter to post.

After the second rafter was in place, we attached the ridge purlin and set the first ridge purlin in place by aligning the pre-drilled holes with the studs of the rafter chevron, and securing the purlin to the rafter using ¼” nuts and washer. The ridge purlins continued to be added as we added more rafters. (During this construction, we waited until all rafters and all ridge purlins were in place before we started to add the support purlins. Looking through the instructions after the fact, we saw that it is best to install all the purlins as the rafters are being installed. For constructing HT2, this point of reference will probably help troubleshoot the stubbornness we experience when attempting to attach the support purlins.)

Raising rafters, attaching ridge purlin, and loosely attaching rafter struts.

To attach the rafter struts, we took a strut and loosely attached it to the ground post flange using the 5/16” fasteners provided. Once loosely attached, we pulled the rafter strut up into position to be aligned with the rafter hole and corresponding hole on the strut. We inserted the bolt through the rafter hole and through the mounting hole on the strut, added a locknut and tightened to secure the strut to the rafter. We would do the same on the opposite side of the high tunnel. It is important to attach the struts before you begin to attach all the purlins. They don’t have to be fully tightened, but having them attached is crucial. In the photo below, AM demonstrating how to loosely attach a rafter strut to the post flange.

AM placing attaching rafter strut to post flange

With the first three rafters assembled, we began to mount the first under purlin and secured it to the rafters using the ¼” x 1” bots and nuts. At some points, a purlin and a rafter are used to attach cable assemblies. To do that, we installed the rafter cable plate between the rafter and under purlin.

Phase 3: Cable Installation and Diagonal Struts
The first step to attaching the cable is to attach the eyebolts to the frame. Next we measured the distance between points A and B and cut one length of cable from the roll to the determined length. It is imperative to account for the turnbuckle and the extra cable needed to install the clamps. After doing that, we created the turnbuckle assembly. To create the turnbuckle assembly, we used the diagram on pg 22 of the instruction manual(see photo below).

Pg22 High Tunnel Construction Document

The assembly uses a combination of thimbled ends, cable clamps, turnbuckle and turnbuckle jaws to create a tensioning cable that ties each rafter to one another to create a seemingly hurricane-proof design. Below, you can see we used the back of a trailer attached to an ATV as our turnbuckle assembly workbench. This works great as the work bench is mobile and moves along with the crew as needed.

Building the Turnbuckle assembly on mobile workbench.

To attach the turnbuckle assembly, open the turnbuckle to its extended position and check to see how it fits from point to point on the frame. From the diagrams on page 22 (here’s a link to the PDF of the instructions) of the instruction booklet, we attached the first cable to the frame in the location the cable was measured and created for. In the first photo below, JT and TH demonstrate how we used the scissor lift to attach cables to the frame.

Using scissor lift to aid cable installation

We repeated this step on all the cables. Finally, when all the cables were attached, we tightened the turnbuckles. It’s important to not over tighten the turnbuckles as you can pull the posts out of plumb. In the photo below, AM is demonstrating how we used a long screwdriver to turn the turnbuckle to tighten it down.

AM tightening turnbuckles with long screwdriver for leverage to turn the buckle.

To attach the diagonal struts for additional support, we used a vise grip to bend each flattened end of the strut to have the ends of the struts flat against the frame when installed. We positioned the diagonal strut in between the end rafter and the interior rafter with the strut bent end nearer to the rafter on the end rafter and the other end near the bottom of the post on the interior rafter post. Think “diagonal” strut if that sounds confusing! We mounted the struts by drilling holes then inserting bolts with washers through the mounting holes.

Phase 4: Frame Check, Poly Latch U-Channel on End Ridge, Attach Ribbon Board & Double Poly Latch U-Channel to Sides
Before moving forward from here, we made sure all frame members were properly secured, that all bolts and screws were tight, and to cover up any sharp edges or fasteners, we cut 12” pieces of repair tape and taped the tops of all interior rafters to protect the plastic. We also cut 4” strips of tape and taped over each rafter splice. To install the Poly Latch U-Channel on the end ridge, from the peak, we attached u-channel to top of rafter every 12”. We made sure to cut the last section flush to the bottom end of the rafter.

Utilizing the scissor lift to attach channel lock to rafters

To attach the ribbon board to the base of the rafters, we used 5/16”x5” carriage bolts. We had to countersink the bolts because the ribbon board we used was thick (a bit wider than 2”). We installed all the carriage bolts then tightened them at the end. We used self-tapping screws and carriage bolts to attach the double poly latch u-channel.

Ribbon board.

RM's flexing muscles and the mounted, double H-channel on ribbon board

Phase 5: Cover Frame with Plastic and Secure
We woke early in the middle of the season with all hands on deck to get the plastic over the frame. We started out by tying ropes to one end of the plastic and throwing the ropes over the HT until we stood on the other side with the rope over the ridge of the HT and attached to the plastic on the ground. All we had to do was pull together, right? Wrong. Heave, ho! We gave it our all, but it proved too heavy and difficult! We stood confounded, MR running up and down the HT trying to push the plastic up into the air with a broom or piece of wood. In the end, we started to flap the plastic up and down – creating the “billowing” effect – which allowed the air to get under the plastic and there it came over the ridge and down to the other side. Success!

Pulling plastic over frame

Crew securing plastic on humid morning

We secured the plastic on the ridge first. We used the JD bucket to access the peak of the HT. From the peak of West end, we wiggle wired to lock the plastic into the channel.

Wiggle wire through H-Channel

Next we secured the opposite end. One person wiggled while the other tried to hold the plastic taut to create tension.

Securing Plastic

Thirdly, we locked in one long side with wiggle wire to create tension for the opposite long side. On the opposite side, JT, TH, AM, and RM worked together: two persons held the plastic taut while one applied the wiggle wire and the other one drove our mobile scaffold (the ATV and trailer). We then cut the excess plastic.

All hands on deck.

Casey & Colleen enjoying morning of securing plastic to HT1

Phase 6: Baseboards & Roll-up Sides
To attach the baseboards, we established the layout, trimmed the boards to length, drilled holes, and then used u-bolts to attach the boards to the posts. To date, we have yet to set up the roll-up sides.

CH with a big smile! Setting baseboards is fun!

Crew measuring out length for next baseboard

Phase 7: Endwalls
To build the endwalls, we started by measuring out the holes for upright posts. Once measured out, we dug the holes, added concrete then set the posts to plumb and level. We backfilled the holes with more concrete and soil we dug up from the holes. Then we set the horizontal cross bracing. After the cross bracing, we started to attach the polycarbonate paneling.

Master carpenter, and friend of the farm, Ric Stang oversaw JT, RM, and TH as we cut and pieced the paneling together. We had to open the H-channel with the hammer on some of the pieces as the paneling had to enter the h-channel from both the top and the side. We also tried a soap mixture to create some lubrication. Paneling was set by self tapping screws and washers. We had to cut the excess paneling around the end walls to fit it flush to the frame. We used a box cutter to lightly score the polycarbonate, then repeated the light scoring until the poly could be bent. After it folded back, we would be able to bend it back and forth until the piece we wanted cut came off. This was very effective.

MR conducting the orchestra.

Setting endwall panels

NSF High Tunnel #1 True Cost:
In hopes of determining the true cost of purchasing and constructing HT #1, we’ve gone back to the receipts and notes to add up the numbers. At first glance, the high tunnel materials (frame & endwalls) were 75% of the total cost. Equipment rentals tallied 8% of the total cost, worker wages was in at 8%, and additional materials cost 8%. Overall, the numbers below tell us that the true cost, albeit high, is justified with the majority of the capital spent on physical infrastructure, equipment necessary to complete tasks (efficiently and safely) and worker hours. To see it through as a profitable investment, we will have to continue to record data regarding the harvests recorded in HT1.