Rather than buying a greenhouse kit for $4,000 or more, I designed and built my own 20-foot-by-80-foot, round- style hoophouse for about $2,000. I used regular 1 3/8-inch chain-link top rails for the hoops, braces and purlins. Each hoop is made from three lengths of 10 1/2-foot steel tubing bolted together with two self- tapping screws. Before screwing the three pieces together, I bent each one using a hoop bender. These are simple jigs available online for under $100. You bolt the jig onto a table and then bend the tubing around it. It doesn’t take a lot of force to use. Each hoop cost about $33. I spaced our hoops 4 feet apart, using 21 hoops in total. The top of the greenhouse is about 11 feet high. The three purlins run the full 80-foot length of the hoophouse to prevent front-to-back sway, tying the hoops together and into the end walls. Every third hoop is braced horizontally with a single 10 1/2-foot steel tube. On our small farm, we dedicate 30 feet of the hoophouse to seed-starting tables, which sit atop a weed barrier. Fifty-five- gallon drums filled with water act as a heat sink, absorbing energy during the day and releasing it during cool nights. The remaining 50 feet provide ample space to grow early tomatoes in no-till raised beds.
HOW IT’S DONE
Step 1. I leveled the ground first. In retrospect, I wish I had sloped the ground to get better drainage. Then I used a backhoe to dig a trench from our nearby barn and ran an irrigation line of 3/4-inch PVC and a power line of 14/2 underground cable. Next, I marked out the 20-foot-by-80-foot rectangular footprint of the hoophouse and squared the corners. Using a Granberg Alaskan chainsaw mill on my trusty Stihl MS 660, I milled 2×6 baseboards from western red cedar. These boards are in direct contact with soil, so I gave them two coats of latex paint. Starting in the corners, every 4 feet on the inside of these baseboards, I pounded in 3-foot sections of 1 5/8-inch steel tubing—to receive the smaller diameter hoops—until they were flush with the top of the baseboards.
Step 2. The next step was to drop the hoops about 6 inches into the 1 5/8-inch steel tubes. I drilled two 5/16-inch holes through both the pieces of tubing and the baseboard and secured the hoops to the hoophouse frame using galvanized bolts, washers and nuts. Galvanized hardware adds considerable cost, but it withstands soil contact and high moisture levels.
Step 3. Next, I plumbed the hoops before attaching the purlins. You can fix the hose clamp around the purlin and hoop loosely, adjust the hoop so that it is plumb and then tighten the ratcheting clamp around purlin and hoop. The top purlin should be centered along the midline of the hoops and the two others spaced about 6 feet to either side. When attaching the purlins to the hoops, do not bolt or screw them in. It is important to allow for some movement as the hoophouse will shift under a snow load, heavy wind and extreme temperatures. I used stainless steel hose clamps. The purlins can be bolted to the end hoops without problems. After the purlins were in place, I installed the horizontal braces every 12 feet.
Step 4. Framing the end walls came next. I milled up more cedar into true 2×4 boards and painted them like the base- boards. Access from both ends of the greenhouse is helpful, so we included a couple of doors, a 6-foot sliding door on one end and a 30-inch man door on the other. These both stay open all summer to promote air circulation. For the same rea- son, we roll up the sides of the hoophouse.
Step 5. The next step was to attach the two plastic sheets over the hoops. We bought two 32-foot-by-100-foot rolls of 6-mil greenhouse plastic from an online retailer. It is UV-resistant for four years. On a sunny day, we unrolled the first sheet and laid it along the length of the hoophouse. After allowing some time for the plastic to expand in the sun, we cut the sheet down to about 84 feet in length, which gave us a couple of extra feet on either end.
Before hoisting the sheets over the hoops, you need a method for securing the plastic. The inexpensive solution is to screw some 1x2s into the end hoops, fold over the excess plastic a few times on either end and then staple the sheet to the wood. This works fine, but it’s a huge hassle to remove the staples later if you want to take down the plastic. Some farmers have told me that they like to expose their greenhouse soil to UV light in the winter, so having a better method for preserving the plastic is important. The commercial solution is to use channel locks and wiggle wire, available from a greenhouse supply company. The channel lock is a grooved piece of aluminum about 2 inches thick that bends over the shape of the hoop; you screw it into the hoop and lay the plastic over the channel lock. The wiggle wire fits into the groove of the channel lock and holds the plastic snugly in place. The downside of channel locks is their cost. For our 20-foot-by-80-foot hoophouse we spent a few hundred dollars on them.
Step 6. We called our neighbors to help us get the plastic over the hoops. Together, we rolled the plastic over itself along the 80-foot edge nearest the greenhouse, punched a few holes for rope and heaved the two heavy sheets over one by one. With the two sheets in place, we inserted the wiggle wire into the channel locks on one end, pulled the plastic taut from the other end and locked it into place with the second wiggle wire/channel lock set. (There should be about 18″ of excess plastic touching the ground along the length of the structure; this will allow you to roll it over some tubing to create roll-up sides described in the next step.) Why two sheets? We use a small blower to inflate the space between the two thicknesses of 6-mil plastic. This air cushion provides additional insulation during the early spring and late fall, and also reduces the condensation that would otherwise build up on the inside of a single layer. (Each plastic sheet blocks 11 percent of visible light that passes through. On the 100-degree days of southern Oregon’s late summer, I appreciate this reduction.)
Step 7. The last step was to fashion roll-up sides. These are critical during the summer months to release the heat and moisture that build up. I had milled some 1×3 cedar boards and screwed them into the hoops running the length of the 80-foot structure, each about 3 feet above the baseboards, then screwed the aluminum channel locks into these boards. After securing the plastic to the end hoops, we pulled it taut along the sides and locked it into place with the wiggle wire. This left 3 feet of loose plastic flapping along each side that we could roll up. We glued 20-foot pieces of 1 1/2-inch PVC together to make two 80-foot lengths, one for each side. There was some excess plastic at ground level, so we folded it over itself a few times and secured it to the 80- foot length of PVC using lengths of more 1 1/2-inch PVC after cutting the circumference in thirds. The circumference is 4.71. Divide it by pi (3.14) and each piece turns out to be about 1.57 inches. It’s easy to rip PVC on a table saw. We used screws with rubber washers to screw the strips, plastic and PVC pipe together.
At the ends of the PVC on both ends of the roll-up sides l glued a 1 1/2-inch slip fitting to which I had glued a 3/4-inch pipe thread connector. I welded a couple of simple cranks with 3/4-inch threaded pipe and these screw right into the PVC. Pound in a couple of T-posts by your cranks, roll up the sides, and loop some baling twine over the cranks and posts to hold the cranks in place. It works!
A collection of helpful hints and easy do-it-yourself projects for the pioneer in you!
by Len McDougall / Jul 6, 2014