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Build Your Own Gothic Greenhouse

Seven years ago, I was about to give it all up -- gardening, I mean. Here in the Colorado mountains the sunlight is too harsh, the season too short and the winters too cold. Using the simple structures described in my article, Gardening in the Fall and Winter (M Y, September 2007) I was able to grow a limited number and range of produce. But it was not enough. I wanted a real vegetable garden with red ripe tomatoes and tall rows of peas. The answer had to be some sort of greenhouse or protective structure. The problem is, I prefer to make things myself – and I keep to a very tight gardening budget.

There followed a couple of years of experimentation, building growing structures from low cost, easily available materials. I started with a rounded hoop house made from ¾” PVC pipe. The plans I found on the web were well designed and easy to follow, so I glued up the plastic joints and covered it with standard 4mil transparent poly sheeting. It looked sturdy. I particularly appreciated the simplicity of the foundation; the PVC ribs slipped over 2 foot lengths of ½” rebar hammered into the ground.

The snows that winter were heavy. I spent a number of sleepless nights with a broom pushing snow off the top, trying to lighten the load on the hoops. Spring roared in with wind gusting around 80 mph. A loud scrunching noise and the greenhouse collapsed into a tangle of broken pipe and pierced sheeting. One positive: the simple foundation held the hoophouse solidly to the earth.

The answer seemed to be, make it stronger. Bending ½” metal conduit into semicircular ribs was tricky, but two trees growing close together came to my assistance. I drew a template on the ground and bent each rib to shape, using the gap between the trees and gentle even pressure. ½” copper tubing fits tightly into the 1/2” conduit to serve as a jointing method.

The main technical problem was how to connect the crossties to the ribs without drilling and bolting each one -- a horribly tedious process. After a long time wandering around my local hardware store handling all the odds and ends, I came up with a solution. Using four standard metal pipe straps with nuts and bolts, I fashioned a kind of slip connector that could be moved around and then tightened into place (see picture).

The new hoop house looked even more robust. Still, it could not stand up to a full scale Colorado blizzard the next winter. I calculated that the poor structure was trying to hold up about half a ton of snow on its broad back – about the same weight as my old Ford truck.

Obviously, I needed a shape that could shed snow easily and withstand strong winds. But why reinvent the wheel? The Victorians designing the glass greenhouse in my grandmother’s kitchen garden in Wales had solved those problems 125 years ago. They knew the Gothic arch shape with its pointed ridge and curved sides was incredible strong and resilient.

I found a company online called gothicarchgreenhouses.com and studied their pictures. More expensive than my limited budget allowed, their greenhouses looked beautiful and functional. I just needed to solve a few technical problems: how to connect the ribs at the top, what to do about a ridgepole and how to construct an end wall and doorway.

After a few more experiments, it came together. Using ribs made of ¾” galvanized conduit, the ends fit tightly into ¾” 90º galvanized iron fittings, the kind used for gas piping. This creates a sharp ridge which can be covered by 1 ½” PVC pipe with notches cut to hold the iron elbows (see picture). The end walls were constructed out of standard 2 x 4 lumber with a used door from the local recycling yard, cut down to size.

To finish the structure, I ordered conventional 6mil UV resistant polyethylene film from gothicarchgreenhouses.com. On past structures, I had tried different methods of fixing the plastic sheeting but I decided to splurge and get the Wiggle Wire installation system. It proved to be the most expensive budget item but is well worth it in reducing the hassle of fixing or tightening the poly film. I attached 2 x 4s along the wall sides using pipe clips, about 12” off the ground to hold the wiggle wire. That allows the plastic to be rolled up to provide extra side venting on hot days.

The first prototype, 10’ x 12’, is still working well after 5 years (see picture). Two other, 10’ x 20’ and 10’ x 24’ joined the collection over the next few years. According to my calculations, the last and biggest cost me less than $350 complete. They stood up to the 2006 blizzard with 40 mph winds and 6 foot snow drifts that locked us in our house for a week. Last winter, I harvested 12 different varieties of greens and salads through October to February with temperatures below zero. The only heating method is 50 gallon plastic barrels of water used to capture the sun’s energy during the day and release it slowly at night.

Interest in this method of greenhouse building has spread through our community. I often have visitors asking how to make their own. A mountain friend has added a number of useful adaptations (see pictures): the conduit is heavy duty, the tops of the end walls have automatic venting mechanisms and the end wall structure and door are much simpler. You can build your own Gothic greenhouse out of easily available materials and adapt the design to your skills and budget. Then you are ready for all year round growing.

Constructing the Gothic Greenhouse
Basic Materials List for a 10’ x 20’ greenhouse:
18 @ 2’ lengths of ½” rebar
18 @ 10’ lengths of ¾” galvanized metal electrical conduit, standard or heavy duty
6 @ 10’ lengths ½” galvanized metal electrical conduit, standard or heavy duty
9 @ ¾” 90º galvanized iron elbows
2 @ 10’ lengths 1 ½” PVC piping
56 @ ¾” metal pipe straps
42 @ ½” metal pipe straps
1 Box ¾” #10 round head bolts
1 Box #10 washers
1 Box #10 nuts
4 @ 12’ lengths 2 x 4
8 @ 9’ lengths of Wiggle Wire base
12 @ 6’ lengths of Wiggle Wire
1 Box ¾” wide head screws
21’ x 40’ 6mil polyethylene film
Lumber and door to construct the end walls.

• Draw your plans before you start and adapt the materials list to your needs
• Choose a flat area 10’ by 20’. If possible, dig a 24” wide center pathway and sure up with 10” boards to create two raised beds, each approximately 4’ wide.
• Measure and lay out the placement of the rebar every 30” (9 to a side) making sure it is square and straight. Hammer them into the ground for 15” leaving 9” exposed.
• Make a template of the Gothic arch shape on flat ground. The ribs are bent only in the middle 4’ leaving the last 3’ of each end straight. Take care to bend slowly and evenly using a commercial bending machine, two trees or heavy barrels of water placed close together.
• Connect two rib ends together using the 90º elbows, hammered or screwed down tight. Place the connected ribs over two opposite rebars, easing them down carefully.
• When all the ribs are up in the air, using the connectors, attach the ½” conduit crossties, one as close as possible to the top and one on each side about half way up the ribs.
• Attach the 2 x 4 side plates 12” off the ground using ¾” pipe clips. Join the plates together in the middle with a screwed on 2’ piece of 2 x 4.
• Build the two end walls out of 2 x 4 with a door in one end. Drill and screw the last end ribs and crossties to the end walls
• Drill the Wiggle Wire base and attach to the wall plates and end walls using the wide head screws
• Cover the greenhouse with poly film, stretch tight and attach with the wiggle wire worked into the bases. Make ventilators at the top of each end using plywood or plexiglass and an automatic venting mechanism, if desired.

Resources
Gothicarchgreenhouses.com