Hoophouse design

A little background. Jeannine and I both love to garden. We've had two small gardens in our front yard for five years now. We're also interested in growing as much food as we can, both for health and financial reasons. After reading some books by Eliot Coleman on four season gardening in colder climates like Maine, we decided we'd like to try that. Seeing the hoophouses on display at the Common Ground Country Fair each year was also inspiring. So with our tax return in hand, I did some research on hoophouses and found quite a variety of information.

In general there are a few main options when building a backyard hoophouse:

- Do I build it with plastic or metal hoops?
From what I read it's much simpler to use PVC plastic pipes to make the hoops because they're flexible enough to bend by hand and very cheap. The downside is that they're not very strong and apparently have a habit of degrading the plastic where they touch. Also, they'll eventually break before a metal hoop would. Metal is obviously stronger and will last longer, but it's more expensive and you need tools or jigs to bend it. The metal that is commonly used in small hoophouses is EMT conduit. I've seen designs using everything from 1/2" EMT conduit to 1 3/4" fence rail. EMT conduit is convenient because of it's cheap price, availability and it is galvanized. While you could use wood to build the frame, it seemed much more complicated to me.

- Do I make it semi-circular or a gable design?
Traditionally, hoophouses have been semi-circular in design. I think this has mostly to do with simplicity in construction. However, you may also lose a lot of headroom or vertical growing room on the sides depending how wide your hoophouse is. A gable design is much like a simple house, with straight sides and then a straight sloped roof to the peak. The benefit with the gable design is that you can have a much higher ceiling at the edge of the building for growing taller plants or simply for headroom. You can also set your roof pitch higher for snow in the winter. I decided on a gable design for the high headroom at the edge and because I'm planning to leave the plastic on all winter to experiment with some four season gardening.
With either design, I would need tools to bend the metal I'll be using. For hoops, there are hoop bending tools you can buy online or you can make a bending jig on a sheet of plywood. For my gable plan, I need discrete bends, so I'll be using a 3/4" EMT conduit bender. I'll need a 90 degree bend at the peak and 45 degree bends at the wall/roof edges. There is also a gothic style design, which is quite similar to the semi-circular design, but forms a sharper peak near the top.

- Do I buy a kit or make it myself?
Many businesses will sell you hoophouse kits, but I've found them costing much more than I want to spend on this project. I should probably mention right now that I'm very frugal and like to do things myself whenever possible, so from the very beginning I've been planning on doing most of the work myself. Shipping on the kits is also quite expensive and they're limited to specific sizes (often fairly small). Many of them don't even include a base frame or end wall framing materials. I found Eden House as a local business that sells and installs small hoophouses that actually look very nice, but they're still twice the price I'd want to pay.

Given all that information, my decision was to build a 12'x16' gable style hoophouse with 3/4" EMT conduit "hoops". The hoophouse will be taking up one corner of a garden, so based partially on that and our desires, we decided on the 12'x16' size. This was also determined somewhat by how wide we could make a greenhouse using no more than three 10' pieces of EMT per hoop. I wanted a 45 degree roof pitch, but didn't want the roof peak to be too high, so we decided on 4' side walls. With 8' of roof from the top of the sidewalls to the roof peak, that actually gave us a hoophouse width of 11.4' and a roof peak height of 9.7'

Now what about the base frame?
We didn't want to use pressure treated wood. Although relatively inexpensive, it's also full of horrible chemicals we don't want leaching into our soil. I priced out naturally rot resistant cedar, but it was quite pricey (a planed 2x6x10 was $24.55). So I went with some rough sawn hemlock 2x8 lumber from a local sawmill. I won't last as long as cedar, but it was much cheaper (a 2x8x16 was $11.10). The hoops will connect to the base frame with a couple conduit hangers on each side, basically u-shaped pieces of metal with tabs you screw in on each side. There will be a ridge purlin that will be 3/4" EMT and go the length of the ridge. At the top of the sidewalls there will be 1x3 boards going lengthwise acting as siderails to strenthen the building and attached to the hoops with the same strapping as on the base.

Constructing the hoops
The "hoops" in my case consist of a 4' sidewall, a 45 degree angle, 8' to the ridge peak, a 90 degree angle 8' down the other side, a 45 degree angle and a 4' sidewall. In all it should be 24' of conduit. The problem is that conduit comes in 10' lengths. D'oh! So how to connect the pieces together? There are EMT connectors available, but I tried both the set screw and compression type connectors and both were utterly flimsy for my needs. After some thinking and testing, I found out that 1/2' IMC conduit has an outside dimension that fits snugly inside 3/4" EMT conduit. IMC conduit is very similar to EMT, but has a thicker wall. So I tested cutting a 10" piece of 1/2" IMC and inserting 5" of it into one end of 3/4" EMT. Then I drilled a 1/4" hole all the way through both pipes about 3 inches from the end of the 3/4" EMT and inserted a 1/4-20 x 1 1/4" bolt through it, securing it with a lock washer and nut. Then I repeated the steps with another section of 3/4" EMT, so that the ends of the EMT were abutting. Once done, it made a very strong splice. One note, I found that not all pipes are the same sizes. For instance, when I tested 1/2" IMC from Lowes and Home Depot, it was just a fraction too big to fit inside 3/4" EMT, but the 1/2" IMC from my local True Value store worked flawlessly!

The EMT was bent using a 3/4" EMT conduit bending tool I bought at Home Depot. It's quite simple to use, but took a few trials to figure out where exactly to start the bend to get the lengths right on the resulting pipe. I used three 10' pieces of EMT per "hoop". Since the whole hoop was 24', I cut two of those pipes down to 7'. Starting from the ground of one side, I used a 7' pipe, bending 45 degrees at 4'. I did the same thing for the other sidewall. Then I bent the 10' piece at 90 degrees right in the middle. Connecting those three pieces gave me my hoop.

How will the plastic attach to the frame?
Wiggle wire! I learned about this while doing my research and it sounded like a great idea. Basically you install this metal channel along the edge of your frame, put your plastic in the channel and then insert this zig-zag tensioned piece of wire in the channel and that holds it in place. You can bend the channels over hoop circumferences, but I only have straight edges which makes it even easier. Then just screw the channel to the frame with some self-drilling metal screws.