Here's a DIY hoop-style greenhouse I built last fall (October/November, 2010).
I learned how to build this greenhouse from Gred Gross in a class he taught at a local Organic Grower's School here in western North Carolina. The basic design is his, but I modified it somewhat, as you probably will if you build one.
This one is 12' x 12', but the plans can easily be modified to make a shorter or longer version if desired. A bare-bones version can be made for just over $300 with brand new store-bought materials, so an enterprising individual could make one for even less. Of course, "bare-bones" means it won't have all the features in the photo above, such as the ventilation system, rain catchment, screen door, etc., but it will still be functional.
Here's the material list:
(10) 3/4" x 10' EMT Conduit
(16) Conduit Clamps
(10) 1/2" x 48" Rebar
(4) 4" x 4" x 8' Posts
(1) 2" x 4" x 12'
(12) 2" x 4" x 8'
(1) 5/4" x 6" x 12'
Lattice Strips (120')
6 mil Greenhouse Plastic (24' x 30')
(4) Tarp zippers
#10 x .75" pan head screws
(2) Boxes Deck Screws
old inner tube
Duct tape (optional, but recommended)
Clear UV-Resistant tape
Here are the tools you'll need:
Post Hole Digger
1" Spade Bit
Stapler & Staples
File or wood rasp
Clamps (optional, unless you're going to build this by yourself)
Here's how I built mine (please note that I’ll be using some pictures from the class and some from when I built my own):
Select and prepare the site for the greenhouse. Because my yard slopes, I decided to use a retaining wall around the front half of the greenhouse.
Mark off the perimeter of the greenhouse and make sure it’s square (check across both diagonals to make sure they measure the same – for a 12’x12’ the measurement should be 17’). It’s not figured in the cost of a bare-bones greenhouse, but you may want to consider using landscape timbers or 2”x4”s along the sides, as it will provide more solid support for the hoops and help to prevent grass or weeds around the outside of the greenhouse from spreading inside, plus it makes it easier to get everything level.
This is a good time to think about how you intend to use your greenhouse. Do as much preparation as possible for what you plan to put inside it. Consider whether it makes sense to prepare your beds, etc. before you build your greenhouse or afterwards. Bear in mind that when you’re working in your greenhouse, it doesn’t take much to cut a slice in or poke a hole through that poly shell, which reminds me – go ahead and buy some UV resistant clear tape to have on hand to patch your poly. You’re going to need it at some point, and it’s better to have it before you actually need it. But we already know that, don’t we?
Dig post holes for the 4"x4"s, two on each end spaced so that you end up with a 36” wide opening (39.5” on center). These will become doorways for the greenhouse. If desired, one end can have a window instead of a door (that’s what I did), but you'll still need to have two 4"x4"s on each end for structural support. Place a brick or flat rock in the bottom of each hole for the 4”x4” to rest on. The effective hole depth (to the top of the rock/brick) should be 2’.
Set 4"x4" posts in place. Make sure the posts are spaced properly, and are plumb and square, and cement or pack firmly into place. You’ll have to decide, based on your particular situation (deed restrictions, local ordinances, etc.), whether or not to cement your 4”x4”s in place. As I understand it, in some situations using cement may mean your greenhouse is considered a “permanent structure” (as opposed to temporary).
Cut two 36" lengths of 2"x4" and temporarily attach one between each pair of 4"x4"s near the top to create a door frame (these will be repositioned and attached permanently after the tops of the 4"x4"s have been trimmed later in the building process. For now, the idea is to make sure the posts are spaced properly at the top).
Bend conduit into half-hoops (two pieces of conduit are used to create one hoop – these will be joined together at the top by a 2"x4" ridge beam). This is the hardest part of the process to explain (and the trickiest to actually do). The idea is to bend each length of conduit into the rough equivalent of an arc, and the instructor did this by driving some of the rebar into the ground in an arc-like pattern as shown below (if you look closely, you can see that he's using five pieces of rebar). He's done this a lot and can position the rebar by eye – I took a different approach, which is described below. Note that the conduit is on the inside of the left-most piece of rebar (the one with a foot pushing on it), while the conduit is bent around the outside of the remaining rebar.
The two pieces of lumber in the above picture are measurement aids. They are positioned perpendicular to each other. The piece that runs roughly from the bottom to the top of the picture is placed 6 feet from the end of the other piece and has a mark at 85 1/2" up from the bottom. This is the point the conduit will be bent to. NOTE: When bending, the conduit has to be pulled beyond the mark because it will spring back when released. It will take some practice to get a feel for how far beyond the mark it has to be pulled. The conduit needs to be bent so that when it's in the relaxed state (i.e., when you've let go of it) it stays at that 85 1/2" mark on its own. It's also important to keep the conduit at ground level while bending it. This is hard to do. The natural tendency is to lift the conduit while pulling, but if you do, you'll end up with a twisted arc, which will end up making your greenhouse rather wonky. Hopefully, you'll end up with ten closely matching arcs.
You'll notice that my conduit is more like a series of angles rather than an arc. That's because I used a different process. I couldn't visualize where to place the rebar like the instructor did, so rather than bending each length of conduit all at once, I bent them in a series of steps as described below. Because I’m very left-brained, I built a jig out of 2”x4”s so I could position the rebar precisely in a pattern I laid out in AutoCAD.
However, after bending the first piece of conduit, I discovered that I had to make each bend separately for the conduit to have all the different angles I intended (I had also laid out the shape of the conduit in AutoCAD – like I said, I’m very left-brained). That meant removing all but the first two pieces of rebar and making the first bend so that the conduit ended up right at the hole for the third piece of rebar (you’ll recall that the conduit has to be bent beyond where you want it to end up because it springs back considerably when you let go of it, and you can’t bend it beyond that point if the rebar is in the way!). The easiest way to do that was to put the first bend in all ten pieces of conduit,
then drive in the next piece of rebar and put the second bend in all ten pieces,
then drive in the next piece of rebar and put the third bend in all ten pieces.
And of course the jig was marked with the location for the end of the conduit to get the correct angle on the third bend.
Check the length of the 2”x4”x12' and trim to 12' if it's longer. Drill 5 pairs of 1” holes in the 2"x4"x12' to create the ridge beam. The holes are for the conduit to slide into. The hoops are going to be spaced 36” apart, so you would expect that the hole spacing to be 36”. However, since the beam is 12' long, that would mean the holes on each end wouldn't be complete holes – they'd be half-holes. So the holes on each end are drilled about an inch and a half from each end. That leaves a spacing of 35 1/4” between each pair of holes.
Note that the holes are drilled on an angle (about 20 degrees), to match the angle on the conduit. You also have to watch the depth to avoid breaking through. Here's a cross-section view:
Cut two short pieces of 2”x4” and screw one onto the bottom of each end. The instructor said this would prevent the beam from splitting on the ends. I'm not sure why that would happen, but he obviously knew what he was talking about, so that's what I did.
Cut the 5/4"x6"x 12' in half lengthwise to create two 5/4"x3"x12' purlins. These will be attached to the conduit along each side of the greenhouse using conduit clamps.
Drive rebar halfway into the ground (24" deep), five along each side of the greenhouse, spaced 36" apart. If you're using lumber along the sides, you'll need to drill holes for the rebar.
Bend each rebar slightly (about 5 degrees) toward the inside of the greenhouse to match the angle on the lower section of the conduit.
Slide conduit over the rebar at each corner of greenhouse. Temporarily assemble ridge beam to conduit in order to mark and trim the top of the 4x4s (the 4"x4"s will provide support for the hoops on each end of the greenhouse). This photo shows the posts after they have been trimmed, with the conduit resting on top:
Permanently attach 2"x4"x36" between 4"x4"s at the top (finish framing window if you're going to install a window instead of a door on one end).
Slide remaining conduit over rebar.
Use wood rasp or file to smooth all the corners that the plastic might come in contact with.
Assemble ridge beam to conduit.
Attach purlins to conduit, about 2' up from ground level, just below the first bend.
Use conduit clamps to attach conduit to top of 4"x4"s
Cut 2"x4"s and frame in ends.
Cover sharp corners with pieces of inner tube.
Don't forget to cover the conduit clamps on top of the 4”x4”s
Cover sharp edges with duct tape.
Install plastic on each end of greenhouse, staple securely and trim excess plastic.
Install tarp zippers to create doors on each end.
Lay plastic over conduit and staple to one of the purlins. Pull the plastic snug and staple to the other purlin (NOTE: stapling is an interim step - you'll later screw lattice strips over the plastic to hold it secure). Two notes about the picture below, which was taken during the greenhouse-building class: 1) Normally, the plastic would extend to the ground (plus some), but the components of this greenhouse were being given away as a door prize, so it was only stapled on one side near the bottom for demonstration purposes only. 2) The black strip is some old, flattened plastic tubing that serves to protect the poly from the lattice strip that will be screwed on over the tubing. I covered the back of the lattice strips with duct tape instead of using plastic tubing, and initially attached the lattice with brad nails before screwing it into place.
Screw lattice strips over the plastic along the purlin and around the framing on the ends.
Note that the bottom sides of the plastic (below the purlin) is left unattached so it can be raised when needed for ventilation, as shown below, or lowered to hold the heat in.
Staple plastic to end frames (don't apply staples to the sides below the purlin – that part remains unstapled so the plastic can be rolled up like in the photo above).
Attach lattice strips and trim excess plastic. Note in the picture below that the plastic on the sides below the purlin is cut at an angle so it can be wrapped around the ends to cut down on air infiltration during cold weather.
And you're done! Now, here are a few things you might want to consider. An addition I made that turned out to be very helpful was two 2”x4”x12's running the length of the greenhouse. These can be used for any number of purposes – I use them to hang light fixtures from. Also note that I used 4”x4”s instead of 2”x4”s between the vertical posts to provide additional support.
I also added bird netting along the sides to keep critters out when the sides are rolled up. This is another advantage to using timbers on the ground along the sides. The bird netting was attached to the timbers and the purlin on the inside of the greenhouse.
Another addition was reflective insulation on the north end. In addition to providing insulation, it increases the light level inside.
I also recommend installing an automated ventilation system unless you can make yourself available to open the side flaps and doorways when needed. Even on the coldest sunny day, the greenhouse can get hot enough to stress your plants or cause them to bolt prematurely.
I still have a lot to learn about how to make the most of a greenhouse, but here's how I'm using mine currently. In the center, I have a home-made hydroponic system (maybe I'll make a separate post about that some time in the future). The hydroponic “tubes” are supported by four black 55-gallon drums that serve as thermal mass. There are also traditional beds along the left and right sides. This is how my fall planting looked in early November, 2011 (the bed on the left had only recently been planted, and only two of the four hydroponic tubes are shown because nothing was in the other two at the time). The greenery growing along the edging (outside of the beds) is chickweed. It came up on its own, along with some lambsquarter. It really likes the conditions in the greenhouse and has really taken off since these pictures were taken.
Here's how things look a couple of months later (January 1, 2012)...
While the weather has been relatively mild so far this season, I was having similar success last year when it was much colder...