How to Make a Cheap Greenhouse
Use These Low Cost Construction Techniques to Build a 12 x 16 Greenhouse
By Jay Rodgers – Two years ago I began to experiment with an idea that was spawned from the pages of Countryside and Small Stock Journal. Someone wrote an article detailing portable and inexpensive utility shelters built from cattle panels. While viewing the pictures supplied, I could only see the greenhouse I already desired, but was too cheap to build. After locating a good source for the poly-cover (which I knew would be the greatest expense) and some rough figuring, my dream fell within my budget. So, I set out to learn how to make a cheap greenhouse, after some time, the 12 x 16 greenhouse was finally completed. What took me months to scratch my head in the process of building, could actually be completed in a couple days now that the kinks have been worked out.
Step 1: Location
Find a rather level surface away from any trees to construct your greenhouse. Consider wind and sun factors; I faced the entrance to the south for greatest light penetration (keeping the main beds running north-south) and tucked the structure behind the barn, which serves as a windbreak. Utilities may also become important, so keep those in mind as well.
Step 2: Supports
Measure and mark the front corners 12’ apart. With a 16’ tape, measure the first sidewall, marking the back corner with a rock or something. Now, the same for the other side. It is very important to get the layout square! I learned that the hard way and ended up with only a 12×12. However, if done right, you’ll get your 12×16 with less effort. To square the corners, keep measuring the diagonals until they are both equal—they should be close to 20’, ensuring the sides stay at 16’. Once the diagonals are equally 20’ and your sides both measure 16’, the layout is square. Now, you can run string along the sides or eyeball as I did to drive the t-posts. Drive four posts along each side to a height of 46” and place them roughly 5’4” apart, bringing each corner post in 2”. Be sure to get the posts plumb (using a level) and in line along both sides. Now is a good time to initially work your beds by double digging or deep tilling, when the sides are up this will be more tedious. Simply run your beds in a horseshoe, leaving about a 3-4’ aisle in the center to work from.
Step 3: Paneling
Double check your sides for squareness and adjust if necessary. I cannot over emphasize this, whether you are building a greenhouse or tackling a DIY fence installation project. Now, you’re ready to attach the side panels. Lean one side against the posts being sure to get the ends equal distance (2”) from the corner posts. Attach to each post with the t-posts wire ties or some heavy wire, making sure no sharp ends are pointing to the outside. Once both sides are attached securely, triple check your squareness and adjust if needed (you won’t want to proceed to the roof panels without a truly square base).
Before placing the roof panels, the side walls need to be reinforced by putting a 16’ 2×4 perpendicular to the inside of the side posts, fastening with plumber’s tape to the panel at each post and between each post. This will stabilize the walls, enabling them to carry the load of the roof. Also, for temporary support, tie a rope to each end of one side and secure to the opposite side’s end posts to keep the walls plumb while raising the roof panels. Start on one end by laying a panel on top of the sides, fastening one end of the roof panel to the inside of either side panel. The wire ties used for chain link fencing work great for securing all the roof panels because they are strong, yet easy to bend (again, be sure to keep all sharp ends to the inside). When secure, you can push the other end of the room panel in, towards the top of the side panel, creating the arched roof. Place this end of the roof panel inside the top of the side panel and secure it with ties. Proceed with the other three roof panels identically. After each panel is up, secure each roof panel, in several places, to the panel beside it with the wire ties. When all panels are up and secure, your greenhouse should be taking shape nicely.
Step 4: The Ends
Measure the width of your door and window. This will be the distance you’ll set the landscape timber supports. First, run a string from the bottom ends of each side panel as a guide.
Measure and mark the midpoint (6’) then center and mark the position of the door and window, this is where the door and window framing will be set. Attach a 2-1/2’ 2×4 (sharpened on the bottom end) about 1” from the ends of all four landscape timbers. The timbers are only 8’ each and this will give the height necessary. Now place the timber supports where the door and window widths are marked and drive them into the ground until flush with the top of the roof panel. Double check the fit of your door and window and then attach each timber to the roof panel with large fence staples or nails. Measure and place a 2×4 at the top of the door framing and the top and bottom of the window and then hang them both. Measure and attach a bracing 2×4 to each timber support to be level with and overlap the side 2×4’s. Once all bracing is in, the temporary rope can be removed and the structure should be solid.
Finally, measure and cut a piece of re-bar to go between the 2×4 on the bottom of the front door support timbers and the side panels. Attach to the outside of the 2×4’s with fence staples and to the side panels with wire. A 12’ piece of re-bar should go across the back in like manner.
Step 5: Wrap and Enjoy!
I found an excellent supplier of greenhouses and supplies, including poly-film coverings. When your roll of poly-film arrives, read the special instructions regarding the unfolding of the film. Before you wrap the greenhouse, tape 1” pipe insulation (comes in packages of 4’ tubes) on all outer edges of the panels to ensure no sharp corners or edges tear the poly covering. Choose a warm morning when there is no wind and a friend is available to help unroll the length of film. Measure the height of the front and back to be sure, but you should need approximately 35’ in length. Once cut, roll up the cover leaving enough to hang down the back of the greenhouse and place the roll centered on the top edge. Now, with your helper on one side and you on the other unroll along the top and down the front. Next, carefully unfold the cover down the sides. Start on one end, and simply consider the structure as a large gift you’re now wrapping. Cut out openings smaller than the door and window, being careful to not cut away too much. It will take some patience and careful rolling up and tucking, but you should get the excess covering to where you can nail to the 2×4 bracing and timber supports using large head roofing nails. Along the bottom, cut off any excess and roll up to attach the covering to the re-bar and to the first run from the ground on the side panels, using the binder clips placed approximately 12” apart. Congratulations! The hard part is over.
Sometimes in the fall, I place square bales of hay all around the bottom of the greenhouse to close off the gap at the bottom and help insulate the ground, keeping the beds on the inside from freezing. In spring when the days begin to warm, I remove the bales for mulch and you can roll up the sides as needed to better control inside temperatures. I also leave the sides rolled up all summer to avoid overheating and so the tomato plants get pollinated.
After completed and used a season, I began to notice some rather unique benefits to this type of greenhouse. Not only had I used low cost construction techniques and built a simple design, but practically all materials are completely reusable. Also, expansion in either direction would be possible simply adding more side and room panels. You’ll also find the usefulness of having the panels available for tying off shelf supports, hanging baskets and lights and even used for trellising. There are many possibilities with such a structure. Enjoy learning how to make a cheap greenhouse of your own. Once complete, turn your sights on researching topics like how to grow tomatoes in greenhouse settings and more. Good luck!
8 6’ Metal T posts, new
6 4’x16’ cattle panels, new
1 Storm door, auction
1 Lg. crank window, auction
4 Treated 2x4x16’, new
4 Landscape timbers, new
1 UV 6mil cover, new
1 Medium binder clip, new
5 16’ 1” pipe insulation, new
1 Plumber’s tape, new
1 20’ re-bar, used
Originally published in Countryside in 2003 and regularly vetted for accuracy.