If you want to build a greenhouse of your own to save money on produce, I applaud your intentions. A greenhouse for vegetables is a nice way to save money and provide good nutrition. Unheated greenhouses can help you raise vegetables year round in most of the country, so they can substantially contribute to lowering your food bill.
You can also save money by building your own, and if that's your goal, then the Quonset hut style is the design I suggest. This is greenhouse #3 for me and it's made from chain link top rail on 2 foot centers, but with a twist, or rather a bend.
The top rail steel tubing is bent into hoops to make ribs for the greenhouse. This approach simplifies construction – a rib replaces two walls and two rafters.
The Quonset hut structure makes use of the natural strength of a semi-circle, and it is easy to brace. It can shed snow reasonably well because of the curvature, and provides lots of usable space under the galvanized arches. If you build it to be a moderate size, you won't have any vertical supports on the interior to get in the way.
The photo above shows a 20 foot wide by 62 foot long greenhouse with most of the structural elements for the "hoop" portion in place. Plenty of preparation went into building this, and I hope to highlight much of this in the details below. Such a Quonset hut or hoop house style is popular for small backyard gardening structures, and it's simply because they're easy to construct and typically give the user lots of room inside for the amount of material used to build a greenhouse.Whether you're trying to be frugal or not, getting a lot of space for the materials used is a key to success. After all, that's why you build anything at all, to get usable space out of it.
Let's talk about planning to build a greenhouse. I'm a deliberate kind of guy, so I like to plan, then work the plan. It's the best approach to success, regardless of the endeavor.
The truth is that many times along the way I modify my plans, including the original plans to build a greenhouse like the one shown above. For our discussion, I'll discuss my original plans and where I made a variation and why.
So, let's take a look at my plan to build a greenhouse using a tubing bender and top rail fencing material:
There were three key assumptions when I decided to build a greenhouse of this size and in this location. They were:
Photo above shows fasteners accessible for inspection and tightening, and use of nylock nuts to help prevent backing off.
The design makes use of 33 ribs, each made from three pieces of 10 foot tubing, bent exactly the same way, and fastened together to make an arch that is about 20 feet wide and 12 feet high. These ribs make up the walls and ceiling.
Four purlins and a ridge piece hold the ribs together. Purlins are horizontal members that tie the ribs together to keep them spaced evenly. The ridge piece is similar to a purlin except it is installed directly overhead, instead of on the walls.
These five horizontal members are necessary as the curvature (up and over) runs about 30 feet. The purlins and ridge piece provide fastening points about every five feet across the curvature of the structure. They also provide more points to tie into with other structural members to make a sturdy shell.
Cross bracing on the walls is necessary to keep the building from swaying on the foundation from end-to-end. Some people use cables or strapping for cross bracing, but these only give strength when pulled - very much like a rope. I wanted rigidity, so I used steel tubing - it offers strong resistance against forces that push and pull.
Photo left shows cross bracing or angle bracing on the walls to add shear strength. The angle brace is shown using colored dots. Notice that it's woven between the ribs and purlin.
At each location where the tubing crosses another tube, a bevel head sheet metal screw is used to secure the tube to one another.
When you build a greenhouse with a large arch like this, you better have diagonal bracing to keep the curvature in shape against wind and snow loads. Diagonal bracing also helps distribute snow loads lower on the structure and they help strengthen the shell.
Proper anchoring of the sides is especially important for high wind areas like Wyoming. I opted for a staked anchoring system for each rib as this was easy to get each rib positioned at exactly the same level and lined up perfectly. My plans were for an overall building anchoring system that pulls the structure down onto its staked foundation. The main anchoring system would employ large turnbuckles and "dead men" buried deep in the soil.
Anchoring with stakes offers adequate support for initial construction, but it's not ideal because the stakes can become loose in the soil just through vibration. The turnbuckle arrangement is also subject to loosening, so the anchoring of the structure will be accomplished with dead men buried near the based of the building and connected to the ends of the ribs. This location for a connection will also get the hardware low enough on the structure to avoid conflict with roll-up sides.
The dead man anchoring system will also provide resistance to pulling and compression forces, so it won't matter which say the wind is blowing. My plan is to secure angle iron across the ends of multiple ribs as a span, and create about six spans per side. A dead man will be buried just outside the perimeter of the structure, centered on a span, and welded to the angle iron. The dead man will have a "foot" welded onto the bottom of a steel rod that will be buried about three feet deep.
Bracing each end will involve 2 rigid steel tube tie-ins between the upper door frame (the center of each end) and the upper purlin. This will help keep the door frame and ends from bowing in or out from high winds. Any pressure on the door frame will be transferred to the purlin which in turn will push the building along its length. Since there are 3 sets of cross braces on each side of the building, there is no way the purlins are going to budge.
When you build a greenhouse of a rounded or Quonset hut design,
the ribs can be assembled as a separate activity. This allows much of
the building to be prefabricated. Photo below shows the tubing bender that I used. It's mounted to an inside wall of my barn and made for easy work when it came time to bend the tubing. When you build a greenhouse with an arched structure, it's important to bend the tubes carefully and all in the same manner.
The ribs are attached to concrete form stakes driven into the ground about 2 feet deep. Four side purlins and a ridge piece are used to hold the ribs together. Diagonal bracing every fourth rib is used to tie the center ridge piece to the lower purlins on each side.
Ceiling joists tie the upper purlins on either side of the arch with the diagonal bracing. When you build a greenhouse this way, the joists and diagonal bracing can be tied together to form sets of triangular shapes adjacent to the shell that make the structure much stronger than they would be without them. This strength helps resist our regular high winds and will distribute snow loads from the ridge to points lower on the structure.
The picture below shows that the joists connect the upper purlins together, and the diagonal bracing runs from the ridge piece down to the lower purlins (not shown). The joists and the diagonal braces are tied together to make 5 triangular shaped sections that create a strong shell across the upper portion of the ribs.
Cross bracing on the sides of the ribs was tricky because the walls are not straight up and down. Instead they are curved to form the roof as well as the walls. To create side bracing, sections of pipe were bent by hand in a gentle arch, then woven between the rafters and lower purlin until the bottom of the cross brace pipe met up with the bottom of a rib, and the top end of the cross brace pipe met with a rib almost as high as the upper purlin.
There are 6 pairs of “V” shaped cross braces, 3 pairs on each side, with one set at each end, and one set in the middle.The cross bracing achieves an excellent angle to resist end-to-end movement, and it ties in 7 ribs and the lower purlin. Reaching fairly high into the arch, the cross bracing adds more strength to the upper portion of the building that will be hit hardest by wind.
The photo to the left shows the angle bracing at one corner of the greenhouse. Notice that is is secured to a foundation stake at the bottom and attached high on the third rib on either side of the center.
Wherever the bracing crosses a rib or purlin, an attachment was made with a machine screw, washer and nylon locking nut. In addition, the cross bracing is woven beneath the ribs and over the purlin, so it is locked in there tight.
Ribs are doubled at the ends and middle of the structure to accommodate composite 1 by 8 boards that will be used to fasten the UV protected poly covering. Composite 2 by 6 boards are run along the sides at the bottom for the same reason.
A type of "C" channel fastener, called "poly fastener", will be mounted to the composite boards. The channel has an insert that is used to "capture" the poly covering inside the "C" channel, much like "wiggle wire."
Photo right shows the underside of the double ribs at the end after the composite decking material was installed. Notice how the tubing is cut off flush with the outside of the outer rib. This is done very easily with a portable ban saw. There is no guesswork on angle or length when you cut the tubing flush after it's installed.
The double rib in the center of the building covered with the 1 by 8 composite material will be used to staple the poly covering in place using plastic lath on the outside of the poly cover. In this way, we only need to stretch the poly covering halfway across the structure (from the middle to the end) and then secure.
Otherwise, we'd have to stretch the covering all the way from one end of the structure to the other, more than 62 feet before securing. Stretching only half way should allow for a tighter fit and should be much easier to do.
When you build a greenhouse, you want it to last - that means the poly covering too. Get one that is UV protected. Here in Wyoming, we're at over 6,000 feet in elevation - closer to the sun than most - and about 72% of our days are sunny. That's a lot of UV exposure. Think about it this way, my rule in Wyoming is 45 minutes in the sun, without sunscreen, and you're burnt. That's how intense our sun is.
Remember that you'll build a greenhouse in a week or so, and the sun will have all day long, week in and week out, year after year to destroy the greenhouse film, so get the good stuff.
Note: You must use UV protected poly. You can't save money by using plastic sheeting that comes in rolls from the hardware store. It won't last more than a year or so in intense sun before it crumbles away. The greenhouse film is more expensive, but it will last much longer.
The woven poly, cinch strap, poly fastener and installation tool for this project were all purchased through Northern Greenhouse Sales where Bob and Margaret Davis are the proprietors. They're nice people to deal with and they've been in business for almost 30 years. They specialize in strong, UV protected woven poly greenhouse covers and also carry pond liners and poly for other outdoor projects. Give 'em a shout and tell 'em Clair Schwan sent you.
Tools that are typically used to build a greenhouse of this type include:
Power tools aren't necessary, but they make everything much easier to accomplish. Your most important tool is a friend to help hold things in place and pass you tools and fasteners. When you build a greenhouse like this, it's a two person effort.
The photo to the left shows me and a friend assembling the structure. When you build a greenhouse this big, it's best to have help.
You can build a greenhouse like this by yourself, but it's much more difficult. Cut down on the time, effort and frustration, and get a friend to help out. Thank them now, and pay them in fruit and vegetables later.
And, it's best to have a nice sturdy platform for working overhead. Ladders work well, but sometime scaffolding is the best way to build a greenhouse because you can work on your feet, have a nice platform to walk around on, and have tools and materials at the ready.
One last word on construction, when you build a greenhouse of any style, the more perfect you conduct each step, the more perfect the end result will be. The most important parts to get perfect are the ribs and the foundation.
The ribs should be assembled on a flat surface so there is no wave in the rib. The foundation has to be level and evenly spaced so the ribs are placed with consistent spacing.
When you build a greenhouse, the word "perfect" means "well made." We want to build a greenhouse in a well made manner, but it isn't necessarily a problem if you have some minor imperfections as the plants growing inside don't care. They won't care if your purlins aren't exactly level or your ribs are spaced a little unevenly.
The key point to remember is that your construction won't get more precise as you build a greenhouse, it will get less precise. Since you aren't going to disassemble the thing and start over if you find a minor imperfection, the minor imperfections will build on top of one another as you go, resulting in an ever less perfect structure.
Therefore, when you build a greenhouse it is important to start out as exact as you can and keep to that standard (without driving yourself crazy). A little imperfection is just fine. It won't hurt a thing. Remember, you're setting out to build a greenhouse. As my neighbor is fond of saying, "You ain't building a Swiss watch."
The underground heating system for this greenhouse is similar to the other two. The system has 2 circuits of 240 feet of 1 inch Pex piping, with one circuit on the south side, and one circuit on the north side. Each circuit is directly under a planting bed, and operates independent of the other. Each will have its own insulated 55 gallon drum buried deep underground and plumbed into 5 flat plate solar water heating panels.
The piping beneath the beds is about 24 inches deep, so it should have lots of soil mass to heat up for energy storage. Each circuit represents a thermal battery of about 930 cubic feet.
Photo above shows my trencher. It's a serious piece of equipment, and it's necessary for doing many of the construction jobs I have around my place. You can't hope to hand dig piping underground, you need to use heavy equipment for these kinds of jobs. One of the keys to success when you build a greenhouse is to do all of the dirt work first because you simply can't get heavy machines inside the greenhouse once you have it erected.
With the piping about 24 inches deep, it's plenty deep enough that garden tools won’t come anywhere close to it. The storage drums and pumps will be located sufficiently deep so that the system drains back when shut off. This will concentrate the heated water in the insulated barrel deep in a pit to help keep the pump from freezing, even during the coldest of weather.
When you build a greenhouse of your own, you might want to build a waste oil heater for it like I'm doing. Each circuit for greenhouse #3 will be connected to a surface level waste oil heater that will be housed in its own "dog house" near the structure. The waste oil heater will allow supplemental heating for limited periods, or when I want to get a “push start” for the solar system. All waste oil heaters and heat exchangers will be homemade, so the cost will be scrap materials, electricity for shop tools, and my time.
Rigid closed cell foam insulation surrounds the planting beds of this greenhouse to a depth of 2 feet. This will help hold in energy from the hot water system and soil warmed by the sunlight. Otherwise, the colder surrounding soil will rob energy away from the planting beds, especially during the winter months.
As you probably figured, trenching for the pipes and insulation must be done first, and well marked so you know where the building has to be set in place. If you don't mark the location of piping and insulation underground, then you might drive a foundation stake into one of them, or you might place the foundation stake so close to one of your trenches, that you don't have enough undisturbed soil to hold it in place with the strength you desire.
The trenching part is a lot of work, but like the rest of the structure, it is a one time investment that should pay dividends each year for many years to come. So, build a greenhouse as best you can one stage at a time and you'll be happy with the results for many years to come.
Completion of greenhouse #3 is on hold because of three issues; adequate ventilation, foundation strength, and support for the shell. Those issues have been resolved with dead men anchors as described above, roll-up sides that will eliminate the need for vents and fans, and making two "ends" within the structure. The interior "ends" will incorporate buried foundational elements so they not only provide rigidity to the shell, but resist lifting up as well.
Other than that, what remains to be completed are the ends, and the underground pumping and storage system.
I'll be a happy guy when I can report on the finished product and offer a guide to construction for those who would like to build a greenhouse of a similar design.
Anyone who gets to build a greenhouse of their own design should be smarter than when they started, right? I am, so let me share with you my insights:
Good fortune to those who choose to build a greenhouse and take more control of the quality and variety of their food supply.