After speaking to some local metal building guys... It seems like one of the higher end carports might be a good base to start from.  they can be braced to take up to 130MPH winds and 60lb snow loads.  My understanding is the roofing provides very little structure so changing 1/2 out to polycarbonate shouldnt cause any issues.

just tossing the idea around, the galvanized frame and roof for a 12x24 is only $1500.  seems like you could add extra hat purlins and wall bracing to really strengthen the structure for cheap.

Yeah, but then, I think most of the expense in a green house is the glass, unless you have a source of used glass. Either way, I think wood to be the cheapest and easiest, but that's because it's the material I have the most experience in. Did mine at $32 for 50 sq ft under cheap plastic. For a 20x20 I think it would cost me about $600.

I have 400 sq ft of 1/2" safety glass i got used and can get 300 sq ft of trip wall poly carbonate for $1200 delivered

All in all of the frame works after i close up the ends I might be into the greenhouse maybe $6k, that's about  $21/sq ft

Definitely more expensive than a hoop house but the poly carb should be 10yr min, the glass is forever (assuming no rocks etc) & the r value of the walls would be like a houses.  

I was just wondering if someone else had beat me to it so I don't have to repeat their mistakes

I have seen it.  The end walls are much easier than hoops.

You could use the metal they give you for the north wall.

i went by a local dealer and my only real concern at this point is the stiffness of the structure.  given, this example unit had no end walls, but you could shake the entire structure side to side.  i would think end walls would rigid it up a good bit though.

fwiw carolina carports will sell the frame and purlins for 75% of the entire building cost (so $1125 on a normally $1500 carport) but then I need to come up with the roofing on the other half.  

We built a lean to greenhouse using mostly recycled stuff 20 years ago . It grows foods through the winter,all my seedlings for the gardens ,over winters tender herbs and trees and I also use it to dry things and save seeds from several crops . I use old pickle barrels for shelves and since they are filled with water passive solar heat .. I also use them as planters and hang racks to hold seedling flats that also dry things . I repLaced the doubled greenhouse plastic on top once at a cost of about $200. THE ORIGINAL BUILDING IS 20 BY 40 FT A AND COST LESS THAN $1,000 TO build. frount and sides are recycled doors and windows . best thing we ever built .

sharon, what did you use as framing material.  

In the book, Health and Light, the author, John Ott explains that plants (and people) need full-spectrum light, which includes UV (both visible and invisible to the naked eye) light in order for proper ripening to take place.  Using glass, as in a greenhouse situation, is not the best.  Glass blocks UV light, at least to the extent plants need to undergo proper ripening and ethylene production.

Clinical trials found sheet plastic a better (as well as cheaper) method to screen wind, rain and airborne contaminants like bugs and particles, while still allowing the proper full-spectrum lighting needed by growing organisms, plants in this case.

I build welded steel buildings on my ranch in North Texas, which is noted for high winds.  I use "red iron" tubes.  Red iron is primed (red) steel and I generally use 2" x 2" square.  I have a pneumatic driver, which pounds the steel deep into the ground, having first dug a 30" hole with an auger, then pounding said steel post into the ground another 4 feet.  Concrete is then poured into the upper 30 inches and left to hydrate before further fabricating.  Once a superstructure, including solid steel gussets at corners and welded trusses at the roof is fabricated and welded into place, the structure is nearly indestructible. The posts (on 10-foot centers) are anchored 6.5 feet into the earth!

Sheet plastic can then be stretched over the building's sides and top, tacked in place with staples driven into "sleepers" (wood boards bolted to the surface of the exterior structural steel), to create the "greenhouse".

The cost is far and away less than using pre-fabricated, bolted- and screwed-together galvanized carports.  Primed red iron resists weather just as well as galvanized.  Many of my buildings are over 30 years old and show no signs of weather-related decay (rust).

Hope this helps someone.

Al,  whats your average rainfall and humidity level?  in Virginia where I live most of the summer is 80% plus humidity and would likely be higher within a greenhouse.  I had considered building a red steel structure but had concerns of rust, especially underground in our clay which holds a LOT of moisture.  that being said i have had several pieces of 1/8'' wall 2x2 sitting outside for about 4 years (use them as lightweight forks for my little tractor) and they are rusty as can be but still appear solid

polycarbonate/glass vs poly sheet - its possible that the UV blocking of glass and polycarbonate causes some loss in photosynthesis but it cant be that much, people have been growing under glass for over a hundred years and polycarbonate is one of the most used greenhouse materials.  For me, the strength of the polycarb and glass to support a snow load, not have to be replaced for a decade plus, and much higher Rvalue outweigh any possible weakness for UV light transmission.

Rainfall near me (Farmersville, TX) is about 43" per year.  Winds are horrendous (but I have windmills, which make electricity, so I'm good with all that wind).  My soil is black CLAY.

As for rust:  I find the vertical steel NEVER rusts.  The horizontal runs allow water to pool and are subject to some decay.  When I see things going south, I touch-up paint it with a burst of sprayed-on (spray paint) primer.  "Rust" being ferrous oxide usually and in some circumstances, the reduced form called ferric oxide will be produced when mild steel is left to a cycle of wet-dry-wet, especially if it's in contact with soil, which introduces a variety of "electrolytes" which will likely hasten the process.

If you're really concerned about decay in the soil, bolt on a chunk of zinc metal or magnesium, which is better even still.  Magnesium has the lowest reduction potential of all known metals.  Your water heater has a magnesium "anode" inside its storage tank.  If oxidation occurs, it does so at the magnesium; it's just how the chemistry works.  As long as your steel building is tied electrically with all the other steel, that magnesium tied into the 'grid' as it were, will do the trick.

As for the business about glass vs poly-carbonate vs polyethylene sheet plastic, all I know is what I've researched through books.  John Ott's book, Health and Light, was my source today.  Hard-core permaculturalists aren't satisfied with "good-nuf" as in glass or polycarb being "okay".  I for one, am on a trek for a personal best, which is why I mentioned the often-overlooked (or unawares) topic of full-spectrum lighting.  I'm an odd lot though, so as Jimminy Cricket would say, "Let your conscience be your guide."

I appreciate the input, gives me more to consider!  

im 100% sure im not as hardcore as a lot of permies.  i like to value engineer my projects for total lifetime cost/maintenance/impact, theres not enough time or money for perfection.

Al, id be interested to know if you could offer some input as to metal structural member sizing.  you say you use 2x2'' tubing on 10' spacing, what wall thickness?   Do you use trusses between the 10' spaced columns or just more 2x2 tube as a top plate?  do you use purlins for roofing to attach?  any chance you have a picture or two of what you are describing.  

Erik , My husband built the greenhouse as an add on to our black garage that he also built . he put dirt in the holes in the blocks to help as a heat sink . The greenhouse has a block foundation and the framing is just regular 2 by 4s .he got free sliding windows and glass patio doors for the walls in his construction work . It has fed us for many years . There's a frost proof faucet in there as well as a small electric heater that I turn on in severely cold weather . I upen windows in the summer as well as put a shade cloth over it so I can use it for drying things .

@ Erik:

I use 14-gauge, 2" square tubing on EVERY aspect of my builds.  This means I get a better price on the steel (buying it in pallet-sized quantities).  I haul it with a 30-foot farm trailer.

I fabricate with both a Lincoln "cracker box" stick welder using 1061 or 6110 rods -- I'm kinda dyslexic at times -- when fabricating everything EXCEPT when I have to weld overhead.  Then I use a Hobart .030 flux-core MIG.  I'm a year short of 70 and holding a heavy stinger above my head, not to mention the shower of molten sparks drives me to use the MIG whenever I can.  I turn the power up to achieve better penetration.  

All walls, both interior and exterior are 2".  If I want insulation, I use sprayed-on, closed-cell foam (wicked expensive, by the way).  If I have an interior wall on a plan, I bury the support posts just as if it were a perimeter vertical support.  My stuff is built like a brick shit house, my having been 'bullseyed' by an EF4 tornado about 5 years back.  Since then, I've become a big fan of concrete and steel construction.

Exterior walls and interior walls (if covered) are done so with "R" panel, screwed to the steel framework.  All electricity is pre-planned using EMT (metal conduit).  It makes pulling new wire (should the case arise) a breeze.  It looks a bit "industrial" but like I said -- brick shit house is my goal.

I fabricate my own trusses and place them atop opposing 10-foot-centered vertical framing members.  I do not "pre-load" the bottom chords (they are not engineered) but I do "gusset" triangle corners with welded 11-gauge steel plate.  "Rafters" are 2" square tubing.  I used to use purlins, but I've found 2" square tubing, properly welded in place is stronger and easier to hang by myself.  Square tubing tends to be more stable (bending forces) on longer runs than perlin.  If I'm doing an attic space, I lay undulated galvanized sheet metal down, screw it to the attic side of the ceiling structure and pour in floatsome, which is air-entrained concrete.  This gives me a ceiling, which will support massive loads of crap and allows me a second roof, should Mother Nature decide to rip mine off.  Winds in North Texas are  brutal and Mother Nature is, well, nothing short of a female dog!

Hope this helps.  As soon as I have something in "frame" I will snap some pictures and send them along.

I am cheap, so I use a Geo Dome Jungle Gym for kids with plastic thrown over the top and the plastic zip tied to the bottom rail. I only get 78 square feet of space, but for $500 (if I bought 4 of them at $135) I would get the same square footage that the carport would offer.

I like the roundish shape, and the vertical space gives more room than what initially appears, and the bars make a great place for hanging hanging plants.

The silly thing is indestructible, quite heavy so it resists the wind, is easy to get into (just lift the silly thing up and step inside, or cut a door in one grid if so inclined), portable, and did I mention CHEAP!

We only use ours for Spring starter veggies for the garden so in the summer it does double duty as a Jungle Gym for kids. (This is not mine, but displays what I am referring too)

You might wanna consider fan inflated double layer polythene. The r value is very similar if not identical to double glass or polycaronate and will cost way less. The fan doesn´t need to be anything fancy, i´ve seen quite large structures blown sufficiently by a 60w unit. Longitudinal runs of galvanised wire can help support the plastic from a snow load.

To Al's good enough comment regarding glass windows: They found in some studies in humans and vitamin D that glass prevents the process. Also recycle code 1 plastic is supposed to be uv penetrable, which is supposed to be good for water purification in emergency situations (if you have access to sunlight). However, from experience, I can tell you that it's not a big deal in seedling starting. I mostly grow babies in windows. I do move things out to an unheated green house, when the weather cooperates, but have spent months with the seedlings in an East or West facing window in an unlit room. I just know these things won't grow huge fast, but that's a small & slow solution. Low energy, and easy. I also, because of running out of Windows, have some grow bulbs. I'm not really impressed. I only turn them on in cases like this upcoming week where the green house is going to be too cold and all the windows are full. Even the pole facing one and glass blocks. Between the windows and the green house, I usually get seedlings to look comparable to those in the stores. When I was first getting started, what hurt my seedlings most was 1: not petting them. Pets build stem strength, against wind and such. 2: not fertilizing them with a liquid fertilizer. Starter mix ain't enough. 3: trying to start seed in a window (windows in winter here are usually about 40 degrees.) Now I start them by a heater vent and then move them to the window.  As for how hard core I am, I'll let you decide.  Oh, and I should mention that I also have a bunch of trees inside all or part of the year that seem to do just fine.