szut, I have built both a wood framed and steel framed GH. First was the wood one. It has survived well but is in need of a repaint and recaulk which was part of the reason I went with steel for the second GH. The other reason was strength. Wood 2x framing is fine for smaller structures. But if you are trying to span longer widths, the size of the wood becomes prohibitive. Especially if you have high snow or wind loads to contend with (I have both). I think you will find Al for structural members is not doable for other than rather small sizes (hobby GHs, etc). Perhaps if you use built up Al girders, but that would be very pricey. Most commercial GHs use steel structures. I ended up going with steel tube arches from a commercial GH, which I made into a lean-to GH on the side of a barn. Given the structural loads it had to meet (50# snow and 135mph wind) it was relatively cheap (no doubt would have been cheaper in less extreme environments). IMO if you can build with wood 2x material, making the transition to steel tubing should not be that hard. You'll need a few new tools (metal saw and drill), but in many ways it was just like working on a big erector set. As a cost saving, you could look into buying a used commercial GH, or at least the frame. That could give you the structural material at a very inexpensive cost. Something that is harder to do with wood 2x material. My steel GH is too new for me to say how it will fair on the maintenance side. But I expect it to handle the moisture better than wood, although it will need some touch up painting no doubt in time.

Have a 15'X 40' quonset style that fits your description. The frame was $400 back in March,1994 when I put it up. Now, 12 seasons later I still have the same plastic on it. The bathroom ceiling fan that I used as an inflater has quit last year(thought it might last one season). I framed the ends with treated 2x4s and installed double used wooden doors on the entry end and a louvered barn fan on the other end. My suggestion to you would be to go gothic style just because I believe they are more sturdy. When I replastic mine I will install a wire-lock channel at the base, ends and even possibly 3' up one side to permit roll-up side for ventilation savings. Purchase the galvanized frame since they are not that expensive but avoid extra perlins since you can buy the cross connecters from a greenhouse supplier and pipes at Lowes or Home Depot cheaper. As you might gather from earlier, I'd save money on the peripherals like fans, inflaters and heaters by improvising used equipment. Erecting the frame should only take a few hours but I'd take the time to square up (equal cross corner measurements) and aviod uneven terrain. The harder part is to install the baseboard and frame in ends.ALWAYS USE ONLY TREATED WOOD. A 2x6" or 2x8" baseboard sunk 2" below ground level should be clamped to each arch and put a tech screw through each clamp into the arch also. The end frames can vary tremendously but I like to have 4x4" posts on either side of the entry door, sunk to below frost level(2'for you anyhow)and connect 4x4" burried to ground level along the entire end. From it I extend a few extra upright 2x4"s ,spaced so as to allow widths for fans, louver openings and flue for your heat source. I use metal plates to fasten the wood to theend frame at the top. They should be cut so as to not extend above the top of the arch, then tech screwed into the arch. When screwing into the wood be sure you have the proper type for treated wood. You can cover your ends with solid plywood, plastic film or any other material. I framed the ends of my larger greenhouses to install cheap auction purchased screen doors and windows. The glass sections from sliding glass doors works great too. Get 1x1" treated strips at Lowes to brace them, sides and top, inside and outside. When you install your first layer of plastic try to pick a calm day when temps are moderate. Too hot out and the plastic might stretch too much. Install one wire over each layer for convenience (if you go with the wire-lock fasteners). The wire-lock cannals will easily hold two wires on top of eachother. I would clamp two plywood pieces, each with a 4" center hole, together at the spot where you want the inflater, One piece on either side. I prefer the fan on the inside, near the ground near one end. you can cut the plastic out of the center whole anytime later. Wire lock channels also formed around the end arches also is much better than relying on batten strips to secure plastic at ends. A cordless drill/driver and circular saws are a must for the project. A mitre saw is helpful for framing ends. Also a level (and even laser level) and square are needed. Try to have a few extra hands available when put the plastic on although my wife and i did a 30x96 without too much effort on a calm day. A helpful hint when installing plastic: form softball sized balls or rocks around the one side(foot from edge) every 12-16' with heavy cord tied at the base of the ball. Throw the cords from one side to the next and use them to pull the film over the framework. Enough said. I'll let others fill you in on their methods.

Hello azee_2009, OK I originally thought it was going to be a rooftop greenhouse, but after rereading your second post I wasn't sure anymore. As far as the construction I used to build mine, it could be applied to a rooftop situation. By just building a sturdy base for the electrical conduit posts/poles. Similar to the baseboards I made for mine. The weight of it will keep it in place. The poles for my greenhouse only go down in the ground about 8-10 inches, and there is no cement holding them down (just the weight of the greenhouse structure itself). I just posted the link/information to give you some ideas. That greenhouse looks much nicer than the one I built. Although much more expensive too. Not including the subterranean system, and the hydro systems inside, I built my whole 10 by 20 foot greenhouse for about $800. Ya, heat will be challenging. Our outside temp today was about 92 degrees, but the temp inside the greenhouse was 108 degrees. When the temps here reach the 120 mark, greenhouse temps would likely be around 140-150 if I didn't have a multi tiered cooling plan. Cross ventilation is great in some circumstances, but it wont help much at all to blow 120 degree air on your plants when it gets hot. Given the best circumstances, swamp coolers may bring the temps down about 20 degrees. But if humidity isn't carefully controlled, they may only provide a difference of between 5 and 10 degrees at most of cooling effect by themselves. Just to clarify, when I mention high humidity, I mean in the greenhouse. That will depend a lot on temperature inside the greenhouse, water vapor given off from the hydroponic systems inside. Not to mention the biggest factor, how many plants your growing, and how big they are. Plants breath out water vapor (called transpiration). The water vapor (relative humidity) in the greenhouse will get higher and higher unless it's ventilated. But ventilating a cooler greenhouse with the (hot) dry outside air will only heat up the inside of your greenhouse. I have searched the web a lot & every where i have read that Double Poly Sheet that makes air pockets between them retain heat within the greenhouse & reduce heating costs about 50% compared to single poly coverings. So how would i be able to use it. As i have to fight against high temperatures. The air pocket in-bitween the two layers of poly is an insulation pocket. Air pockets are the best insulation there is. Insulation will protect against heat just as easy as it does against cold. If your greenhouse is cooler than it is outside, the insulation (air pocket) will help keep your expensive cool air inside the greenhouse stay cooler longer. If you have no way to cool the inside of the greenhouse, then it wont help much. The air pocket is just insulation, not a cooling or heating layer. It just helps block the heat transfer from one side to the other. Blocks heat from getting out during winter, and blocks against heat from getting in during summer. The 4-5 inch's between both layers you describe should be plenty. Typically it's only about one inch between layers. Though I would expect that you will still get much better results with a air blower between the two layers. The reason is if you blew the cooler air in between the layers, it won't give the air pocket a chance to heat up. Thus keep the heat transfer between the inside, and outside layers to a minimum. I want to know the link & name of software you are using to create 3D images. I want to download it to make someone understand what i mean. I use a free program called Google SketchUp. They have a pro version you can pay for, but I've always used the free version, and like it very much. I save all my drawings into a folder so I can expand on them later without starting over. Before you post your drawings, you'll need to remember to export and save it as a 2D image. Otherwise it wont be in jpg. format, and people that don't have Google Sketchup wont be able to view it. Before I built my greenhouse, I did a lot research online to figure out what would work the best for me, as well as my budget. Here are a bunch of the links I have got bookmarked from those searches. I know it's a lot to read over, and it's been a long time sense I looked through it myself. But a lot of it is in lists of topics. I just read through what was of interest to me. Arched Greenhouse PRESENTATIONS REFERENCE SOURCES Horticultural Engineering Greenhouse Energy Cost Reduction Strategies POLYTHENE GREENHOUSE FILM FACTS Chapter 11- Greenhouse Site Selection Chapter 12- Greenhouse Structures Chapter 13- Greenhouse Control Systems Chapter 14- Greenhouse Energy & Resource Alternatives "Greening" the Greenhouse Greenhouse Glazing Greenhouse Energy Conservation Strategies Greenhouse Energy Conservation Strategies: Temperature and Scheduling Managing Greenhouse Temperature

Costco is selling an 8' X 12" Palram for $1200 delivered, and it looks like it gets rather good reviews: Palram 8 X 12 When I looked, there were 48 reviews, and 47 of them were either 4 or 5 stars. As for shelving, I bought a couple of heavy duty, chrome plated wire shelving units at Costco for my greenhouse. They have been selling these for years in the warehouse for about $80-90. For that you get a unit that is 6' tall and has 6 shelves, but you can also make two 3' tall units out of it. The shelves are 18" deep and 4' wide and they are the perfect size for trays. You can get four standard trays on a shelf, so one of these units can handle 24 trays. I've had these in my greenhouse for 8 years and there is only a trace of rust in a couple of places as the chrome plating offers good protection from moisture. I'll add a picture here when I get a chance.