Built a roof for my 24' x 15' x 8' green house. Nearly everyday there are wind gusts up to 26 mph, and I'm in some sort of rope tornado alley. So I wanted to build something robust, but I have no help, so I figured I could build a maker pipe roof on the ground and raise it. Well it worked quite nicely. It doesn't even sway in the strongest winds and it didn't break a sweat when it snowed. My only issue is the 6 mil plastic sheeting that been taking a pounding from the wind. It's only been a few months and it is all stretched out. As a result, it now make a lot of noise. I should have went with the 12 mil.
So a 12' x 12' greenhouse is no little job. If I were doing it, I would seriously consider going down to 10'x10'. It would save some cutting and make things a little simpler. Then, my next concern would be what external forces will be exerted on the greenhouse. Snow? Wind? Tsunami? Etc. Assuming yes to just wind and snow. You should probably do a little math, and that will begin to tell you what you need. For instance, the snow load in my area is 35 pounds per foot squared. I have wind gusts of no more than 26 mph and that calculates to 1.7 pounds per foot squared. Again, these are non civil engineer/architect calculations. So I built for the snow load plus extra and my calculations said in order to meet my load requirement my conduit could not be more than 4' long. If you look at my framework, you will see that about every 4' there is at least one support system, that directs the load to the base. There should be 2 or 3, but that's a whole other topic.
Lean-to roof or A-Frame roof? For snow I like a steep A-frame because it keeps snow from accumulating too much. Plus, I don't need to put columns for support in the greenhouse. But it does use a lot more resources. For rain only and no snow load to worry about, I'd do a lean-to roof, it is simpler and it gathers rainwater well. Depending on how much rain you get for a 12'x12', there may need to be some columns. You have to do the math. A 10'x10' is less likely to need columns or need fewer columns.
Now let's talk about the base. My 4' requirement also extends to the walls. That means if I used conduit for the walls, they could only be 4' high. Any taller I would have to put in supports, and that would take up garden space. In addition to the greenhouse looking like spaceship crash-landed. 4' high walls aren't as bad as it sounds with the A-frame roof, because the A-frame has about 2' of headspace. Still, I think it would look too much like a jail. Plus I wanted to grow bamboo and a few trees. So I went with 4x6 posts, allowing the base wall to be over 8' and have a wider space between posts.
Wiggle wire would have done the same thing. The plastic film just stretches because of the wind load. I can and will tighten it, but I fear that it will affect the longevity of the plastic film if it stretches again.
I don't have pictures of how I extended the conduit with a metal filler. It would require me to put my latter in the greenhouse soil and I don't want to do that. So you'll have to settle for a quick sketch, I'll post.
I got the "bright" idea to make this a more year round structure, hence discovering Maker Pipe. I can appreciate the 10x10 suggestion, as it would be an easier build and allow for a better slope. 12x12 came from expansion of the current frame which has raised beds along both sides. I would have to empty one of the established beds and move it or make more cuts. Personally, making more cuts was the lesser of the two evils.
I live in central VA, so I don't have a high snow load, but enough that I do need a decent slope. I plan to make 2 more of the "walls" just like the first 3. That would give me 5 sidewalls 3ft apart. My intentions are the connect them along the roof on both sides and along the sides at ~6 ft. I'll prob come back and connect at 3ft as well if build goes well. 8ft poles connected with the 45's ~3ft away from corners giving a 6ft door (I may adjust to 3.5 and 5ft door). I made a zipper door in the plastic last year and would like to still utilize it.
I bent at 7ft because I'm 6ft tall and it just felt better. I'm considering coming down to 6ft to give a better slope without it getting too tall. I was surprised at how sturdy my shade frame was, but I know I have to do more with this one. Trying to get the best build I can with fewer pieces (should have paid attention in physics). Thank you for your help and that illustration would be much appreciated.
Your project sounds great. Experimenting is the basis of all science, so there is nothing wrong with your approach. Plus, I believe one of the primary purposes of physics is to turn your dream into reality. Dream first, physics later is how it should be. Just take some safety precautions. Has it snowed on your shade structure yet?
Light snow is 5 to 20 lb/ft2. In Virginia... I will assume 20 lb/ft2. If I visualize your structure correctly, the effective snow load area is slightly less than 12'x12' because of the pitch, but for the sake of time, we will call it 12'x12'. That makes it 144 ft2 at 20 lb/ft2. Now, if and when it snows at Virginia's "assumed" max load, the weight on your structure's roof will be just under 2,880 lb. That is a Toyota Corolla sitting on your structure's roof. Really take the appropriate precautions.
May I offer some suggestions, again remember my disclaimer. The great and terrible thing about a pitched roof is that it doesn't take the full vertical load directly, it sends some weight horizontally. It's the same reason we take small steps on slippery surfaces. Too big a step, like a pitched roof, and our feet head off in opposite directions. A fix for this problem is to install a crossbar. The height of the crossbar and the strength of its attachment matter. That takes care of the roof doing the splits to the left and right, but it doesn't stop it from falling backwards or forwards. Think of a table with 2 legs... right now you have 3 of those tables making up your shade structure. What gives a table its stability is the strong connection to the other 2 legs. You ought to connect your arches to one another at multiple points. It sounds like you have the bottoms of the arches attached very well, but keep in mind the amount of leverage generated by a 9' or 10' pole that's leaned on by a Corolla. Finally, will your knees buckle? If you can't fall in any direction because of bracing. You have 6 conduits as the base, will they buckle if you place a Toyota Corolla on top of them? You can add more legs, or use stronger legs, or trim down the Corolla (smaller roof).
I hope that helps.