I was over at a friends house a while back checking out his green house and he gave me a good idea. I had always planned to use metal tubing for the greenhouse frame (electrical conduit or chain link fencing top rail) Conduit is cheap and strong I use it for trellises already. Anyway The roof on his green house is inflated sort of like those bouncy houses for kids. Which offers more strength and better insulation. So I did a little research because I had seen some inflated forms that people used to make concrete domes that were similar and I thought why not do the same with a green house for better insulation any way I found you can seal up seems or weld greenhouse plastic together with an Iron and some wax paper or soldering iron.
My Friends roof has to have constant pressure with a pump like a bouncy house but I thought why not have a sealed section with a valve so you can inflate it like an air bed and it stays inflated and maybe have to add a lttle air once and a while but no constant pressure needed. So I plan to experiment with this and will let you know how it works out.
Anyway here is a video of how to bend metal tubing:
And here is a site with more info and videos on how to bend the tubing. They also bend a peak into the tubing to make a parabolic or catanary arch which is stronger and sheds snow better. (One of these days I am going to build a concrete house this shape) 😉
Here is a site on making inflatable bladders of course this is just a small one but could translate to green house size: http://www.rusticgirls.com/how-to/inflatable-bladder/
Here are some more links on variations of welding plastic and making various forms out of it:http://www.instructables.com/id/Welding-plastic-bags/