Yea. It's top rail. 1 3/8th. Ground posts are 1 5/8ths fence post. 12 foot wide ribs are 2 pieces. 20 foot wide ribs are 3. Jigs work. Diy or store bought. Hand roll benders suck. Electric benders work the best.
I built a bunch of expensive kits before I started bending my own. The kits used same top rail. In fact I steal unused pieces from the kits if I need it.
This is a veg green house so it won't be auto black out, but at some point I'll do a thread on cheap and easy auto depps. Maybe I'll even make this one an auto depp just so you can't see it from outer space when it's lit up at night.
This will be winter month veg. 2 layers of poly with inflation blower inflating an air gap between the two for insulation. A pellet stove with auto ignition and thermostat for heat and dehumidifying. Lighting will be basic cfls for extending day and also hid for pushing growth on those really overcast days. Im off grid so this is all running off my solar system (and diesel generator when I'm running hid).
That's building the foundation. I use the larger posts that the ribs side into. I use a string to line them up. When you look down the line they should all be dead on and you should just see the first pipe when you look down. This greenhouse is not being build on completely level ground. It's rainy season in norcal and I could not level a pad. It would just turn into a muddy mess this time of year. It's on a slightly southern hill with a bump in the middle and the low point is 3 feet lower then the highest point. The foundation is still 99.99% level.