Built mine from house windows removed during remodel.

I used a lot of french doors,they are easiest as they are tall enough for floor to roof. Other windows will need to be paired with twins (width wise) so they can be deployed one over the other if desired.

Windows that are hinged can be used with temp controlled auto-vents. I rigged it so that 1 vent control could open 3 windows using pulleys and a counter-weight.

I did not use any windows on roof, they were all wood frame for one thing, and then would hold rain and rot fast, instead I got clear corrugated poly-carbonate panels. I do not have a roof vent, the roof pitch is only about 12" over 8 feet. I could always add a vent at the top of the roof gable...

I would line up the "tallest" windows for the South Side and make a short "cripple" wall for the other sides so the windows only need to be on the upper half.

That is great! We believe that everyone should have a greenhouse. My biggest concern is your roof. Not to open a can of worms, but if they are old windows the chances are that they do not have tempered glass in them. Then again they may. There should be a "sticker" etched into the glass in the bottom corner if they are tempered. I would be hesitant (not to mention it doesn't really meet codes) to put regular float glass above my head in the greenhouse. If you go up a couple of rows with cinderblock there is no problem at all on the walls. The glass does not need to be tempered unless it is with 12" of the ground. Pavers make an excellent floor in a greenhouse as they provide the needed drainage. For your roof (if you have it in your budget) the polycarbonate sheets or maybe even a UV protected film would work. Hopefully you will find some tempered windows in your salvaged materials and won't need to explore other options.

I have no idea what may be tempered or not- hadn't even thought about it as a thing. as to how far off the ground. Good to know. I wanted the cement foot for the greenhouse just because I like a solid base for a greenhouse. But now that you mention "regular float glass"? I assume that means the stuff that breaks like shards and would suck to have overhead... hadn't thought of that, ouch.

Some of the windows in the pole barn do have stickers and such in their corners so maybe some of them are good to go. All of them are the nicer newer windows from 10-5 years ago. None are newer than 5 years, that's how long they have been in the pole barn. I know all of them are good quality expensive windows- the guy does houses for the oober-rich tourist mansions around here. It's why he wanted to store them in my pole barn in the first place, they were the really nice ones he didn't want to risk getting damaged in his barn. I'm not sure how many there are yet, most of them are pretty large. Takes up about 20' of wall, stacked 3 deep or so of various windows. Biggest problem with them are their sizes- there might be 2-3 pairs, but otherwise they are all different sizes.

The ones I spotted were kind of a mix of older vinyl frame stuff and wooden frame stuff. Those were leaning here and there all over the place. I'm pretty sure those ones will be older float glass and not nice tempered or UV stuff. Though there was a glass patio door out there- if so I'll keep those in mind for actual door use since it would go down to the ground. There's enough pole barn glass that I was already figuring on using some of the older ones for more light duty cold frame lids.

If I do find that I have windows of roof-managable size that have the temper marking, can I use those for the roof? Or maybe a better question is- just how big can the windows be before it becomes moot to use them as roof, tempered or not? There's some with the sections in them to make smaller squares, some that are almost door sized.

Yes, I will get out to the pole barn soon to measure and really look at the windows. And I do plan on talking with the building inspector again once I have my plans drawn up- just to be sure. Our local ordinances are generous- but I know part of the reason why it's initially a go is because I plan on not running electric or plumbing for the greenhouse. And want to do a cement foot/brick buildup instead of an extensive poured slab and framework thing.

And the cripple wall thing- intriguing idea. My north facing wall of the greenhouse will be very close to, but not adjoining the stable- that I'm cleaning up and converting to a garden/harvest shed. The south facing stable door will be facing the north wall of the greenhouse. I did need a solution for the matching north wall of the greenhouse. Instead of crippling the wall by making it shorter, I'll use a set of french doors there to accommodate the stable door size. But that does remind me I could cripple a lot of the north wall too because it will be so close to the stable. The other 3 sides of the spot are free and clear of any obstructions.

Thank you so much everyone for the suggestions so far!!

Yes, if you find tempered windows you should be able to use those in your roof. Yes, regular float glass is the glass that will break in shards. My job used to be going to homes and doing bids. I remember 2 greenhouses in particular that I couldn't wait to get out of. Both of them had rotting wood frames and regular float glass in the roof. It sounds like you have this pretty well thought out. It is so nice to hear about a building inspector who is working with you to achieve your goal. Looking forward to pictures when you are done. Tammy

Oh gosh. Fun project. I’m very jealous!