Single pane glass was used to glaze GH's for generations and is still in use, but while it has it's advantages, it also has some drawbacks, especially old glass, which shatters easily and as few if any older windows were glazed with safety glass, there is the potential danger that can entail, especially on a roof.

However, if the frames are sound and you will be getting enough of them of the same size to allow some rationallity in your construction plans, a single pane glass GH would serve very well to jump start spring growth and extend fall harvests. One of the advantages of glass is the ability to whitewash for shade to extend use into the warmer months, then power wash it off for fall.

Also, with glass, if you run into some ill and need to make the GH safe for new plants, you can use bleach to remove mildew and/or sterilize; you can't with polycarb.

If you do end up using old plain glass on the roof, taping it with clear shipping tape might be a good idea. Criss crossing glass panes with tape helps contain shards if it's broken. Or lay a sheet of clear plastic over the roof to reduce breakage. It would also help insulate a bit.

Or, and even better; a solar pool cover. It would not only give the glass a lot of protection, it would also give you far more insulation to allow even earlier spring starts and longer fall extensions.