I've built several covered ship docks, but all of them have been stone walls, with a large peaked roof. All of them are at the build limit in the center of the top peak, but I did not use any supports other than core wood. I also started the walls near water level in most cases.

I believe this is the largest example:

https://steamcommunity.com/sharedfiles/filedetails/?id=2616846356

I've also make some buildings with large areas covered with stone roof, although sometimes with iron reinforced beams.

My guess is that, if you used terrain/stone as much as possible on the sides, followed by stone walls (using the large 4x2 stone), and then used stone floor pieces across the top, likely with iron reinforced beams integrated into it, you could build a 100% stone structure.

Either that, or you will ALMOST be able to build it, which, of course, is very frustrating. :-)

One thing to note with the stone roofs is that you will likely have to build some sort of support beams while you're building the roof, and cross your fingers that it will not collapse when you remove the beams. The stone tiles are not their most stable until filled in on all sides.