Sure, if you treat the ceiling like a subfloor and install 3/4" plywood on it and then nail the flooring to it. I'd want a bead or two of construction adhesive as well to keep it from rattling. It will be absolute hell trying to stain and finish it though. Most people that do this would use prefinished. It will all have to be hand sanded rather than using a floor sander. You'll need a drywall lift for the ply, along with the screw guns to screw it down and then a pneumatic nailer to nail it to the ply. You'll probably want scaffolding in order to finish it, as it's easier to do that on a platform than it would be a stepladder.

What kind of flooring are you talking about? I can see unfinished tongue and groove wide pine flooring working just fine.

live-wire-oak - Why would it have to be installed like flooring is installed? Why does it need a "subfloor" (so to speak)? Couldn't it be nailed to the floor joists or rafters? How is unfinished flooring that different from car siding? If the ply is needed to achieve some effect (like not having the seams all lined up along a joist) wouldn't the same be needed for car siding, anyway? What am I missing?

You can buy it and finish it before putting it up. Depending on the size of the ceiling, that wouldn't be all that difficult. (The difficulty, in my mind, comes in when you have to find a place to lay all the boards for staining and sanding.) And because the purpose of a floor is very different from the purpose of a ceiling, you can be a lot less careful than you would be if you were laying floors. I might not even sand it at all. And, it could also be finished in place with a long-handled tool, depending on the finish.

I agree, though, after painting just one ceiling in my house, that any ceiling work generally seems hellish. :-)

You have to have something to nail it to. Regular random width flooring won't even be close to line up with the joists. Unfinished flooring also can be slightly different heights, so you will get a bit of lippage when installed. A normal sanding with a floor sander usually levels that out just fine for flooring. You would not be able to do an aggressive sanding like that on a ceiling, so the lippage would remain, and depending on lighting, could look great (if there's enough to look deliberate) or it could just look amateur sloppy and bad.

Wood needs to be sanded in order to take the stain uniformly. Otherwise the rough spots will become dark blotches. If you are not staining but merely clear coating, the rough spots will snag your finish applicator. A ceiling full of fuzzy fibers isn't really an attractive look either. It's got to be sanded with at least and 80 grit.

If it is perpendicular tp the joists you could get away with nailing to the joists.
Ends that do not fall on a joist may be a problem.

If the strips are parallel to the joists you will need a nailing surface.

Old flooring was usually composed of very long boards. I lived in an old house that had zero butt joints in the 14' wide master bedroom. They were 6" single boards, all nailed to the joists directly with no subfloor.

Modern flooring is a mix of lengths, short, medium and long. Bargain wood flooring is usually mostly shorts and medium lengths and no long ones. They aren't going to be the exact right length to exactly work with your joist spacing. You would not be able to use that to nail to the joists without a lot of work cutting it to the exact lengths needed to hit the joist edges with two pieces. And then routing a tongue and groove or shiplap onto the ends if you didn't want to see big gaps due to seasonal moisture changes. That would be a lot of labor and a lot of waste.

It's much easier to just create a proper nailing surface with the plywood and then install. That only solves half of the problem if you're dealing with unfinished wood though.

I bet you got that floor on the ceiling by now. Does it make you dizzy when you walk into room? I've heard of crawling up the walls, but...

What's with all the people who tried to make this sound so hard? You didn't need a "sub ceiling" did you? Paralell to joist use furring strips, perpendicular no problem at all. A dab of glue in T&G will hold the butt joints fine. A few nailer blocks might be needed. A few face nails could be puttied. People, the cuts are made at ends where molding covers nails.

" find a place to lay all the boards for staining and sanding." You had some ladders I'll bet, set close together, after sanding and staining you set them across rungs. A pro floor layer stains his planks before installing, so when gaps open, unstained tongue doesn't show.

" Unfinished flooring also can be slightly different heights," He means thicknesses, why I've seen it vary as much as a 64th, even a 32nd, you get more than that just from uneven under-layment, on a floor it might matter, not on a ceiling.

Do you polish it with sealing wax? Have you finished with putting carpet on roof?

I bet you got that floor on the ceiling by now. Does it make you dizzy when you walk into room? I've heard of crawling up the walls, but...

What's with all the people who tried to make this sound so hard? You didn't need a "sub ceiling" did you? Paralell to joist use furring strips, perpendicular no problem at all. A dab of glue in T&G will hold the butt joints fine. A few nailer blocks might be needed. A few face nails could be puttied. People, the cuts are made at ends where molding covers nails.

" find a place to lay all the boards for staining and sanding." You had some ladders I'll bet, set close together, after sanding and staining you set them across rungs. A pro floor layer stains his planks before installing, so when gaps open, unstained tongue doesn't show.

" Unfinished flooring also can be slightly different heights," He means thicknesses, why I've seen it vary as much as a 64th, even a 32nd, you get more than that just from uneven under-layment, on a floor it might matter, not on a ceiling.

Do you polish it with sealing wax? Have you finished with putting carpet on roof?

I am curious for an update on this. I need to also do a "floor on a ceiling", or more correctly a "floor behind a ceiling". Reading this post brought up a few things I hadn't thought of. Hubby and I have been discussing some of the same issues except our beams will be in front of the "floor" we need to finish. I would love to hear if you decide to buy and take on this project and how you accomplished it if you did.