You got me beat... My darkroom has a concrete floor, about 3 x 7 feet of it. It's a "re-modeled" coal cellar that was used to store coal when that once was fuel for the big gravity furnace the house came with.
But it works. For prints up to 11x14. I would like to print 16x20, though. But any larger than that, I would have to make some serious money first. I have no idea how people can afford those large sheets of paper. When you start hitting eight or nine bucks a sheet... Geez. It would break the bank in no time.