After battling with a photograph of a flat-roofed building where views into the windows exposed blue sky upstairs (where there was supposed to be a flat roof), I remembered the glass double-sided ceiling tiles I'd made. I expect you don't remember those so here's a link: LOOK AT THESE TILES NOW. In theory, the same approach should work for solid tiles too, and if it does, lo and behold, no more toplessness! Ceilings at last! BUT (there's always a 'but' in my blog posts) there are a few problems. (1) modified floor tiles don't reflect light, so lamps and windows have no effect on them, making them no good for ordinary indoor floors, and (2) lighting can be a bit of a fiddle: build on top of the ceiling, and it goes dark; don't build on top, and it can be VERY light!