Frequently, if you or a follower walks across an area while trying to place a block, the game will get confused into thinking that space is occupied. Switching to another building piece and then back again will generally correct this and let you place the original block.
You can also put square or wedge-shaped foundations atop ceiling pieces of the same shape. Of course, if something knocks out whatever is holding that ceiling piece in place, then anything atop it will crumble too. Still, it can be a handy trick if you want a really thick floor on an upper story of your house.
In addition to the square and wedge foundations, there are also Fence Foundations and Pillars. Fence Foundations are like walls that you can stick directly in the ground, and they can support walls, fences, etc. atop them.
Pillars are interesting because you can actually extend them downward from a ceiling piece until you eventually touch the ground. In this way, you can make bridges by extending ceilings outward to their limit, extending pillars downward to the ground, and then continuing across with more ceiling tiles.