See Stacking Seforim -- the linked answer.
It says that a chumash goes on top of a Nach. (See the other answer for a dissenting view.)
According to that view... what about a loose binding? Say, the bound cover on a chumash falls off. If, for whatever reason, it is still kept with the chumash, should it be placed with it in stacking as well, or does it have no kedusah whatsoever and should go on the bottom?
What about if it's totally loose, not even around the chumash?