This question already has an answer here:
Is there guidance in any of the source books on damage taken when something falls on a creature?
For my new Loxodon Monk (you may have read about him) I'm planning on Shadow Stepping to a ceiling (which we verified is possible) and simply dropping on my enemies head.
Our current "homebrew modifier" is 1D10 per 100lbs if at least 10 feet in the air. We haven't been able to find anything official. Maybe we just missed it.