When we talk about height, usually we mean the distance from an object to an external zero reference such as the ground.  I'm looking for a single word that describes how much height is *resting on* or *hanging from* an object, where the object itself is the zero reference.  For example:

> If the top floor has a ____ of zero, then the third floor of a five-story building has a ____ of two.

If there's a word for this concept in mathematics, that would be ideal.  Or a word from genealogy expressing the number of generations of descendants a person has.  A word from architecture or shipbuilding might also fit.  I considered the word *freeboard*, which is roughly the height of a ship above the waterline, but the reference always seems to be the waterline.  I don't think you'd speak of the freeboard above a particular deck.  I also thought about the word *load*, but I don't want to express the total mass or volume of stuff, only the dimension of height.