Ohalos 14:7

If the projections of the previous mishnayos didn’t have an opening of a handbreadth, then regardless of whether there is an opening of a handbreadth between them or not, if there’s ritual impurity beneath, between or above them, the uncleanliness extends upwards and downwards. The same is true of two curtains that are a handbreadth off of the ground.

Ohalos 15:1

A thick wool coat and a thick wooden block do not convey ritual impurity unless they’re raised the space of a handbreadth from the ground. Folded garments one above another don’t convey ritual impurity unless the topmost garment is raised the space of a handbreadth from the ground. Tablets of wood stacked one above another don’t convey ritual impurity unless the topmost tablet is raised the space of a handbreadth from the ground. If the tablets are of marble, then the uncleanliness extends upwards and downwards.