Ezekiel 41:8

The foundations — The lowest chamber had properly a foundation laid on the earth, but the floor of the middle, and highest story must be accounted here a foundation; so from the ground to the ceiling of the first room, was six great cubits; from the first to the second, six great cubits; and from the third floor to the roof of the chamber, a like number; to which add we one cubit for thickness of each of the three floors, you have twenty — one cubits for height, ten yards and a half high.