Ezekiel 41:8

Of the house; of the chambers, or the three stories of them; it is not the temple. 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 beams, joists, and boards to the second; six great cubits; and from the third floor to the roof of the chamber, a like number: to which if we add one cubit for thickness of each of the three roofs or floors, you have twenty-one cubits for height, ten yards and half high.