Mishnah 9

If one constructs walls from the top down, then if the wall is three tefachim above the ground, it is invalid. [Hoever, if one makes walls] from the bottom up, then if the wall is ten tefachim high, it is valid, [regardless of the vertical gap between it and the s'chah.]

But R'Yose says: Just as a wall built from the bottom up is valid provided it is ten tefachim high, [regardless of the gap between it and the s'chach,] so too a wall built from the top down is provided it is ten tefachim high, [regardless of the gap between it and the ground.]