Query (R. Hoshiya): If a person brought a board (that had the dimensions of a valid Sukah) and rested it on top of a pile of earth in the middle (or on the side) of a Sukah that is too tall - if measured from the board to the Schach, it is less than 20 Amos -

How do we view the Schach above the area to which the board does not extend? Is it viewed like air space or invalid Schach? If we view it like air, it will invalidate the Sukah if it is three Tefachim wide; if we view it like invalid Schach, it will only invalidate if it is four Amos wide (like the law of Dofen Akumah, as follows)...?

Answer (R. Yosa bei R. Bun quoting Chizkiyah): Why does invalid Schach only invalidate if it is four Amos wide? Because if it is less than four Amos, it is considered to be part of the wall of the Sukah (Dofen Akumah).

Question (R. Maisha): I am surprised that Rav Hoshiya had this query - why did he not learn it from the statement of R. Ba bar Mamal, who taught the Mishnah -

One who wove his walls from the top of the Sukah downwards, if the wall is three or more Tefachim from the ground, it is invalid.

R. Ba bar Mamal commented - This applies when he does not sit and eat in the shade of the walls (as he is sitting on the ground and is not protected by the walls as they do not reach so low) but if he does (as he is sitting on top of something and the walls are therefore shading him) it is valid.

R.Yosa: This statement of R. Ba could not be used by R. Hoshiya, as the Mishnah (in Maseches Eiruvin) teaches -

A ledge that is above the water - one may only draw water on Shabbos if he had made a ten Tefachim tall partition around it, whether above or below the ledge.

R Zeira/R. Yehuda quoting Rav - as long as the partition dips into the water a depth equivalent to the depth of a bucket (that he is dipping to fill).

Rejection: They are not comparable - the sea is a Karmelis, neither a private nor a public domain (so even a minimal partition is sufficient); but here, the Torah said "you shall dwell in Sukos" - from the bottom of the Sukah (which is the place that you actually dwell) you should measure 20 Amos.