Rif Succah 5a {Succah 9b continues ... 10a}

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5a

{Succah 9b continues}
A succah atop another succah -- the top one is valid and the bottom one is invalid.
Rabbi Yehuda says: If they do {=can} not live in the top one, the bottom one is valid.

{Succah 10a}
Gemara:
And how much space must be between the two succahs {=how tall must the top succah be} for the bottom one to be invalid?
Rav Huna said: A handbreadth.
And Shmuel said: Ten {handbreadths, the minimum size of a valid succah}.
Rav Chisda and Rabba bar Rav Huna said: Four {handbreadths}.