Sukkah 18A

ומודה רבי מאיר שאם יש בין נסר לנסר כמלא נסר ⁠— שמניח פסל ביניהם וכשרה.

And R' Meir concedes that if there is between one board and another board a gap the complete width of a board, then one places fit roofing from the waste of the threshing floor and the winepress, and the sukka is fit.

בשלמא למאן דאמר בין באמצע בין מן הצד בארבע אמות ⁠— משום הכי כשרה. אלא למאן דאמר באמצע בארבעה, אמאי כשרה?

The Talmud clarifies: Granted, according to the one who said: Both along the side and in the center a sukka is rendered unfit with a measure of 4 cubits of unfit roofing, it is due to that reason that the sukka under discussion is fit, as none of the boards is 4 cubits wide. However, according to the one who said that a sukka is rendered unfit with a measure of 4 handbreadths of unfit roofing in the center, why is the sukka fit? Each board is capable on its own of rendering the sukka unfit.

אמר רב הונא בריה דרב יהושע: הכא בסוכה דלא הויא אלא שמנה מצומצמות עסקינן, ויהיב נסר ופסל ונסר ופסל ונסר ופסל מהאי גיסא, ונסר ופסל ונסר ופסל ונסר ופסל מהאי גיסא,

Rav Huna, son of Rav Yehoshua, said: Here, we are dealing with a sukka that is exactly 8 cubits, i.e., 48 handbreadths, wide, and one began placing the roofing from the side. And he places a 4-handbreadth board and then 4 handbreadths of waste, and another board and waste, and a board and waste, from this side, so that the total measure of roofing from that side is 24 handbreadths. And then a beam and waste, a beam and waste, and a beam and waste, from that side, so that the total measure of roofing from that side is 24 handbreadths.

דהוו להו שני פסלין באמצע, ואיכא הכשר סוכה באמצע.

The result is that the sukka has 6-handbreadth stretches of waste in the middle of the sukka, totaling 8 handbreadths. In that case, there is the minimum measure of fit roofing required for fitness of a sukka in the middle, and everyone agrees that the unfit roofing in the rest of the sukka cannot render it unfit. Since the unfit roofing measures less than 4 cubits on either side, the sukka is fit both according to the principle of curved wall and according to the opinion that unfit roofing renders the sukka unfit with 4 cubits.