Sukkah 14B

שני נסרין אין מצטרפין. רבי מאיר אומר: אף נסרין כסדינין.

However, two boards placed on the sukka do not combine. R' Meir says: Even boards are like sheets, in that they join together to constitute the measure of unfitness.

בשלמא לשמואל דאמר בשאין בהן ארבעה מחלוקת, אבל יש בהן ארבעה דברי הכל פסולה, מאי ״מצטרפין״⁠ — מצטרפין לארבעה.

The Talmud elaborates: Granted, according to Shmuel, who said that the dispute between R' Yehuda and R' Meir is with regard to boards that do not have 4 handbreadths in their width, but if they have 4 handbreadths in their width everyone agrees that it is unfit; what is the meaning of that which R' Meir said: Boards join together? It means that boards less than 4 handbreadths wide combine to measure 4 handbreadths, which renders the sukka unfit.

אלא לרב, דאמר בשיש בהן ארבעה מחלוקת, אבל אין בהן ארבעה דברי הכל כשרה, היכי דמי? אי דאית בהו ארבעה, למה להו לאיצטרופי? אי דלית בהו ארבעה, אמאי? והא קנים בעלמא נינהו?

However, according to Rav, who said that the dispute is with regard to boards that have 4 handbreadths in their width, but if they do not have 4 handbreadths in their width everyone agrees that it is fit, what are the circumstances? If each of the boards has 4 handbreadths in its width, why must they join together to render the sukka unfit? If each board is 4 handbreadths wide, each is capable of rendering the sukka unfit on its own. And if each of the boards does not have 4 handbreadths in its width, why would R' Meir prohibit their use? But aren’t they merely reeds according to Rav? Just as one may roof the sukka with reeds, one should be permitted to roof the sukka with these narrow boards.

לעולם דאית בהו ארבעה, ומאי ״מצטרפין״⁠ — מצטרפין לארבע אמות מן הצד.

The Talmud answers: Actually, explain that there are 4 handbreadths in the width of each board and each renders the sukka unfit on its own. However, what is the meaning of: Boards join together? It is with regard to a completely different matter. They join together to constitute 4 cubits from the side. If one placed these unfit boards adjacent to one of the walls of the sukka, they do not render the sukka unfit, due to the halakhic principle of curved wall, which views that roofing as an extension of that wall. However, that principle applies only up to 4 cubits of unfit roofing. If these boards join together to measure 4 cubits, the sukka is unfit according to R' Meir. According to this explanation, the Mishnah can be explained in accordance with the opinion of Rav as well.