GEMARA

(Mishnah - R. Yehudah): Beis Shamai say, if there is no cement on a roof (of boards) he moves the boards and removes every other one (and puts Sechach in its place);

Beis Hillel say, he moves the boards or removes every other one.

R. Meir says, he removes every other one. It does not help to move them.

Question: We understand Beis Hillel. They expound "(Chag ha'Sukos) Ta'aseh" - not something Asuy (made in a way Pasul for Sukah, e.g. the roof of a house). Therefore, if he moves the boards or removes every other one, this is a new action. But what is Beis Shamai's reason?

If they require Ta'aseh v'Lo Min ha'Asuy, it should suffice to move the boards or remove every other one;

If they decree Gezeiras Tikrah (not to use boards, lest one rely on his roof), it should suffice to remove every other one!

Answer: Really, they decree Gezeiras Tikrah. Beis Shamai say, even if he moves the boards, it is Kosher only if he removes every other one.

Question: In the Seifa, R. Meir requires removing every other one; it does not help to move them. This is Beis Shamai's opinion!

Answer: Indeed, R. Meir teaches that Beis Shamai and Beis Hillel do not argue about this (all require removing every other one).

Question: Why does our Mishnah teach that R. Yehudah and R. Meir argue about Gezeiras Tikrah? They argue about this in the previous Mishnah!

Mishnah - R. Yehudah: One may use boards for Sechach;

R. Meir forbids.

Answer #1 (R. Yochanan): The previous Mishnah discusses smooth boards. They argue about whether or not we decree lest one use Kelim for Sechach.

Question: Rav Yehudah said that Rav allows 'male' arrow shafts for Sechach. He does not decree lest one use 'female' arrow shafts. Similarly, he would not forbid smooth boards lest one use Kelim!

Answer #2: He must say that they argue about Gezeiras Tikrah in both Mishnayos.

Question: Why must they argue in two Mishnayos about this?

Answer: In our Mishnah, R. Yehudah asks R. Meir, why do you make Gezeiras Tikrah and forbid boards? That is Beis Shamai's opinion, but Beis Hillel do not decree!

R. Meir answers that all make this decree.

Question: This is like Rav, who says that R. Yehudah and R. Meir argue about boards that are four Tefachim wide. But Shmuel says that they argue about boards less than four wide, but all disqualify boards that are four wide. How does he explain the argument in our Mishnah (why does R. Yehudah say that Beis Hillel allow them)?

Answer: They argue about what is needed to Mevatel the roof. R. Yehudah says that Beis Hillel say that it suffices to remove each other board. R. Meir says that this does not suffice.

RISHONIM

Rif (7b): The Halachah follows Shmuel, for Rav Papa and the Sugya are like him.

Rif (8a): Beis Hillel are concerned for Ta'aseh v'Lo Min ha'Asuy. Therefore, if he moves the boards or removes every other one, this is a new action. This is the Halachah.

Objection (Ba'al ha'Ma'or): Rav and Shmuel agree that our Mishnah discusses one who made a roof for Diras Keva from boards that are four wide, and reconsidered to use them for Sechach. Rav says that according to R. Yehudah, Beis Hillel hold that moving the boards or removing every other one is a new action; all the more so one may use them for Sechach of a Sukah l'Chatchilah. R. Meir, and Beis Shamai according to R. Yehudah, make Gezeiras Tikrah not to use such boards. It is not enough to move them; he must remove every other one. R. Yehudah and R. Meir are consistent with their opinions in the previous Mishnah. According to Shmuel, all forbid to use them l'Chatchilah because of Gezeiras Tikrah. They argue about b'Di'eved, whether or not it suffices to remove each other board to Mevatel the roof. Since the Rif rules like Shmuel, he should not say that Beis Hillel's reason is Ta'aseh v'Lo Min ha'Asuy, for this connotes that l'Chatchilah one may use such boards for Sechach!

Defense (of Rif - Milchamos Hash-m): The Gemara says that according to Shmuel they argue about what is needed to Mevatel the roof to avoid the Pesul of Ta'aseh v'Lo Min ha'Asuy. All agree that once he seeks to Mevatel it we are not concerned for Gezeiras Tikrah.

Note: Presumably, the Ba'al ha'Ma'or explains the conclusion like Rashi, that they argue about what is needed to Mevatel Gezeiras Tikrah. R. Yehudah says that Beis Hillel say that it suffices to remove each other board. Since the person knows Ta'aseh v'Lo Min ha'Asuy; we need not make Gezeiras Tikrah. R. Meir holds that we are still concerned for Gezeiras Tikrah.

Question (Milchamos Hash-m): The Rif holds that the Mishnah discusses one who made the roof for the sake of a house, and this is Pasul mid'Oraisa, even if the boards are less than four wide. According to Rav and R. Yochanan, the boards are four wide. If so, why does it help even to remove every other board? They are Pasul Sechach, and one who sleeps under one is not Yotzei! Also, why did the Gemara assume that the boards are four wide and thereby challenge Shmuel? Rashi says that ceiling boards are four wide, but this is not necessarily so. It is reasonable to say that they are less! In some places reeds are used (Bava Metzi'a 117a). Also, we must be concerned that others who enter the Sukah will not realize know that any Tikun was done. If bundles of reeds were put on the roof to dry, and now one wants to use them for Sechach, it is not enough to move them, even though he is concerned for Ta'aseh v'Lo Min ha'Asuy. He must untie them!

Answer (Milchamos Hash-m): The Mishnah did not specify the size of the boards; everyone establishes it according to his opinion. Beis Hillel are concerned for Ta'aseh v'Lo Min ha'Asuy. Beis Shamai make Gezeiras Tikrah. All agree that after the Tikun (moving or removing or both) it is as if he put the boards l'Shem Sechach. Therefore, Beis Hillel allow even if the boards are four; Beis Shamai require the boards to be less than four. We ask, according to Shmuel, what was R. Yehudah's argument against R. Meir? R. Yehudah himself agrees with Gezeiras Tikrah (when the boards are four)! The Gemara answers that they argue about Bitul of the roof.

Rambam (Hilchos Sukah 5:8): If a roof consists of boards without cement, it is Pasul for it was made for the house, not for a Sukah. Therefore, if he moved the boards and removed the nails l'Shem Sukah it is Kosher, as long as no board is four wide. Similarly, if he removes every other board and replaces it with Kosher Sechach it is Kosher.

Question (Rosh 1:29): Why does the Rambam say that no board may be four wide?!

Korban Nesan'el (3): The Rosh understands like Rashi, that since the person seeks to fix Ta'aseh v'Lo Min ha'Asuy, Gezeiras Tikrah does not apply. Also, in the Mishnah the boards must be four wide, otherwise the Gemara could not challenge Shmuel!

Answer (Korban Nesan'el 3): The Rambam holds like the Milchamos Hash-m, who answered these questions.

Rosh (1:29): (Removing the nails and) moving each board, or removing every other board is considered an action (to fulfill Ta'aseh).

POSKIM

Shulchan Aruch (OC 631:9): If a roof consists of boards without cement, to make it Kosher for a Sukah one must remove all the nails l'Shem Sukah, or remove every other board and replace it with Kosher Sechach.

Beis Yosef (DH Bayis): The Halachah follows R. Yehudah against R. Meir (Eruvin 46b), according to Beis Hillel.

Magen Avraham (7): It seems that if there was cement, one may not remove the cement and move the boards. It is not clear why. This is a better Tikun than moving boards (when there is no cement)!

Mishnah Berurah (22): The Acharonim hold that it suffices to remove the cement and move the boards.

Mishnah Berurah (24): (If one only moves the boards,) he must move every one.

Kaf ha'Chayim (36): If there was Pasul Sechach on top of Kosher Sechach, it suffices to remove the Pasul Sechach.

Shulchan Aruch (ibid.): This is even if the boards are four wide.

Taz (9): The Rambam holds that if the boards are four wide they are Pasul like Kelim that are Mekabel Tum'ah; Bitul does not help.

Mishnah Berurah (25): If there was not a roof to begin with, it would be forbidden to use boards four wide lest one think that his house is just as valid. However, Gezeiras Tikrah does not apply to one who seeks to fix Ta'aseh v'Lo Min ha'Asuy. Some say that removing every other board does not make the remaining boards Kosher if they are four wide, and he must remove two adjacent boards in the middle and rely on Dofen Akumah. Others say that all the boards become Kosher.

Shulchan Aruch (ibid.): Some say that the boards may not be four wide.

Magen Avraham (8): The Rambam holds that removing every other board helps even if some are four wide, for one who sits in the Sukah sees that other Sechach was needed. The Gemara says that it suffices to move them, i.e. when they are less than four wide. The Gemara supports the Rambam. It said that either Tikun helps for Ta'aseh v'Lo Min ha'Asuy, but only removing every other one helps for Gezeiras Tikrah!

Rebuttal (Kaf ha'Chayim 40): Many argue with the Magen Avraham and say that the Rambam does not allow boards four wide in either case.

Mishnah Berurah (26): Nowadays the custom is not to use boards of any width, unless one is in pressed circumstances.