QUESTION: The Chachamim in the Mishnah maintain that the roofs of different residences are considered separate domains (in contrast to the opinion of Rebbi Meir, who maintains that the roofs are all considered one domain). Rav and Shmuel disagree about whether the Chachamim permit one to carry in each roof alone. Shmuel says that one is permitted to carry throughout each roof, since we view the Mechitzos that are below the roofs, which separate the houses from one another, as though they rise above the roof to separate each roof from the next (and each roof is viewed as surrounded by Mechitzos). Rav maintains that one is permitted to carry only four Amos on the roof, because we do not view Mechitzos as rising above the roofs and, consequently, each roof opens into an area in which the residents of this roof are forbidden to carry ("Parutz b'Milu'o l'Makom ha'Asur Lo").
Why do Rav and Shmuel disagree about the opinion of the Chachamim? Both Rav and Shmuel rule like Rebbi Shimon (91a) who says that all roofs are considered one domain, and thus one certainly may carry more than four Amos on any of the roofs, and even from one roof to the next.
(a) TOSFOS (DH Rav Amar) writes that in a case in which one wants to move utensils that were located in the house when Shabbos began and now are located on the roof, even Rebbi Shimon agrees that one is forbidden to move the utensils to another roof, because at the time that Shabbos began he was forbidden to move those utensils to the roof of another house. The argument between Rav and Shmuel applies in such a case. According to Shmuel, Rebbi Shimon permits the utensils to be moved throughout the roof of the house from which they were taken, since the roof is considered surrounded by Mechitzos. According to Rav, the utensils may not be carried on the roof more than four Amos.
(b) Tosfos suggests another answer. We know that one may not carry in an area that is larger than a Beis Se'asayim and that was not Hukaf l'Dirah (enclosed for residential purposes). Accordingly, if the total area of the adjoining roofs is greater than a Beis Se'asayim, the argument between Rav and Shmuel applies even according to Rebbi Shimon. According to Shmuel, one may carry on the roofs because they are considered to be surrounded by Mechitzos, and thus none of the roofs are considered larger than a Beis Se'asayim, since the Mechitzos separate them from each other. According to Rav, the roofs have no Mechitzos that separate them from each other, and thus together they constitute an area larger than a Beis Se'asayim that is not Hukaf l'Dirah. Therefore, one may not carry on the roofs more than four Amos.