The tenth and final perek of Massekhet Bava Metzia, Ha-bayit ve-ha’aliya, focuses on one specific situation – when a two story house that is owned by more than one person collapses, either completely or in part. How are the bricks to be divided up? What if the person who owned the bottom floor does not want to rebuild, but his upstairs neighbor does?
Two Mishnayot appear on today’s daf. In the first Mishna, the case that is presented has a two story building that collapses, and the Mishna rules that the two partners divide up the rocks, bricks, etc. equally between them. If one of the partners recognizes that certain bricks came from his part of the house, he can claim them as his own, but his partner will get an equal amount, even though he is not sure which bricks belong to him.
Early manuscripts of Rashi appear to indicate that the two partners will divide the materials equally, even if one floor was larger or taller than the other. Since we cannot tell which bricks belonged to the bottom floor and which to the top floor, we cannot identify some as belonging to one partner and some to the other. Our only choice is to divide the materials equally. The Ramban quotes the Tosefta and Talmud Yerushalmi as ruling otherwise. According to this approach, the materials are divided between the partners, but it is divided fairly, according to the size of the floor that each one of them owns.
The second Mishna in the perek discusses a case where only the top floor collapsed. According to the Mishna, if the owner refuses to pay to have the top floor fixed, the tenant on the top floor has the right to move in with the owner of the bottom floor until such time as the top floor is fixed. As is clear from the Mishna and the Gemara, the case discussed here is one where one person owns the entire house and the second floor is rented to someone else. Only in such a case would the owner be obligated to fix the roof apartment, and only in such a case would people upstairs be allowed to force the owner to do so.