Here we'll talk about a two-story house owned by one person. The owner lived on the bottom floor, and the renter lived on the top floor. The Mishnah says that it is a two-story house lived in by two different people: the owner on the bottom floor and the renter on the top floor. The floor of the upper story falls in. So it is not that the whole house collapses; imagine the person is up there, and then the floor falls out from under him. Now there is nothing between the bottom apartment and the top apartment. The owner of the house does not want to fix everything. The renter who lived on the upper story is allowed to go down and live in the ground-floor apartment until the owner fixes the floor of the upper story for him.
The Gemara elaborates and says that this applies only in a situation where their deal was that the owner told the renter, “I'm renting you this upper story, which is on top of this ground floor.” In other words, when he threw in an extra line—“which is on top of this ground floor”—there is an indication that if the upper floor becomes unusable, he is going to provide the lower apartment for the renter. That is understood from their agreement. We have to comment that it is not in every case where the floor falls in that the renter could come to the owner's apartment. It is only when he added that line to their agreement. But the owner would be really obligated to fix that top floor and provide him with everything he needs in order to have it fixed.
Rebbe Yosi omer (Rabbi Yosi says). So, according to the previous Tanna (Mishnaic sage), who said that the owner has to fix the floor above, which is really his ceiling—the ceiling becomes the floor of the upper apartment—he has to provide the wood to make the actual ceiling and, in turn, the floor of the upper apartment. He also has to provide the ma'azivah (plaster) that would cover the wood. According to Rebbe Yosi, no. He only has to... They give him the wood that is going to serve as the ceiling for the bottom apartment and the floor for the upper apartment. But the ma'azivah, the plaster that covers the wood, is the responsibility of the renter of the upper apartment. The plaster is not needed to really strengthen the floor, but it ensures that it is smooth and flat. Therefore, according to Rebbe Yosi, since the upper apartment is now livable without that plaster, the landlord has no obligation to provide it. The renter could put it there on his own if he'd like to.