Case:


Ruling: Beit din’s research indicates that it is plausible but highly unlikely that cold weather caused a crack in special insulated glass. The first question is: who has to provide the proof when neither side claims to know precisely what caused the damage?
In general the status of a renter in relation to damages is as a shomer sachar (paid watchman), who is obligated unless that which happened was fully beyond his control (Shulchan Aruch, Choshen Mishpat 307:1). On the other hand, regarding damage to that which is part of a structure that is connected to the ground, there are not obligations stemming from being a watchman (ibid. 301:1).
However, in this case, the rental contract states: "The renter will look after the "wholeness" of the apartment and its accessories ... and return it in the same condition that it was in when it was received." When a kinyan is made to specially obligate oneself, this is binding even regarding watching land (ibid. 4). Based on standard convention, the stipulations of a rental contract qualify as a kinyan.
In general, if damage was caused to the object that is being watched, the watchman has to bring support for his claim that it occurred in a manner that he is not culpable. Classically, this is done by an oath, and when the watchman is unable to swear because he does not know what happened to the object, he has to pay. However, this may not apply to a case of "land," where one specially obligated himself. The K’tzot Hachoshen (301:3) says that the special obligation relates to the monetary obligation and does not by itself turn the person into one with all the laws of a watchman. Therefore, unless he obligated himself explicitly to be a watchman, he does not have to pay just because he does not know what happened.
Nevertheless, the majority opinion obligated def for a combination of the following reasons: 1) The language of the contract does not talk simply about paying for damages but more broadly requires returning the apartment in the same condition, so that only certainty of a lack of liability should exempt him. 2) Despite the distinction above, according to most opinions, when there is a doubt as to whether the obligation to pay for damages should be applied, the burden of proof is on the watchman even regarding land. 3) Since there is a likelihood that the damage was caused by def’s family, there is at least a moral obligation to pay.