This Mishnah talks about different precautions a person has to take to prevent a house fire from happening in his own home, because a house fire could damage his neighbor’s house as well, spread, and damage the property of others. So a person has a responsibility to set up his own home properly. In the times of the Mishnah, they would also have apartments, one on top of the other. The close proximity of one apartment to the next could cause damage to a neighbor’s apartment, whether the downstairs or upstairs neighbor’s.
The Mishnah says that a person is not allowed to set up an oven inside his house unless he has four amot (cubits) of open space above it. There must not be a ceiling, an item, or a roof right above it, because he wants to make sure that the flames and heat of the oven won’t cause a fire in the ceiling. So he has to have open space. It's sort of like fire. It's a code that he has to have in his house. If a person was setting up an oven in the upper story of his house, there had to be plaster three tefachim (handbreadths) thick under the oven as well, to protect the floor. We want to make sure that the fire wouldn’t go downwards, heat the floor, and then cause damage to his downstairs neighbor’s apartment.
But if he wasn’t setting up an oven but a small kitchen—a kirah, a type of stove—the heat isn’t as intense, so he doesn’t have to have three tefachim of plaster under it. He would only need one tefach (handbreadth), and that would suffice.
Now, what happens if he followed the proper code and there was a fire in his house anyway that caused damage? If he took all these precautions but the oven caused damage anyway, he has to pay for whatever damage he caused, according to this opinion. Even though he did everything he needed to do and took the proper precautions, he is still liable. So you may ask, “What’s the point of doing all this?” The answer is that we want to make sure this person isn’t left penniless. We want to take every precaution necessary to avoid his having to pay people for damage he caused to their property. We’ll do everything we can to make sure he’s okay. If it still doesn’t work and he does cause damage to other people, we’re still going to make him pay for the damage he caused.
Rabbi Shimon says, “I disagree with Tanna Kamma (the first tanna). All of these different measurements, all these different code requirements that we’re making him follow, we’re only making him follow them because if he does cause damage after taking the proper precautions, he would be exempt from paying. That’s the point of all of this. It would be considered an ones—something that was unavoidable, an unavoidable mishap—if he took the proper precautions. And it’s not a good thing. There still was a fire that caused damage, and therefore he would not be liable to pay.”