Siman 159

It is a known matter that in the time of the sages of the Shas, the roofs were not sloped like ours, but rather were flat and level and fit for use. Nevertheless, they do not possess damage of sight as much as a courtyard because their use is not frequent, except for roofs made for dwelling which are covered; and regarding these, their law is like a courtyard according to the opinion of the Rambam, of blessed memory. Therefore, two houses side by side where the roof of one is not higher than the other and they are made for dwelling, this one makes a parapet four amot high for half of his roof on the east side, and this one for half of his roof on the west side. And even if the roofs were on two sides of a public domain, where regardless of this they have damage of sight from the people in the public domain, nevertheless, one compels the other to make a parapet four amot high. For from the people in the public domain he is only damaged when he stands on the roof and during the day, but when he does his work while sitting or at night they do not see him; and even when they see, they see only in passing, whereas his fellow sees always and permanently. Therefore, they make the half-parapets not opposite one another, and each one must also exceed by extending his parapet slightly even against the parapet of his fellow so that they do not see one another, for if not so, when they stand from a distance they will still see one another as is understood by the sense of sight. And if the roof is low to the point that the people in the public domain see him just like one standing on the roof, there is no more damage of sight here than that of the owner of the roof, and they do not compel one another.

And all this is regarding roofs that are made for use, but ordinary roofs whose use is not fixed and frequent do not require a parapet, for people do not dwell in them as in a courtyard, even if they are not sloped and there is no damage of sight. Nevertheless, they must make a small partition between the two roofs when they stand one next to the other, so that he may catch him as a thief if he enters his domain. And this is the approach of the Rambam, of blessed memory, in the third chapter of Laws of Neighbors.

And the opinion of many of our teachers, the Rishonim, is that there is never a distinction between one roof and another roof, for a roof never has frequent and fixed usage like a courtyard. Rather, when they are adjacent to one another, we do not require a parapet of four cubits, because he can see when his fellow ascends to his roof and will be careful of him, since they do not have fixed usage, and it is only necessary to make a partition of ten handbreadths so that he would be caught as a thief. But if they stand distant from one another and airspace intervenes between them—whether the airspace of a private domain or the airspace of a public domain—we require a parapet of four cubits, and this one builds for half his roof and that one for half his roof, and they overlap one another as has been written; for when they are not adjacent, one does not sense the arrival of his fellow and will harm him with the damage of sight. Our teacher, the Rema, decided in accordance with this opinion. And regarding our roofs, these laws are not relevant at all, for they are sloped and are not fit for usage; however, even now there are countries where their roofs are flat.

In a place where they must construct half-parapets, one not opposite the other, if one says, "I will build my half on the eastern side," and the second says, "Not so, rather I will build it on that side," they shall cast lots. And if one comes before us and says to his fellow, "Either you build the parapet and I will give you half its cost, or give me half its cost and I will build it," we listen to him, for they gain by this the excess that they would have otherwise needed to construct. However, if one waited until his fellow built his half and said to him, "Since you have started, finish it, and I will give you half the cost," if the other does not wish to, he cannot compel him; for he can say to him, "Why did you not start, and you would have built across the entire length and received half the cost from me? Rather, it is because you did not want to damage your wall due to the weight of the parapet, and therefore I also do not wish to burden my wall by building it in its entirety." And if the roofs are very far from one another in a manner that the damage of sight is not removed until he builds across the entire length, and in a situation where each one builds half and adds an excess the damage of sight would not be removed—as it is known through the sense of sight that at a distance the vision is narrowed, and as long as it is not across the length of the roof it will be possible to see—if they can agree that one will build everything and receive half the cost from his fellow, it is good; and if not, they shall cast lots between them as to who will build everything and take half the cost from his fellow. And it appears to me that if one says, "I will give you such and such in addition to the costs of half the parapet so that you will build it, or give me and I will build," the authority is in his hand.