Parashat Ki-Tetze 5766/ September 2, 2006
the weekly Torah reading by the faculty of
“A parapet for your roof”
Prof. Yair Goldreich
Department of Geography and Environmental Studies
One of the rational mitzvot in this week’s reading, so replete in commandments, is the positive commandment  to build a parapet for one’s roof, as it is written: “When you build a new house, you shall make a parapet for your roof, so that you do not bring bloodguilt on your house if anyone should fall from it” (Deut. 22:8). The regulation height for such railings is 10 handbreadths (about 100 cm.). 
From Sifre Deuteronomy we learn that this refers not only to newly built houses, but also to existing houses that a person has acquired or inherited, etc., since the same danger of falling pertains there as well. Silos, barns and storehouses also require a railing.  But not all buildings require railings:
1) A house smaller than 4 cubits by 4 cubits is exempt from the requirement of a mezuzah and a railing around the roof. For what reason? It is written “house,”  and a roof on such a small area is not considered to be covering a “house.”
2) A house less than 10 handbreadths tall is exempt from having a railing because it too is not considered a “house.” 
3) Synagogues and houses of study are exempt. 
On the face of it, it is hard to see why the danger of falling off the roof of a synagogue is any lesser than the danger of falling off the roof of a house. This halakha, however, becomes clear when we understand how roofs were used, for which purpose we turn to the rules of building a synagogue. We learn that the roof of a synagogue must be higher than the other buildings in the city:
Rabba bar Mahasya said: Rav Hama bar Giora quoted Rav: Any city whose roofs are higher than its synagogue is destined to be destroyed, as it is said: ‘to raise again the House of our G-d, repairing its ruins’ (Ezra 9:9). This applies to houses, and not to towers that were built for beauty and are not inhabited (Shabbat 11a).
The Tosafot  note that the towers referred to were not intended as residences, and their roofs were not suitable for use in the way that residents of houses customarily used their roofs in the time of the Talmud. Therefore, also today it is not necessary to have the synagogue roof be higher than all other roofs, if these are on ornamental towers. Also the Sefer Mordekhai  discusses the importance of roofs, citing Moses ben Jacob of Coucy (Semag): “Specifically in their day,” when it was customary to make use of the roof, then the roof of the synagogue had to be taller than others; but in places where the roofs of the houses were not in use, there was no issue of the synagogue roof being taller.
Following these authorities, the Shulhan Arukh ruled: 
Synagogues are to be built only in relation to the height of buildings in the city, elevating them until they are taller than all the houses in the city [whose roofs] are used, excluding citadels and towers, [whose roofs] are not used. In the case of a roof that is slanted and is not fit for use, one estimates the height to which it is fit for use, i.e., if it has an attic, the attic must not reach higher than the roof of the synagogue.
The Torah Temimah already
noted that in
Thus we learn that the roof of the synagogue had to be taller than other roofs which were used or fit for use. To understand the use of roofs, we need not reconstruct the way roofs were used in the talmudic era. While in Jewish Israel roofs are not generally used as they were in the past, in Arab villages rooftops are used much as they were in the time of the Talmud.
The houses in an Arab village are typically built on a hillside, so that often the roof of one house serves as the entrance level of the house above it. On hot, humid summer days, or in the transitional seasons of spring and fall, when there are often days of hot dry wind, the rooftops of houses in Arab villages are often used as the sleeping-place of the family, and when sleeping danger of falling off a roof that has no railing is clearly greater. The roof of the synagogue, in contrast, is not used for sleeping, and since “human beings are forever liable”, there is no need for a railing.  Thus we can also understand why a roof whose area is less that four by four cubits (1.28 x 1.28 meters) is exempt from having a railing, since it is not large enough for sleeping a family. The same applies to structures lower than 10 handbreadths, where it would not be safe to sleep due to the danger of unwanted two- or four-legged visitors.
 This command, “you shall make,” also subsumes a negative commandment, “do not bring bloodguilt on your house” (Sifre).
 Mishnah, Bava Batra 4.1.
 Sifre infers from the word bayit, or house, that also pits and caves require railings.
 Babylonian Talmud, Sukkah 3b.
 Babylonian Talmud, Bava Kama 51a.
 Babylonian Talmud, Hullin 136a.
 Tosafot Yeshanim, Babylonian Talmud, Shabbat 11a (these are not actually chronologically older tosafot, rather they are texts that did not appear in the first printed editions but were added to later editions [so I learned from Prof. Ya’akov Spiegel]).
 Par. 228; compendium on talmudic problems by Mordecai ben Hillel ha-Kohen (1240?-1298).
 Orah Hayyim 150.2.
 Torah Temimah, Deut. 22:8, note 75.
 Cf. also Torah Temimah, loc. sit., note 77.