Sukkah 12B

דאין מסככין משום גזרת אוצר, הא דאורייתא שפיר דמי. התם דקתני ״אינה סוכה״ אפילו דיעבד ⁠— מדאורייתא נמי אינה סוכה.

that one may not roof, due to the decree of the storehouse issued by the rabbis; but by Torah law, it seems well to do so. There, in the Mishnah pertaining to the stack of grain, where it teaches: It is not a sukka, it means that not only by rabbinic decree, but even after the fact, by Torah law as well, it is not a sukka.

אמר רב יהודה אמר רב: סככה בחיצין זכרים ⁠— כשרה. בנקבות ⁠— פסולה.

§ Rav Yehuda said that Rav said: If one roofed a sukka with convex arrow shafts, which are made of wood with a protrusion on the end that is fitted into the socket of the metal arrowhead, the sukka is fit. These shafts are flat wooden utensils, which are not susceptible to ritual impurity. Therefore, they are fit roofing for a sukka. If, however, one roofed his sukka with concave arrow shafts, which have a socket into which a protrusion from the metal arrowhead is inserted, the sukka is unfit. Since these shafts are wooden utensils with a receptacle, they are susceptible to ritual impurity. Therefore, they are unfit roofing for a sukka.

זכרים כשרה, פשיטא! מהו דתימא: ניגזור זכרים אטו נקבות, קא משמע לן.

The Talmud asks: The fact that if one roofed a sukka with convex arrow shafts, the sukka is fit is obvious. It is no different from roofing with straight, smooth reeds. The Talmud answers: Stating this halakha is necessary. Lest you say: Let us issue a decree and prohibit roofing with convex shafts due to the prohibition against roofing with concave shafts, therefore Rav teaches us that no decree is issued, and convex shafts are fit roofing.