Thus, we learn from here that there is no issur d’oraisa in opening an umbrella on Shabbos. The Chasam Sofer argues further that both opinions in Yerushalmi would agree that opening an umbrella is not considered boneh since umbrellas don’t have walls! Furthermore, an umbrella is not a stable structure. It is carried around, and cannot be considered a proper structure. (The Chasam Sofer also discusses whether opening an umbrella is assur d’rabanan; see Chazon Ish, O.C. 52:6; introduction to Tiferes Yisrael, Kalkeles Shabbos; and Orchos Shabbos 9:6).
Unlike our umbrellas, the umbrellas discussed by the poskim were difficult to open and close. “They must be stretched out and then tied with straps or fastened with metal wires,” the Pri Megadim and Biur Halacha (ibid.) write. Nevertheless, it is still forbidden for us to use umbrellas on Shabbos. “The great Torah leaders of previous generations forbade opening and closing umbrellas on Shabbos and Yom Tov. Even if an umbrella is already open, a person must not use it because of mar’is ayin; it may appear that he opened it on Shabbos” (see Shemiras Shabbos K’Hilchasa 24:15 and the footnote there).
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