Sukkah 17a
סוכה י״ז א — ד"ה בַּיִת שֶׁנִּפְחַת וְסִיכֵּךְ עַל גַּבָּיו —
Sukkah 17a:2
Chachmei Bavel state that the principle of דופן עקומה permits a sukka only when the distance from the wall to the valid s'khakh, or the width of invalid roofing, is less than four amot; at four amot or more, the sukka is invalid.
בַּיִת שֶׁנִּפְחַת וְסִיכֵּךְ עַל גַּבָּיו — אִם יֵשׁ מִן הַכּוֹתֶל לַסִּיכּוּךְ אַרְבַּע אַמּוֹת, פְּסוּלָה. וְכֵן חָצֵר שֶׁהִיא מוּקֶּפֶת אַכְסַדְרָה, סוּכָּה גְּדוֹלָה שֶׁהִקִּיפוּהָ בְּדָבָר שֶׁאֵין מְסַכְּכִין בּוֹ — אִם יֵשׁ תַּחְתָּיו אַרְבַּע אַמּוֹת, פְּסוּלָה.
In the case of a house that was breached, creating a hole in the middle of the roof, and one roofed over the breach, if from the wall to the roofing there are four or more cubits of the remaining original roof, it is an unfit sukka. If the roofing is less than four cubits from the wall, the sukka is fit, based on the principle of curved wall; the remaining intact ceiling is considered an extension of the vertical wall. And likewise, in the case of a courtyard that is surrounded on three sides by a portico, which has a roof but no walls, if one placed roofing over the courtyard between the different sides of the portico and the roof of the portico is four cubits wide, the sukka is unfit. Similarly, a large sukka that was surrounded at the edge of its roofing with material with which one may not roof a sukka, e.g., vessels susceptible to ritual impurity, if there are four cubits beneath the unfit roofing, the sukka is unfit. The principle of curved wall does not apply to unfit roofing that measures four cubits or more.
