Pi Tikra Yored v'Sosem – Eruvin 25b
The Gemara here introduces the concept of “pi tikra yored v'sosem” (literally: the lip of a roof comes down and closes; see also 94b). The principle, as defined in the Shulchan Aruch,(Orach Chaim, 361:2), is that, when a roof is at least four tefachim by four tefachim and set atop two complete walls, we view the thickness of the roof as an imaginary wall for the remaining two sides. (To employ the principle of pi tikra the structure must have two walls adjacent to each other connecting at a corner, not two parallel walls, see Rama, Shulchan Aruch, Orach Chaim 361:2).
In one of the early works on eruvin in modern cities, the Tikvas Zecharia, Rabbi Zecharia Rosenfeld, the first Chief Rabbi of