As we open a new chapter in Eruvin, we leave behind interlocking courtyards and move on to rooftops. If you’ve visited Jerusalem’s old city, you may have walked on the rooftops and noticed that they create a world of passageways unto themselves. You can’t always tell where one house ends and the next begins. (And if you haven’t walked on Jerusalem’s roofs, maybe you’ve seen the rooftop scene in the movie Mary Poppins.)
Of course, one is allowed to carry within a single domain (for example within a single courtyard) and you only need to erect an eruv in order to carry between two different domains, between a house and a courtyard or between two people’s homes even if they are interconnected. So how do we evaluate the air space of interconnected rooftops that have no visible distinction between them? Here’s the mishnah for today’s page:
All the roofs of the city are considered one domain provided that one roof is neither ten handbreadths higher nor ten handbreadths lower than the adjacent roof. This is the statement of Rabbi Meir.
And the rabbis say: Each and every one of the roofs is a domain in and of itself.
The Gemara begins by discussing the rabbis’ position, which is the simplest. Just as every home is its own domain, the rooftop is owned by the person whose house it covers. An invisible boundary line defines the borders of each individual roof and one can carry only within a single rooftop.
But Rabbi Meir, whose opinion led in the mishnah, says all of the rooftops are one domain, and presumably one may carry between individual rooftops as long as they are continuous. In trying to explain Rabbi Meir’s opinion the mishnah invokes a leniency about air space
“any place above 10 handbreaths off the ground is considered one domain.”
Air space is by nature interconnected and therefore the roof network is considered all one space.
Certainly, for the purpose of building, Rabbi Meir would accept that the home owner maintains the rights to her own roof and may eventually build on it. However, for the purpose of eruv, Rabbi Meir puts the emphasis elsewhere — a bit higher, if you will. He is more concerned here about user experience, hence his caveat in the mishnah: if the rooftops are divided by a step of 10 handbreadths or more, then the division acts as a threshold and then one may not carry between individual roofs.
The Gemara will continue to discuss various kinds of dividers inhabitants might put on their roof and other complexities of rooftop geography, complicating the simple case of the mishnah where all the roofs are contiguous and uninhabited. Still, these two different kinds of boundaries, legal ownership, and visual or experiential divisions will continue to compete for our attention.