שולחן ערוך אורח חיים · סימן שס״ח

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Laws of When the Eruv Becomes Invalidated After It Was Made — 5 Sections If an eruv was made and afterward became invalidated, and one of the courtyard residents comes to fix it again — if he comes to make the eruv from the same type of food as the first, even if the original is completely gone, he need not inform the others, and he may even make the eruv on their behalf using their food. If he comes to make it from a different type of food: if the original is completely gone, he must inform them if he is using their food, but if he is using his own he need not inform them at all. If the original has not completely gone but merely diminished, even if he is using their food he need not inform them.

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If new residents were added to the courtyard — if a house opens to two courtyards, he must inform them, since perhaps they do not wish to make an eruv with these others.

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What is the minimum amount required for an eruv at the outset? When there are eighteen or fewer participants, the amount is a dried fig's worth per person. If there are more than eighteen — even a thousand or more — the amount is two meals' worth, which is eighteen dried figs, equivalent to six eggs. Some say it is equivalent to eight eggs.

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If the eruv was reduced below the minimum amount after the first Shabbat began, and any amount at all remains, it is valid for all subsequent Shabbatot as well.

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If the eruv bread became moldy and unfit to eat, it is as if it were completely gone and the eruv must be remade. Rema: Therefore the custom developed to make the eruv from matzah, which does not mold quickly, and furthermore it can be kept through Passover and used as the eruv all year long. This is better than making a new eruv each Shabbat, lest people forget to make one. However, if one wishes to make a new eruv each Shabbat eve and eat the eruv each Shabbat, one may do so. See below Siman 394.