915. How to Make a Shittuf for Alleys

Eiruvin 1:17

The shittuf for an alley is made as follows: we collect food in the volume of a dried fig from each resident, or less than this if there are many residents. All of this food is placed in one container in one of the courtyards of the alley, or in one of the houses. The shittuf may also be placed in a portico or on a balcony but if it is left in the open in the alley, it is not valid. When placing the shittuf in one of the courtyards, the container must be raised at least a handbreadth above the ground (about three inches) so that it will be evident. When making a shittuf, one recites the bracha that God commanded us regarding the mitzvah of eiruv, followed by the declaration that "Through this shittuf, it will be permitted for all the residents of this alley to carry in and out from the alley to the courtyard and from the courtyard to the alley on Shabbos."

Eiruvin 1:18

If one divides an eiruv or a shittuf, it ceases to be effective, even if all the pieces are in the same house. However, if a container is filled by the food of the eiruv with the result that the overflow must be placed into a second container, it is acceptable.