743. If an Alley is Too High or Too Low

Shabbos 17:16

If the height of an alley from the ground to the bottom of the beam is 20 cubits, it is valid even if the thickness of the beam causes it to extend higher than 20 cubits. If the alley is higher than 20 cubits and one wants to reduce the height by positioning the beam lower than the top, then the beam must be at least a handbreadth wide. If the alley is lower than ten handbreadths high, one should excavate an area four cubits by four cubits (about 6’x6’) and deep enough so that the alley is a full ten handbreadths.

Shabbos 17:17

Let’s say that a breach was made in the wall of an alley close to the entrance. If a section of wall four handbreadths wide (about 1’) remains standing at the entrance, one may carry within the alley so long as the breach does not exceed ten cubits (about 15’). If a section of wall four handbreadths wide does not remain, we may not carry in the alley unless the breach is smaller than three handbreadths wide (about 9”). This is because a gap of less than three handbreadths is considered to be closed up.