932. Reducing the Width of a Trench

Eiruvin 3:13

If there is a trench between courtyards and the residents reduce its width by placing a board or reeds along the length of the trench, they may only make a single eiruv and not two separate eiruvs. Anything that may be handled on Shabbos, like a basket or a cup, is not considered to reduce the depth of a trench unless it it fixed in place to the extent that one would need to dig with a spade in order to remove it.

Eiruvin 3:14

If the residents place a board of at least four handbreadths in width (about a foot) across the trench, they may make a single eiruv or two separate eiruvs. Similarly, if two balconies face one another and a board reaches from one to the other, the residents may make a single eiruv or two eiruvs, one for each balcony. If two balconies are on the same side of a public domain but one is higher than the other, then if they are within three handbreadths of one another (about 9"), they are considered a single balcony and the residents may only make a single eiruv. If they are farther apart than this, then they must make two eiruvs, one for each balcony.