Building and Destroying: How Goodly are Your Tents? (II)

Even though it is prohibited to make a halachic tent, there are various cases in which is could be allowed.  The first is mosif al ohel array.  This means that it is permitted to make a temporary addition to an already existing ohel.  This can be done in one of two ways.  The first is that if you have a tefach (between 8 and 10 cm) of covering, it is permitted to extend it.  The second is that if one has an ohel using the halachic principle of lavud, you can fully cover it.  This means that if one has an open roof, but it has cross beams (or even fine string) stretched across it, at intervals of less than three tefachim, you can cover over the space.  According to the Mishna Brurah one can combine these two concepts, and even if there is only one place that has two cross beams within three tefachim, but covering a total of more than one tefach, one can extend from this to cover the entire area.

The second way in which it is mutar to make an ohel is melma’aleh lemateh.  This means that if first one builds the top, and then one brings the wall, it is mutar to make.  While I do not have a clear proof, it seems to me that this would apply even with an ohel without walls.  If you bring the top before the supports, I believe that it would also be mutar.  The proof for this is a gemara that discusses making a covering over a dead person to protect from the sun.  The gemara says that one may hold a talis over his head, and then bring standup beds to support the talis.  The last way in which it is mutar to make an ohel is avid v’kay.  This means that any ohel that is not made out of parts, rather is collapsible, is mutar to put up on shabbos.