“A folding chair is permitted to open on Shabbat.”
The Torah forbids making an ohel — a tent-like structure — that is of a permanent nature (not intended to be taken down that day or very soon). The Rabbis made a decree to prohibit even a temporary ohel so as not to come to (mistakenly) transgress the Torah prohibition against making a permanent ohel.
Our gemara teaches that opening a folding chair on Shabbat is permitted although this act creates a sheltered space underneath the seat part of the folding chair. It follows that in this case the prohibition against making an ohel on Shabbat does not apply. Does this mean that it is also permitted to open an umbrella on Shabbat? (Of course, it would not be permitted to carry the umbrella outside on Shabbat in a place where there is no eiruv.)
While a few poskim have permitted using an umbrella on Shabbat, the vast majority have prohibited opening it on Shabbat. And this is the widespread and accepted halacha. Why is opening an umbrella “worse” than opening a folding chair? One reason is that the ohel of the chair is meant to sit upon and not to serve as shelter for underneath it. Another reason is that the folding chair simply slides open and stays that way by its nature, whereas the rods of the umbrella need to be affixed open as an ohel by means of a mechanical process. (See Shulchan Aruch Orach Chaim 315:7 and the Bi’ur Halacha there, and Shemirat Shabbat K’Hilchata 24:15 and footnote 53 for a more detailed treatment of this subject.)
Regarding the question of whether one may use on Shabbat an umbrella that was open before Shabbat, there are also two main reasons to not allow this. One is the issue of marit ayin — that an onlooker may see this act and mistakenly think that it is permitted to open an umbrella on Shabbat. A second reason is that a person is considered as continuously making a new ohel as he walks, making a new protected space under the umbrella in any new space he occupies.
- Shabbat 138