Is it OK to open an umbrella inside the Sukkah? I know that covering the Sukkah with something plastic/weather resistant changes invalidates the Sukkah. But what about opening an umbrella or covering oneself with a plastic sheet within the walls?
This question actually shows up in the Mishnah. There it discusses canopy beds. It says one cannot have a four-posted canopy bed in the Sukkah, but one could have a single post at the middle of the head and foot of the bed, creating a tent-shaped shelter over the person. This is because there is a minimal amount of space (less than a hands-breadth) between the person under this sort of “tent” shelter.
Similarly, a plastic sheet over the person is like a raincoat or anything directly overhead, but an umbrella would probably be too far above the head of the sheltered person.
However—all of this is theoretical. If it is raining hard enough that one needs a raincoat, one is exempt from the requirement to sit in the Sukkah..
May we all have a dry Sukkot!