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!