itchesrulik- The issur behind using an umbrella is not clear. The node beyehuda said it’s boneh middoraysa, but many poskim have an issue with that because it’s meant to be open and closed. The chazon ish said it’s uvdah dichol. Some poskim have said it’s muktzeh, but if that were the case one should be able to use it litzorech gufo. Personally, I would feel much more uncomfortable using the local eiruv than using an umbrella.
