Thanks for providing more details. Who manages the money in the “rainy day” fund?
It would appear that, based on the configuration of the Lots, your “rainy day” fund would pay for the repairs to Lot 2.This “fund” should also cover repairs to your Lot 1 – as you are both in the same situation with a collapsed ceiling. Make sure that your ceiling is repaired properly at the same time rather than merely “propping it up”, which is not really a satisfactory repair.
Send all the details to the other owner to keep them informed, keep the records, and at your next informal meeting you can both put a bit more money aside to top up the “fund”.