The swimming pool atop the roof makes a lot of sense.
As the North African swarm continues to descend on France, the more fire-proof Notre Dame can be, the better. As more mudslimes learn their arson trades and apply as they can more and more to the Cathedral, an upper pool connected via piping to sprinkler systems, with a hair-triggered smoke/fire detectors will go a long way to protecting the newly repaired structure.
We have something called sprinkler systems. The building is only about 300 ft high.