After a very heavy rain over several days, I discovered a big leak in my garage ceiling. I estimate a couple of gallons leaked through. My handyman cut into the ceiling drywall and found very wet insulation above it, which he cleaned out and left the ceiling open to dry. We have tried running water steadily from various sources within the house to see if that starts the leaking again, but nothing. At this point, we are waiting for another rain to see if we get another leak, and to hopefully figure out the source. There is a flat ceiling under a deck on the level above and partially over the garage, that is covered with the tar/rubber lining, but it appears to be in good shape from what we can tell. My handyman is reluctant to start pulling up decks boards or extracting hardy plank siding until we have explored all other possibilities. Anyone have any other ideas?

Put a lawn sprinkler up there, don't wait for another storm because you can't shut that off like a sorinkler

My handyman did turn on the hose on the 2nd floor deck from the outside faucet, and ran it a good while in order to simulate rain. No leak.

After reading posts from others on ceiling leaks, I am beginning to suspect that it could be coming from a spare 3rd floor bathroom that a guest was using for about a week before the leak was discovered. The garage isn't used regularly, so there is no telling how long it had been occurring. This guest takes long, hot showers, so it could have created a lot of water. Other than that rare use, this bath/shower is almost never used, which could explain the lack of a leak like this before. This was the only other thing that was different during this time leading up to the leak--the heavy rain could have been a coincidence. Perhaps the leak is coming from cracked caulk/grout/seals around the pipes/tiles/floor in this bathroom. My only question with this theory is, can a leak bypass an entire floor? I have not seen any evidence of a leak anywhere on the floor just below this bath--only in the garage two levels down. I am a complete amateur in home repair, so any ideas would be most appreciated!

Running a hose is not like rain or a sprinkler , I'd try it and then also run all the fixture in that bath on another day. Its one or the other, good luck.

Will do--many thanks!