Answers

Doing a water test to determine where the water is entering may locate the issue.  Also checking your roof vents, flashing, chimney, gutters and downspouts

Are you asking how to repair or solve the problem?  Always need to look at flashing gutters and downs and roof penetrations to locate the leak.

If you are asking about how to repair the roof damage, then I would like to suggest you to go for insurance claim process if you are already done with an insurance of roof damage. Because the plumber will cost you more, so you take some helpful hints from the Public adjusters about how to get the maximum claim amount on the Roof damage.