So, you could seal that with vapor-proof foil tape, but in your position, I would first clarify the cause of the cracks to avoid getting more later on.
Well, the cause of the crack probably occurred during construction and must have been accidental. The problem is that I only noticed it after the roof tiles were installed, so now I can at most try to seal it from the inside.
Yes, this is a bit fiddly since you don’t have proper access anymore. There is this adhesive tape— I can’t recall the name right now— that should provide a long-lasting seal.
