Obviously where a joint is but the only way forward is to clean the roof and mastic with White external window framing mastic run strips of mastic along all joints/alluminium strips. When you do don't cut the nozzle on the mastic tube leave it coming out thin it's easier to handle and you end up with a neat job and not a BIG BLOB build up layers if necessry.Including around the skylite/s
Can't advise where it's coming in can't see it.
Bob, April 2009
