Its important to know that roof leaks often dont show up in your ceiling. In fact, by the time they show up there, the damage is severe. Flashing replacement caused by squirrels is expensive, probably well over 500 dollars for a 4 vent home, because the shingles have to be removed and replaced, and the plywood replaced if damaged by rain rot. It can easily be more. Or you can get some vent shields and save yourself both the aggravation and expense.