Home Improvement Questions? Ask a Handyman for Answers
Hi. This is hard to say without seeing the building. Here is a good ressource on the problem:
Unlike squirels, rats or racoons, they will not come back in by damaging the house to get back in (they always use aready done holes, but they can use holes made by other animals).
Once you got rid of them you can use different strategy to prevent (more reduce) them in the future. You can build external shelter for them (as they are very useful insects controller). You can also close for the night any sentinel lamp you may have so that it does not bring night insects around the house.