Another fine answer above.

Squirrels will be able to get into small openings of which you may not be aware. My neighbor next door swears that he sufficiently covered an opening under a dormer of his home with metal fencing where I noticed squirrels getting in. To this day, I still watch a squirrel climb right up his stucco siding and go around the barrier that he had placed there.

In an old farmhouse that I used to live in for eleven years, I had squirrels everywhere (except directly in the house.) They were in the walls and in the ceilings. I even had urine spots on the ceilings.

One of the ways that they were getting in was by means of an old wooden eaves trough. A hole had formed and they found it. When I covered the hole with wood, they next day I found that they had chewed a round hole in the wood that I had placed over the hole! So, it was best that I not use wood to cover the hole.