This photo shows a one way door, installed right where the squirrels go into the attic. After the squirrels have left, the door will be removed but the solid wire left to keep them from getting right back into this very thin roof vent. You should not install such a one way door in the spring when there risks to be a nest full of babies in your attic. Wait until they are a few weeks old and then lock them out.
This was a TV segment shot in Toronto with Brad Gates of AAA Wildlife Control in Toronto, Ontario.