In this picture:
http://i.imgur.com/xItN2UX.jpgWhere my mouse is there is an invisible wall blocking the path to the coaster before you press the button. After you press the button that invisible wall is removed and you can pass through that bush.
Maybe you can use similar mechanic like that to put the invisible wall on the roof and then remove it when they move to certain position like cookie said.
Using a trigger that survivors pass through sounds good, I just wanted to show an example of the walls being removed if someone wanted to look at how it works.
