Important note from pg6: "At the beginning of the turn, before any initiative cards are dealt..."
This just means that characters in the targeted building must use actions to leave the roof. If that is one action by climbing down the side, that's fine. If it's two actions to move to a staircase then go down it, that's fine. If a character cannot leave (e.g. he gets a stumble card played on him), then that is also ok; the character doesn't leave or do anything that turn and probably just gets really spooked by all the crows eyeing him!