Left 4 Dead/Rooftop Finale

You start in the emergancy ward of the hospital, and need to reach the roof.

Paths
There is one path to the radio. However, this is the first time you need to bust open a grate and crawl through.

After the grate, you need to climb two ladders within the elevator shaft to the top. If you encounter a witch within this shaft, you can safely attack her provided that you stand directly on the ladder she is trying to climb.

At the top of the shaft is the roof, overlooking the helipad. The radio is located to the right.

Once you summon a rescue team, you need to hold off for about "15 minutes". The attacking waves are guaranteed to have at least two tanks, they will come after the news chopper reports 10 minutes remaining and that you are his final pickup. The arrival of the helicopter is based on the number of waves defeated rather than actual time spent on the roof.