Stuck at the end of "Follow THE Freeman",how do I get to the rooftop?

[drizzit]

Hi mates, I have that question:
I shut down all three generators and Barney told me to reach the roof-top.
My dear, I cannot find it. How can I get there?Killed every combine, but there isn't a ladder or a fan-tunnel at all.
Thanks in forward....
Greetz,
drizzit

 

[pharamir]

in the centre their should be stairs going down, where the combine came from after you killed the 3rd generator go through there.

 

[drizzit]

I was just too blind! Hope it was the script and not me...
I was there before, but after sending everybody to sleep I headed in the wrong direction.
WHAT A NICE GAME.