One of our party is from maintenance, and he's shown us where the fire doors are, and how to access the building's PA system. Working on the assumption that there are people in the building who are safely in hiding, we plan on getting on the PA system to let them know that we're going to trip the fire alarms, which will cause the fire doors to drop and lock the zombies as much in place as we can. We've found the exit to the roof, and our maintenace guy assures us that our way won't be impeded once the fire doors drop.
We'll stay inside the building as long as possible; we've stocked provisions by the exit, and the fire doors should solve our impending zombie invasion. Which is good, because it sounds like they're making their way up the elevator shafts.
If we hear any aircraft, however, we'll be on the roof in less than 30 seconds.