I don't know how accurate this is, but I think I have something in my memory that said that many rooftops can't handle the weight of a helicopter. Even a little Bell Jet Ranger weighs about 1800 lbs with just a pilot, and with a max passenger load of five (say another 1000 lbs), you're talking over 1.5 tons of weight resting on a small area. A Blackhawk weighs six times that... empty.

And, as mentioned, can you even get to the roof? In the infamous MGM Grand hotel fire, not only was there no plan for aerial rescue, but the access to the roof was locked.

[Just some interesting info on the MGM fire: the fire was electrical in origin, and started in/near the kitchens. The kitchens and hallway areas were tiled (floors and walls both, I believe) with ceramic tiles affixed with a flammable adhesive. Once the flames were going, the fireball that surged through the kitchens and down the access areas were moving about 17 to 21 feet per second.]

Sue