I had heard that no one was on the roof because the access from inside had been blocked so as not to invite jumpers, stunt people etc. I'd heard that there were helicopters looking for people, though they'd had no orders to go in.
I would have thought it would be too hot from the burning fuel going up any elevator shafts, etc.
99% of the fuel burned up outside of the buildings, within the first 20 seconds or so. - What was burning inside was the carpets, upholstery, and paper, and the fire was not particularly hot, but very toxic.
The buildings didn't fall because of heat, but due to explosive charges that had been set prior to the attack.