It was a 5 foot ladder, used in conjunction with an HVAC unit from what I've read so far. Maybe he put the ladder on top of the A/C unit to access the roof?
Anywho, taking the ladder with him in view of how many people he must have been noticed by may have been accidentally genius. If I saw some dude carrying a ladder right outside a manufacturing facility, I'd probably just assume he was a maintenance worker or something. Now the rifle? That's a different story.