Upload & Sell: Off
I've always contacted the building owners or managers first, assuring them that I know enough to stay on the roof walkways (never walk on the roof surface, and if you use a tripod, put some 1x1-foot plywood squares under the feet to prevent damage to the roof membrane). It helps to have a business card, and a portfolio of sample photos, and to offer to donate a few nice building photos for their use. If you are nice, chances are they will allow the building maintenance to escort you to the roof. Giving that person a tip always is helpful, especially if a return trip becomes necessary.
Don't try sneaking in -- chances are that access to the roof has been locked.