David,
I was a Security Supervisor on graveyard shift at a major mall for 3 years. We were the ones to allow access to the interior, so the cleaners could get to the store and also to the roof access.
Mall management did not care about the stores or their service needs, as long as the rent was paid and if not, they would evict you. Management did not want anyone on the roof after dark, or before they were in the office. This changed a lot, as one month we were told to allow any cleaner that needed access and the next month that it was restricted access. It seemed to vary as to who you were talking to in the office.
A crazy example of their thinking is after our elevator kept breaking down, they said that some contractor who had regular access was tampering with the computer brain, and to not let anyone up on the roof without an escort. Well, our elevator was only for two floors and did not reach the roof and the control room was locked up and not even on the roof. Their was no way that anybody on the roof could tamper with anything related to it, but that was their story aand they stuck to it.
I believe that some bad contractors did some damage to the locations that you work in, and now you are being punished for their mistakes. They may use 9-11 as a reason, but that is only a cover.
After much debate with the General Manager of the mall, we adopted this procedure. All contractors must check in with the Management office during office hours on the day that they are scheduled to work and then management would let Security know you would be there that night. When you arrived that night, Security would have you sign a waiver for being on the roof after dark and this would serve as a logged in status for the property.
Since you have an account at the mall, their should not be a Security issue, since you come their for more than a one time cleaning, and they know who your company is. And as far as the chem/water issue, share the NFPA 96 with them and the danger that can happen to their mall without your services.