Interesting enough, many fire departments won't allow roof mounted generators anymore because storing fuel along with it is considered hazardous. We had an awful time getting our 250 kw generator installed and ended up with it on the ground level next to the parking garage with an integral fuel storage tank; it was the easiest to get permitted.
At PAIX they put the generator on the roof and the fuel tank in the basement. The fuel pump has its own UPS and it's also on the building UPS. It takes a minute or two to pressurize the fuel line, but the UPSs could run the whole kaboodle for a lot longer than the generator's startup time. The permits would have been a huge problem had this building not once housed the Pac Bell CO for Palo Alto. But all we had to do was more or less exactly what they had done and the city agreed that if it had been OK for Pac Bell to do it, it was probably OK for Digital to do the same things.