We solved that as PAIX by having the main fuel tank in the basement,
in a steel-reinforced cinder-block vault, with a "day tank" up on the
roof; a redundant pair of pumps draw fuel from the basement up to the
roof. The fire department liked it because there wasn't too much fuel
on the roof, and the (whatever that other department was) liked it
because the generator was on the roof (and the noise at ground level
wouldn't be too great during weekly tests).