It's best to take a look at the piping layout, as some areas will be much more heavily loaded tahn others. Seems to me that the maximum I've found is 6 psf. Wet pipe weights:
I've just completed a school with a sprinkler system - on the advice of the supplier I included a UDL of 0.3kN/sq.m which is approximately 6 psf.
I would agree with jmiec however and examine the pipe runs - there are primary pipes which are larger diameter (and hence heavier loaded) than the smaller secondary runs.