I have recently replaced my flat roof when I was removing the felt it appeared that some of the stone chippings may have been mixed with cement I think to artificially over come a level problem for drainage. I have replaced the rotten boards and employed a contractor to re-felt however I now find I have ¾ inch of water remaining on ¾ of the roof my contractor suggest adding more chipping to help disperse the water is this the best solution to the problem, he also says that water left on the flat roof is no problem is this correct.