I know that this thread is most likely dead, but I would like to tell a story related to this topic. A close friend of mine bought a house recently which had a story door leading to a flat roof just like in your case, but he didn't notice how old the roof was. One day he made a barbecue party on that roof and at one point it cracked. Nobody fell through, but the consequences could have been much worse, so I would recommend you check the quality of your roof and get a new one in case your roof is old and bad.