FrenchGrant

Hi can anyone here give me some advice? i am building an extension here in France and I am going to put on a flat roof with a terrace for those long summer night sundowners but I am not sure it is the right thing to do, I am not a roofer but common sense tells me that in winter we get a lot of rain and flat roof with Grantplenty water don’t go hand in hand..... but is there anyone who knows a good waterproof/water repellent system to be able to do a flat roof with a deck area on top??? 8m length by 4.5mwidth!!! Am relying on one of you guys to point me in the right direction...??? Grant