How do I set up drains for a flat roof?
In my country, people prefer scuppers and PVC pipes. Make the floor sloped to four corners. Make gutters at the four edges. That's to say, the center point is slightly higher than the four corners and edges.

Edit: Do you want exterior eaves? If you do, you can have scuppers there. If parapet is solid, you might have to make scuppers on the parapet. Exterior pipes are suggested as you can easily handle the clog. Not only make the roof sloped, but also the gutters inclined towards the scuppers.