Lucid... Although I apprecaite you trawling the web to prove me wrong the building you're showing there is a modern building made with a metal frame and a metal roof, the average 'hut' is going to have either a tiled roof (where you cannot have it flat) or thatched (where you cannot have it flat) or like a garden shed that will probably be roofing felt, now this can be flat, however, it usually pitched in one direction so water runs off it.
Furthermore you'll probably find that roof is not completely flat and actually has a network of low pitches and drainage gullies.
Furthermore you'll probably find that roof is not completely flat and actually has a network of low pitches and drainage gullies.