Google up "hip roof", guru... Then look where the two planes intersect... That's called the valley which gets water from two planes, rather than one meaning twice the water... Think of it a s creek between tow hills... What happens at the bottom in a heavy rain is that with twice as much water coming down the valley as you'd have with a standard pitched roof that it tends to overshoot the gutter... So if you take a piece of sheet metal you can use that as a divertor to keep the water in the gutter...

B