I have a glass-roofed lean to conservatory with a shallow roof pitch. After heavy rain there is standing water at the lowest part of the roof where it enters the gutter. What would be the least acceptable pitch of the roof be to ensure complete runoff of the water?
As Jimmy said, apparently the area in question effectively has no pitch. It's a low spot possibly due to poor workmanship on the drip edge. Some roofers install it incorrectly, creating a "ski jump" at the eave.
As you have a glass roof, I'm really not sure what your situation is. A photo would help us offer more specific advice.