In the 70s, in particular, it was the fashion to hide the fact that there was a slope on the roof. Architects designed little parapets to do this but then couldn't have a gutter! So internal gutters (troughs) were born with internal rainwater outlets going through the roof. Not a good idea. Especially when used with roofing felt because the felt is so thick it reduces the bore considerably. Wherever possible Seamless Roofing install hopper heads and cut a small notch in the parapet to let the water off the roof. This means that if leaves or dirt clog the outlet it's the hopper that chokes and floods and NOT the roof. The hopper can be readily cleaned out. Easy peasy with Permadeck.