About 8 years ago we had our tar & gravel roof replaced by a torch-on system.Since then, and for a few years, after just about every rain resulted in a plugging of the drain apertures on the roof, requiring me to disconnect the downspouts to dislodge the accumulated roof granules. Now that the roof is free of the grit and quite slippery, the overflow events only occur after quite heavy rain, but the wooden substructure is still soaked. The roofer has attended a couple of times but insists everything is normal. I am not a hydraulic engineer but isn't is a fact that reducing the internal diameter of a pipe, even slightly, can greatly cut down the flow volume? The reason I ask is that the roofer ran the roofing material into the drain apertures thereby reducing diameter.