Wrong plumbing vent stack

Today took us out to Tsawwassen to replace some plumbing vent stacks. The original installer used sloped leads, meant for a minimum 4/12 pitch roof, on a low slope shingle roof. The usage of this hardware resulted in the flange creating a negative slope back up under the shingles on the top side of the stack. It was obvious this was an issue. A flat roof lead should have been used. It was an easy fix once we determined the cause of the leakage the home owner was encountering.In the photo below you can clearly see the debris that was being carried backwards, and up slope, by the rain water. If debris is up there then that means water was too!