I often receive requests to write a column on certain issues. The most requested subject matter, by far, is the new sizing method for roof (storm) drainage systems. Since the ICC International Plumbing Code adopted the new sizing methods there seems to be plenty of confusion.
For the last 100 years, storm drainage systems have been sized incorrectly by simply assuming that a pipe of a given size flows so much water based on the amount of rain falling from the sky. Hence, the storm drainage systems have been sized using the stack equation and assuming a 33% capacity of water while the horizontal drains have been sized using the Manning equation and assuming full flow (while sanitary drainage is half flow). So why is this wrong?