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Roof area:drain pipe volume/size calculation?


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I'm helping out a friend, overseeing some work she is having done and the guy putting in the drainage systems is going to run all the gutter drains underground to a public storm drain. The house has a really large roof area and he asked me if I know of a calculation to figure the appropriate pipe size to handle the volume of water that will come off it. I told him, I didn't, but I know where to pose the question with a high probability of getting a good answer.

Any takers?

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Chapter 13 of the National Standard Plumbing code would be his starting point.

Everything is dependent on the area & slope of the roof; once you get underground it depends on the slope of the pipe.

I took a course many, many years ago at NJIT called Hydraulics & Drainage and was actually pretty good at it; but if you asked me today to calculate something, I'd spend more time re-learning what I forgot than figuring out the answer.

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"The size of the roof drain system is based on local rainfall

rates. Both the International Plumbing Code (IPC) and the

Uniform Plumbing Code (UPC) publish rainfall rates. These

rates can be found in Appendix B of the IPC and Appendix D

of the UPC. The rainfall data in the UPC is based on National

Weather Service Technical Paper No. 40, Chart 14: 100-Year

60-Minute Rainfall. The IPC bases its data on “a storm of onehour

duration and a 100-year period,â€

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