Storm drainage sizing: IPC or SMACNA?

[PatrickGSR94]

Trying to design gutters, downspouts and subsurface drainage for a large industrial building. I often use the SMACNA calculator, which is based on the charts in the SMACNA architectural sheet metal manual, and I always use worst-case 100-year storm rainfall rates. However, my plumbing/mechanical engineers often design internal roof drains using the IPC maps and tables. The difference is that SMACNA gives 100-year rainfall rates lasting for 5 minutes, versus IPC which just gives a 100-year rainfall rate of total rainfall in an hour. For my area, SMACNA says 10 inches per hour lasting 5 minutes, while IPC says 3.5 to 4 inches total in an hour.

The problem is that those two sizes lead to VASTLY different sizes of gutters, downspouts, and underground pipes. I'm trying to determine if the existing drainage systems on an existing building are enough to handle a large building addition. Per SMACNA it's a hard no, such that a bunch of additional subsurface work would be required. Per IPC, it's a definite maybe.

So at this point I'm not sure which way to go here.

 

[TheCommish]

The Conservative approach use SMACNA when the drains can not handle the water and it builds up on the roof and the roof fails there ie a bigger problem than the extra site work needed.

Or design a totally separate system to handle the addition with a build-up between the old and new roof to keep them separate

 

[Paul Sweet]

The rainfall in the first 5 minutes is 5/60x10 = 5/6". The rate of rainfall slowly decreases after this so that the total rainfall in the first hour is 4", and not 10". Most roofs are designed for a 20 PSF live load, and the momentary backup is just an inch or two and lasts less than an hour, so it won't overload the roof.

Your highway dept. might have rainfall charts that plot accumulation vs. time.

 

[ADAguy]

good to "know" this stuff.