roof drain storm flow rate per IPC

[Steve Sylvest]

When using the IPC 2021 to compute flow rates to size roof drains and leaders and piping, it refers to a 100-year 1-hour rainfall intensity (given in Figure 1106.1). It implies, but does not specifically clarify, that this should be used regardless of the size of area drained (that is for small areas with short time of concentration, or large ones, either way). Is that correct?

One reason I am unsure is because some other codes I have used in the past did account for different storm intensities, and certainly civil stormwater design (drain pipes) do too.

 

[Paul Sweet]

Here in Virginia (4" per hour for one hour duration, 10" per hour for 5 minute duration) I plotted the accumulation vs. drainage and found that the ponding depth would be less than 2". This is only 10 PSF, which is less than the usual 20 PSF live load.

 

[ADAguy]

Good info to consider, thank you