Roof Drain Calculator

Aug 19 '08 5 Last Comment
Aug 19, 08 6:49 pm

Does anyone know of a reliable online roof drain calculator? I tried this one but the number seems a little low. Thanks!


Aug 19, 08 8:23 pm

if i remember right (its been a while since i had to do it...)
but it is just a surface area to drain ratio set up in the
I P(lumbing) C

Aug 19, 08 10:34 pm

your link seems fine if leaves or ice aren't an issue.

to manually calculate the required drainage:


Q = quantity in cubic feet/second
C= coefficient of runnoff - use .9 to 1.0 for a impervious roof
I= Design storm rain intensity in inches/hour per local jurisdiction
A = area in acres (1 acre= 43,560 sf)

don't ask me how the units work on this one, but trust me, it works for most site under 100 acres for calculating the volume of run off.

then you need use an nomograph to calculate pipe diameter by the slope of the run.

you can just connect the dots.

there are also more complex calcs that you can leave to an MEP to figure out the drain intake grating area.

Roofs are small and don't need as much drainage as you'd expect.

remember that you need an overflow drain/scupper about 1" higher then the primary.

Steven WardSteven Ward
Aug 20, 08 7:41 am

i have to relearn this every time i do it. 'where's the smacna?'

Aug 20, 08 8:33 am

Shouldn't this be designed by an MEP or Civil engineer?

Aug 21, 08 5:10 pm

Thanks guys. This helps.

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