# Verifying GPM for Roof Drainage



## jar546 (Jun 22, 2021)

I remember back in 2008 when I took the commercial plumbing inspector then plans examiner certifications that the tables were set up to use the map figures to determine the amount of gpm based on square footage.  When I look at the storm drainage section today I don't see a formula listed or a table that helps determine the gpm.  Maybe I am missing something but in order to do plan review, I had to look up a formula to determine gpm based on square footage and rainfall amount.  What I came up with was GPM= .0104(constant) x Roof Area (sq ft) x Rainfall  (1 hr per figures)

Am I missing something here?  In the past I believe this was simplified and now I had to look up a formula.  How rusty am I?


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## jar546 (Jun 22, 2021)

Here is an example of a recent one from a single family home on the upper roof areas. It is very conservatively designed and more than adequate.

We are at 4.5" of rainfall per hour but that is not shown in the calculation so I needed to verify it which brought this subject up.


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## Mech (Jun 22, 2021)

Your formula appears to be correct.

The constant 0.0104 is calculated from a string of conversion factors: (7.48052 gal / cu.ft.) x (1 ft / 12 in) x (1 hr / 60 min) = 0.0103896.

And yes, there used to be tables correlating rainfall rates and roof sizes to gutter sizes and pipe sizes.

The flow rate values in the example above are about 1 to 2 gpm higher than what I calculate with the formula.


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## Paul Sweet (Jun 22, 2021)

Virginia amended the old tables back into the code.

I don't know whether the IPC was trying to create more work for plumbing engineers, but that is the result.


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## jar546 (Jun 22, 2021)

Paul Sweet said:


> Virginia amended the old tables back into the code.
> 
> I don't know whether the IPC was trying to create more work for plumbing engineers, but that is the result.


OK so I am not crazy, the tables were there.


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## ADAguy (Jun 22, 2021)

For flat roofs take into consideration if a parapet exists? Number of overflow openings in parapet faces? vs roof drain/overflows provided?
What if ice and rain both accumulate?


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## north star (Jun 22, 2021)

*% ~ %  ~ %*




> *" OK so I am not crazy, the tables were there. "*


*Rembo, ...look back in to the **2003 Edition of the IPC,
Ch. 11......There are Tables **there.*

*I do not know if this Link to the `03 Edition will work or not.*
*





						2003 INTERNATIONAL PLUMBING CODE (IPC) | ICC DIGITAL CODES
					

ICC Digital Codes is the largest provider of model codes, custom codes and standards used worldwide to construct safe, sustainable, affordable and resilient structures.




					codes.iccsafe.org
				



*
*As to whether or not you are crazy, well the jury is still
out on this question !  *




> *% ~ % ~ %*


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