roof ponding

huangyhg

roof ponding
can someone help to clarify the "primary drainage system" as described for roof ponding requirements?  
i have a roof with 1/4" per foot slope to the interior of a building, where roof drains are located (the interior is 1'-3" lower than the permeter).
my question is that if we provide a secondary, or overflow scupper, and this is elevated like 2" above the primary drain, can this drain be tied into the main drain stack below the roof?  if so, can i only design the roof for 2" of water, vs almost 1'-3".  or do we need to provide a completely separate drain system (ie 2-full runs of pipe) to be able to do this?
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the overflow needs to be fool proof, i.e. if your drain stack blocks up then it still needs to work.
thanks.   i went to the icc and their code chat had a post citing a code reference that the secondary drain needed a separate point of outlet.  
"ipc 1107.2 separate systems required. secondary roof drain systems shall have the end point of discharge separate from the primary system. discharge shall be above grade, in a location that would normally be observed by the building occupants or maintenance personnel."
don phillips
we have our overflow drains thru the wall like a scupper so that any flow would be noticed!
if your secondary is 2" above the primary, you may have to design for more than 2" anyway.  you have to account for the hydraulic head as well as the static.  you'll find this in the rain loads section of the ibc or in the same section of a state code based on the ibc(e.g. 1611 in the fbc).  you can get hydraulic head information from the asce 7 and the design rain event from the plumbing code (chapter 11 i think).
and remember--what this thread is discussing is "blocked drain design," not "ponding."  if a roof slopes more than 1/4" per foot, ponding is not an issue, because the   
quote:
if a roof slopes more than 1/4" per foot, ponding is not an issue
dave, not sure about that.  in most cases it is true (and in fact aisc k2 mentions "flat roofs") but in some cases you can have 1/4" foot slope into discrete and closed-off valleys which have only one primary drain....say with a 4" ridge height, with scuppers way off in the distance.
the primary plugs up and this bath tub fills to the brim.
you'd design for the weight of the water, yes, but even with the 1/4"/ft slope i'd be very concerned about ponding in the discrete valley areas since the water there would flow to the lower areas first, pond there, and if your framing system wasn't stable per aisc k2, then you could possibly have a real issue with second-order load application of the water.
jae
in the situation you describe, there needs to be a secondary roof drain there. i often try to make sure that the architect and the plumbing designer follow thru on their design properly so that it does not come back on the structural guy.
the secondary drain is the scuppers beyond.  they design for the extra weight of the water in the "tub" and then assume that any excess water goes over the ridges and finds its way to the scuppers.