Secondary (Emergency) Roof Drains

[daaguirre]

Does anyone have experience designing storm drainage systems using scuppers only?

I am designing the storm water drain system for the roof of a two story building (I-2) and our client requested replacing the roof drains by scuppers. We are using the 2012 International Plumbing code and I am sizing the scuppers according to Figure 1108.3 in the IPC Commentary which is based on Francis formula:

http://www.engineeringtoolbox.com/weirs-flow-rate-d_592.html

My question is, since we are only using scuppers, do we need a secondary (emergency overflow) system, i.e. additional scuppers discharging above grade?

From IPC 2012:

SECTION 1108 SECONDARY (EMERGENCY) ROOF DRAINS:

1108.1 Secondary (emergency overflow) drains or scuppers.
Where roof drains are required, secondary (emergency overflow) roof drains or scuppers shall be provided where the roof perimeter construction extends above the roof in such a manner that water will be entrapped if the primary drains allow buildup for any reason.

1108.2 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.

Thanks in advance!

Attached is a picture that I found on the internet depicting scuppers as primary and secondary roof drainage systems.

 

[PEDARRIN2]

Be careful about just using the IPC. Make sure the jurisdiction hasn't edited it for use in their area. it could cause mistakes.

In general, you have to have secondary drainage of some sort because of the parapet as stated in 1108.1. If one scupper gets plugged, the area covered by the scupper could accumulate water and cause issues depending on the roof design (IPC 1101.7).

Normally, secondary drains are designed to allow for 2"-4" of ponding on the roof before water enters, assuming the structure of the roof area can support that much water. So the bottom of the secondary scupper would be higher by that much.

Make sure the scuppers are located per 1106.5

Although the argument could be made that it is highly unlikely a scupper (or all scuppers) would be plugged since it does not have a strainer. Also the argument could be made that any accumulating water would flow over the top of the roof pitch point and be directed to a nearby scupper. The structural engineer would have to prove the roof structure could support the accumulated water.

But that argument might be lost with the plumbing code official.

 

[cvg]

I live in a house with flat roof, parapet and scuppers. they can and do get plugged. it doesnt matter if all scuppers get plugged. each scupper handles a portion of the roof independently from the next one. based on my past experience with this structure, I highly recommend that the scupper design should have a very large factor of safety. Prefer at least two scuppers for each roof area and each one should have at least double the required capacity. Roof crickets should provide the largest slope possible to drain to the scuppers. and provide a down spout at each scupper if possible.

 

[daaguirre]

Thanks for your replies, I ended up using two scuppers for each roof area, one for primary drainage and another one for overflow purposes. The primary drain is connected to the storm sewer and the secondary discharges to the floor surface at ground level. Although civil designed the roof to allow 4" of water ponding, I sized the scuppers using the min water height in Figure 1108.3 of the IPC Commentary.