Overflow Scuppers

Another question on overflow scuppers. I'm working with a team that has proposed a creative solution to providing overflow scuppers, but I'm not sure whether it meets the intent of the 2006 IPC Section 1107.

It's a small building and a single 3" drain pipe is sufficient, but we are providing two scuppers for primary roof drainage.

There is very restricted space on the ground on the drain side of the roof slope (lots of ground-mounted electrical equipment), and so locations for scuppers are limited, too.

The proposed design has two roof scuppers, at the two corners of the building. The scuppers are oversized, 8” square. The idea is that this meets all requirements for both main and overflow in a single scupper: the bottom half of the opening is considered to be the main drain, oversized at 4”x8”, the top half serves as the overflow, is 2” above the level of the main scupper, and the overflow exceeds the minimum 4” requirement.

The main drains are scuppers with downspouts that daylight at grade, and the overflow is provided a separate path by a cutout in the conductor head.

Everything is oversized and redundant. In concept it seems to work, but something just seems odd about this. I’d appreciate your thoughts on this approach.

Thanks!
Michael Montgomery, AIA IVI Assessment Services

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Secondary drain piping needs to be separate, and if combined must enter below the main piping so any stoppage in the main scupper or leader does not cause ponding on the roof.

1101.11.2.2.2 Combined System. The secondary roof drains shall connect to the vertical piping of the primary storm drainage conductor downstream of any horizontal offset below the roof. The primary storm drainage system shall connect to the "building storm water that connects to an underground public storm sewer. The combined secondary and primary roof drain systems shall be sized in accordance with Section 1106.0 based on double the rainfall rate for the local area.
It appears that roof run off volume calcs will also be needed to verify proper leader size.
Ok but check out Section 2006 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."

No provision for a combined primary and secondary drain.
Per IPC 1107.2, the scupper in the first post with diagram is missing a secondary roof drain. The scupper is primary only even though the opening is large enough, there is no way to keep the roof from ponding over 2" deep if the main rainwater leader from the catch bucket gets clogged.
ok, now i'm confused jim. the opening in the wall is 64 square inches, times two scuppers. the scupper has a secondary opening at the front face, who's bottom is roughly the same elevation as the roof.

so how can it hold more than 2" of water if the drain pipe gets clogged?
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