Roof drain/ scuppers?

[steveray]

Do scuppers (as ONLY roof drainage) negate the requirement for secondary drains? By definition you already have a parapet. Do we count on scuppers not being blocked so no secondary is required? Thanks!

[P] 1502.2 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. The installation and sizing
of secondary emergency overflow drains, leaders and
conductors shall comply with Sections 1106 and 1108, as
applicable, of the International Plumbing Code.
These requirements alert the roofer to the need for
providing properly sized secondary drains or scuppers.
A secondary drainage system is required where
the building has parapet walls or other construction
that could restrict drainage at the roof’s perimeter.
This section requires all buildings to have some
method for preventing the accumulation of unplanned
excessive rainwater.
The intent here is to limit the amount of ponding
water that can accumulate on the roof due to rainfall.
If the roof is constructed so water cannot pond on the
roof, such as with roofs sloped toward the edge of the
building, secondary drainage is not required. This criteria
should be applied to all portions of the roof, so
that if any portion is designed so water can pond,
secondary drains would be required.
Bear in mind that a roof could be intentionally
designed to allow ponding to some design depth such
as for a rainwater harvesting system.

 

[Paul Sweet]

The area of the scupper above the maximum ponding depth must be at least as big as the code requires an overflow scupper to be.

 

[steveray]

Just trying to split the hairs between "roof drain" and scupper and whether or not we account for scupper blockage....I've never seen one blocked and maybe that is why it is written the way it is...

 

[Paul Sweet]

At one time an overflow scupper was required to be 3 times the area of the primary drain. I size scuppers based on the scupper being clogged (with branches, leaves, & other debris) up to the maximum ponding depth the roof is designed for (4" for a 20 PSF roof load), then going up another 4" or more to provide the necessary overflow area. If the scupper discharges into a leaderhead, the overflow area begins at higher of the ponding depth or the top of the leaderhead.