ASCE - Ponding Instability
ASCE - Ponding Instability
(OP)
Hi all - wondering if you would consider the framing in the attached sketch a "susceptible bay" in accordance with ASCE7-10 8.4. The issue being that while the slope is 1/4" per ft, the overflow scuppers are by their nature controlling drainage of the rainwater in the case of the primary drains being blocked. So section 8.5 applies, which sends you back to section 8.4 and requires you to investigate the framing for instabilities.
Or would you consider the overflow scuppers "free drainage" so that instability does not exist (since the roof slopes?
I appreciate the input.
Or would you consider the overflow scuppers "free drainage" so that instability does not exist (since the roof slopes?
I appreciate the input.






RE: ASCE - Ponding Instability
RE: ASCE - Ponding Instability
In your case, you must design for blocked primary drains, but not ponding.
DaveAtkins
RE: ASCE - Ponding Instability
RE: ASCE - Ponding Instability
In my opinion, an overflow scupper is not a "point of free drainage" unless it is very, very large. I believe "a point of free drainage" is intended to be a free edge of the roof.
I would say that ASCE 7-10 requires you to consider ponding instability for the roof system shown in your sketch.
RE: ASCE - Ponding Instability
RE: ASCE - Ponding Instability
I did not realize ASCE 7-10 is a little more definitive.
DaveAtkins
RE: ASCE - Ponding Instability
RE: ASCE - Ponding Instability
Why would a scupper not be free drainage? What makes it "not-free"? To me there is nothing blocking the drainage (i.e. a grate or some other means to screen debris). Although I suppose if you were to make the scupper 1" wide, then it doesn't seem so free.
EIT
www.HowToEngineer.com
RE: ASCE - Ponding Instability
RE: ASCE - Ponding Instability
But with the current weather conditions in upper New York State...
Mike McCann, PE, SE (WA)
RE: ASCE - Ponding Instability
RE: ASCE - Ponding Instability
"The authors of this digest concur with the requirements of points 1 through 5.
Calculations presented in this digest indicate that ponding instability can occur
with roof slopes of 1/4 in. per ft. and greater. Thus, it is recommended in the
digest that ponding instability calculations be made even for roof slopes greater
than 1/4 in. per ft., unless the roof slope is greater than 1/4 in. per ft. and is free
draining (free to drain over the edge of the roof)"