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Ponding - Less than 1/4" per foot

Ponding - Less than 1/4" per foot

Ponding - Less than 1/4" per foot

(OP)
If you have a roof that is pitched all one direction toward a free edge but is less than 1/4" per foot, is it still required to be analyzed for ponding?

The code reads "Roofs with a slope less than 114 in./ft
(1.19") shall be investigated by structural analysis to assure that
they possess adequate stiffness to preclude progressive deflection
(i.e., instability) as rain falls on them or meltwater is created from
snow on them."


Ok so it should be checked....
It then goes on to say

"The larger of the snow load or the rain load shall
be used in this analysis. The primary drainage system within an
area subjected to ponding shall be considered to be blocked in
this analysis"


Should you assume the entire edge of the roof is blocked?

I can see that ponding may still be an issue as there can be a low spot in the joist due to dead load deflection.

EIT
www.HowToEngineer.com

RE: Ponding - Less than 1/4" per foot

When I read "primary drain system" I'm thinking a drain with a pipe, not a free edge.

The IBC requires a primary drain pipe and then a secondary drain located 2" higher than the primary inlet.

At < 1/4" / ft on a free edge roof I might be inclined to see what the ponding check shows in any case but the loading on the roof would not be based on the entire free edge blocked off.

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RE: Ponding - Less than 1/4" per foot

Snow accumulation has little to do with shallow slopes. The difference between 1/4" per foot and flat for snow loading is insignificant; however, snow accumulations might be more than the ponding effect, so check both.

As for rain, you should check for ponding, particularly if your slope is in a single direction. If your slope dies into a parapet, all the more reason to check for ponding as the accumulations at the wall due to slow exit of water through scuppers can be significant.

If the roof has a free edge overflow, then you do not have to consider that the edge is obstructed. If the roof has a raised edge with drainage scuppers, consider ponding to be the height of the raised edge plus a little more if your rainfall intensity is high.

RE: Ponding - Less than 1/4" per foot

And if you have an unheated overhang at the low end, an ice dam could form, then you might want to consider ponding. Your call.

Mike McCann, PE, SE (WA)


RE: Ponding - Less than 1/4" per foot

Is the roof you are referring to framed of bar joists? If so, be careful to consider the camber that is built into them. If your design dead load is more than the actual dead load applied, and the joist is fabricated with camber in the upper range of dead load camber, ponding could be an issue.

RE: Ponding - Less than 1/4" per foot

My high school was supposed to drain off one side. It didn't. Every rainstorm, the janitor had to put out buckets to catch the leaks. Since this was in south Louisiana (60" rain/year) that was rather often.

I don't know whether the building settled unevenly, or the roof deflected, but I'd be careful with such a flat pitch. It may not behave as intended.

RE: Ponding - Less than 1/4" per foot

How big of area? At 63 pcf weight of water and 20 psf LL you have (20psf/63pcf)12(in/ft)=3.5 in of water before you exceed the 20 psf ll. I have never work on a job that had this much ponding.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

RE: Ponding - Less than 1/4" per foot

(OP)
@woodman that is a good point.

For this particular situation the building is rectangular 80' left/right by 100' front back. There are two lines of steel that run front/back (span approx 33'-4" between columns and outside walls). There are steel bar joists which bear on outside walls and the interior steel beams (spanning left/right, 3 bays, each span about 26'-8")). The bar joists of the outside bays are sloped toward the center of the building. The middle bar joists are flat. The beams are pitched from front to rear at 3/16". The rear is a free edge.

I'm actually reviewing the project and wondering if requiring them to check for ponding is a legit comment. I suppose it is easy enough to request that but I'm also curious if it is actually warranted.

EIT
www.HowToEngineer.com

RE: Ponding - Less than 1/4" per foot

What kind of deflection is expected on the roof under lets say half of the live load? if it's more than your slope I would be inclined to say yes. If less than the slope, no.

RE: Ponding - Less than 1/4" per foot

woodman88...have seen plenty of 3-4 inch deep ponding. Have seen buildup at downslope parapet exceed 12 inches....broke the perimeter angle attachment.

RE: Ponding - Less than 1/4" per foot

Ron.

Yes and I have repaired/replaced trusses due to that.
But the code requires the owner to maintain the building propertly. As such a building with 24 inch parapets should never have to support more than the depth of water than what the roof was designed for.
The fact that they do sometimes, is a building maintance problem. I have no problem with designing a roof to support 24" of water. But few owners do.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

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