Ponding on flat gym roof
Ponding on flat gym roof
(OP)
I have a 68x100 gym with flat roof (metal deck on LH joists). Support walls are precast with 2' parapet (may change). Ground snow load in the area is 25 psf. Per ASCE-7, as I understand it, I am going to check the lowest point on the roof for ponding with primary drainage system blocked. But what is the maximum water height that I should be using. Is it supposed to be height of the parapet?






RE: Ponding on flat gym roof
I would say to check for that two inches of water on the roof. If the roof is sloped, you shouldn't have to check for the load on the whole roof, just the region unitl the roof slopes up two inches.
If you have to check the roof for the two feet of water (125 psf)on it, it's going to fail. But it does solve any ponding issues!
RE: Ponding on flat gym roof
JC's example is conservative and certainly works.
RE: Ponding on flat gym roof
RE: Ponding on flat gym roof
RE: Ponding on flat gym roof
RE: Ponding on flat gym roof
Mike McCann
MMC Engineering
RE: Ponding on flat gym roof
Michael.
Timing has a lot to do with the outcome of a rain dance.
RE: Ponding on flat gym roof
The calculation for ponding is quite simple, if your deflection under dead load and applied loads is more than about 40% of the total fall over that span then you have a problem.
RE: Ponding on flat gym roof
Csd: sorry I didn't get what you meant by "40% of the total fall over that span".
RE: Ponding on flat gym roof
I am assuming you are using rafters that fall with the span rather than purlins which run across.
RE: Ponding on flat gym roof
As for the ponding, you would consider the height of water which could occur if the primary drain is blocked. Add that weight to the roof, look at deflections, again look at the added water weight resulting from the deflection, add that weight if it is significant, and so on, hence "ponding instability". This local deflection due to local impounding of water is the mechanism for excessive load development. Look at the commentary to ASCE 7 for examples and guidance.
RE: Ponding on flat gym roof
But out of curiosity, from what I read in chapter 8 of ASCE-7, ponding applies only to relatively flat roofs and the ponding load does not have to be combined with snow. Correct? Sorry I am writing this from the phone while in a boring meeting, so I could be wrong.
RE: Ponding on flat gym roof
RE: Ponding on flat gym roof
RE: Ponding on flat gym roof
June 2010 - 7'' of rain that month - no problems.
Jan 2011 - 3-4'' of rain - much colder - leaks like a sieve???
Driving us crazy....
RE: Ponding on flat gym roof
I investigated a collapse where there were very strong winds which may have impounded water under the sustained wind pressure, or may have flipped some TPO over all of the drains at the corner of the building, or may have damaged joists causing either ponding or directly collapsing the joists. The roof was 1/4"/ft pitch and we figured that the water could possibly have been 10+ inches deep.