Snow Drift Loads on Roof Depression

[EngiNinja]

I was wondering what the common practice was for determining snow drift loads on a roof depression. If you have a 3' deep trough around the egde of your roof that is surrounded by a parapet wall, I would think that more snow could build up in the trough than the amount calculated with the standard drifting equations. I suppose the highly conservative approach would be to assume the trough fills with snow to the level of the roof, then add the usual drifting load on top, but I didn't know if there are allowances in the code to be a little more efficient than this.

 

[EngineeringEric]

I would do the conservative, but i have no idea on the code. I would also think that this snow here would be denser (from water draining and freezing) than other areas so conservationism isn't bad IMHO.

 

[KootK]

Interesting question. Of course, when it comes to snow drift, anything other than a big box beside a little box tends to be interesting. I don't have a definitive answer but I do have some observations to share based on my understanding of aerodynamics:

1) with unlimited snow available to contribute to drift, I expect that the trough would fill up first then drift would deposit against the parapet in the usual way until a snow ramp formed that was gradual enough to prevent additional snow deposit.

2) with limited snow available to contribute to drift, I would expect the trough to fill first then drift of the usual profile would accumulate such the volume of drifted snow would equal the volume of snow available less the snow in the trough.

So I guess that my answer would be that I'd go with the conservative answer (1) for a large roof and perhaps a less conservative answer (2) for a small roof. That being said, I have no non-arbitrary basis for distinguishing between a large roof and a small one.

I also wonder if a wind parallel to the trough could fill it after which wind perpendicular to the trough would superimpose drift. Perhaps that scenario is already built into the ASCE provisions.


I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Guastavino]

I'd assume it was filled up and then add the drift load. And then I might even increase that a bit because I think that the snow could be more dense as it gets packed in there. Ultimately, and the cop out answer is it's your judgment. However, I wouldn't stake my career (may sound dramatic, but we do it on every project when you think about it) on a gray area. And, I think you have ample reason to justify being conservative.