Snow drift on sloped roof
Snow drift on sloped roof
(OP)
We're designing an addition to an existing building. The existing building has a flat roof. The addition is a lower roof which is sloped (shed roof). The lower roof slope is 4 in 12. The difference in height between the two roofs is 12 feet. The calculated drift height is 3.5 feet. The drift width is 14 feet.
My question concerns the drift width. Is the width a horizontal projected distance or does it follow the slope of the roof?
My question concerns the drift width. Is the width a horizontal projected distance or does it follow the slope of the roof?






RE: Snow drift on sloped roof
RE: Snow drift on sloped roof
RE: Snow drift on sloped roof
I also read 7.4 about sloped surfaces. In the past, I've always used project area, but then I read the commentary which seems to imply that sloped surfaces have a clearing effect due to wind, hence the ability to use the horizontal projected load. However, because this is a lower shed roof, you won't have the same clearing effect, so that's why I asked the question. I'll probably still use the projected area though.
Thanks for the responses.
RE: Snow drift on sloped roof
It should be conservative to use the snow load along the slanted axis of the beam. The figures 7-3, 7-4, and 7-5 all show the snow load presented on a horizontal beam even for curved and other roofs.
RE: Snow drift on sloped roof
RE: Snow drift on sloped roof
RE: Snow drift on sloped roof
RE: Snow drift on sloped roof
RE: Snow drift on sloped roof
strguy11, ASCE 7 says that the drift load is superimposed on the balanced snow load. If your roof has a slope of 70 degrees or more, then the Cs factor is 0 and you have no balanced snow load, hence no drift load. Up to that point, I would assume that the code wants the drift load to be the full amount, not reduced by slope. However, you bring up an interesting question, which brings me back to my original post. It seems that it would be more realistic to apply the drift width parallel with the roof slope versus a horizontal projected area. This would act as a reduction of the drift load based on roof slope.
RE: Snow drift on sloped roof
RE: Snow drift on sloped roof