Snow drift: flat roof adjacent to gable
Snow drift: flat roof adjacent to gable
(OP)
Howdy...
Given: a gabled roof with an attached, flat lower roof immediately adjacent to the low end of the gable on one side.
Desired: is the drift height (hd) specifically limited by the vertical difference between the top of the low (flat) roof and the bottom of the gable above; or can (should) it "creep" up past the end of the gable, if the calculations indicate such a drift height? ASCE's diagram (fig. 7-8, chapter 7.0, ASCE 7-02) only demonstrates an aerodynamic drift situation between flat roofs of different heights, separated only by vertical distance. And the language in section 7.7 doesn’t seem particularly definitive on this matter, either.
A logical extension of that question would be whether or not sliding snow would be included on top of the drift; as was indicated by figures back in the good ol' UBC. Although ASCE 7 states, in section 7.9, “Sliding loads shall be superimposed on the balanced snow loads," it seems to me like if there were a substantial (vertical) difference between upper roof eave and lower roof surface, you could expect (aerodynamic) drifting AND sliding loads to occur at the lower roof. Would you consider such an opinion horrendously over-conservative?
Opinions…?
Thanks,
walterbrennan
Given: a gabled roof with an attached, flat lower roof immediately adjacent to the low end of the gable on one side.
Desired: is the drift height (hd) specifically limited by the vertical difference between the top of the low (flat) roof and the bottom of the gable above; or can (should) it "creep" up past the end of the gable, if the calculations indicate such a drift height? ASCE's diagram (fig. 7-8, chapter 7.0, ASCE 7-02) only demonstrates an aerodynamic drift situation between flat roofs of different heights, separated only by vertical distance. And the language in section 7.7 doesn’t seem particularly definitive on this matter, either.
A logical extension of that question would be whether or not sliding snow would be included on top of the drift; as was indicated by figures back in the good ol' UBC. Although ASCE 7 states, in section 7.9, “Sliding loads shall be superimposed on the balanced snow loads," it seems to me like if there were a substantial (vertical) difference between upper roof eave and lower roof surface, you could expect (aerodynamic) drifting AND sliding loads to occur at the lower roof. Would you consider such an opinion horrendously over-conservative?
Opinions…?
Thanks,
walterbrennan






RE: Snow drift: flat roof adjacent to gable
DaveAtkins
RE: Snow drift: flat roof adjacent to gable
Up here in Canada, we have a clause that allows us to max out our snow load at 3x ground. There are still cases though, similar to what you described here, that we don't implement this clause.
RE: Snow drift: flat roof adjacent to gable
What if there is NO eave height above the low (flat) roof (i.e. the gable "dies" into the lower flat roof... does that mean there's no drift...? And, according to the sliding provisions (7.9, I think), we seem to be able to determine for ouselves whether sliding might be reduced or eliminated, under just such circumstances... (i.e. "...unless a portion of the snow on the upper roof is blocked from sliding onto the lower roof or is expected to slide clear of the lower roof.")
Whadayathink?
walterbrennan
RE: Snow drift: flat roof adjacent to gable
1. From a leeward direction (i.e. wind blowing away from the gable across the flat roof) the wind would likely be deposited on the gable just as it passed the ridge, resulting in an unbalanced load condition as in ASCE 7-02 Figure 7-5; with little or no snow actually drifting down onto the flat roof.
2. From a windward direction (i.e. wind blowing across the flat roof toward the gable), with a steep enough roof gable, the situation might be treated as in Figure 7-8, with the gable peak taken as the high roof elevation. On the other hand, he said, if it were a “flat” enough gable, there would probably be no windward drifting at all. When pressed on what he might consider a reasonable range of slopes to validate this approach, he responded that 75 or 80 degrees seemed reasonable for the steep condition. When asked what should be done when the situation fell between a “flat” enough gable and the “steep” enough gable, he demurred; once again flatly stating that ASCE simply did not address snow loading for a gable roof with no vertical eave distance above an attached flat roof.
We are left, I believe, to draw our own conclusions. What I’m wondering is, what are the rest of you doing for this (relatively) common geometric scenario…?
Thanks,
walterbrennan
RE: Snow drift: flat roof adjacent to gable
RE: Snow drift: flat roof adjacent to gable
DaveAtkins
RE: Snow drift: flat roof adjacent to gable
Did you read the response posted just ahead of yours...?! This guy only has a 4/12 slope and can confirm waist-deep snow drifting where the gable dies into the flat roof...
I'm not sure where you live, but having spent more than 12 years in Fairbanks, I've also shoveled a few roofs. It’s all the more curious to me that this scenario hasn’t been covered by ASCE, yet; considering that they treat so many others.
Just thinking out loud...
walterbrennan
RE: Snow drift: flat roof adjacent to gable
RE: Snow drift: flat roof adjacent to gable
RE: Snow drift: flat roof adjacent to gable
Have a good 4th.
walterbrennan