Snow load on leeward side of 45 degree gable roof
Snow load on leeward side of 45 degree gable roof
(OP)
I have a gable roof where the rafters span from the exterior wall to a ridge beam at the roof peak. The rafters each side of the roof peak are sloping at 45 degrees. According to the NBC of Canada, the snow load can be reduced for sliding, but according to the User's Guide it also needs to be increased to account for snow blown from the windward side up over the peak and coming down on the leaward side (User's Guide, Fig 1, Case 2; I am not sure if it is still Figure 2 in the latest NBC User's Guide).
Therefore,
the slope factor Cs is (70 - slope)/40 = (70-45)/40 = 0.625
and for roofs of slope > 20 degrees the accumulation factor Ca is 1.25
Then CsCa = 0.625 x 1.25 = 0.78
The snow on the rafter would be based on a snow load calculated with this 0.78 factor (and the other factors in the Code snow load expression).
Do you agree?
Therefore,
the slope factor Cs is (70 - slope)/40 = (70-45)/40 = 0.625
and for roofs of slope > 20 degrees the accumulation factor Ca is 1.25
Then CsCa = 0.625 x 1.25 = 0.78
The snow on the rafter would be based on a snow load calculated with this 0.78 factor (and the other factors in the Code snow load expression).
Do you agree?






RE: Snow load on leeward side of 45 degree gable roof
RE: Snow load on leeward side of 45 degree gable roof
Also, per ASCE, you don't get drift loads on the leeward side of a gable roof unless the roof slope lies between 1/2 on 12 and 7 on 12.
RE: Snow load on leeward side of 45 degree gable roof
RE: Snow load on leeward side of 45 degree gable roof
Jayrod - why do you say that? That is not what the National Building Code says. Is that your personal observation?
RE: Snow load on leeward side of 45 degree gable roof
Unless the code has changed recently, there should be a reduction for all roofs with a slope exceeding 30o but a larger reduction for slippery roofs.
BA
RE: Snow load on leeward side of 45 degree gable roof
RE: Snow load on leeward side of 45 degree gable roof
RE: Snow load on leeward side of 45 degree gable roof
RE: Snow load on leeward side of 45 degree gable roof
If your roof is slippery then Cs = (60-alpha)/45 = 1/3 (NBC, 4.1.6.2(6b) on pg 4-15)
Cs*Ca = 1/3 * 1.25 = .417 for a slippery roof.
RE: Snow load on leeward side of 45 degree gable roof
All your comments are very interesting indeed. Unfortunately I don't have an up-to-date ASCE unfortunately, but anyway I am governed by the Ontario Building Code. But if ASCE has revised to something significantly different, be interesting to know the justification. ASCE used to be quite similar to NBC in this respect. Thanks.
RE: Snow load on leeward side of 45 degree gable roof
That being said, most of the roof components are designed and sealed by the suppliers of the roof trusses and I'm sure they are accounting for the reductions in snow load.
RE: Snow load on leeward side of 45 degree gable roof
Is there anywhere in Part 9 of the Ontario Building Code where it says how to determine the required thickness of a spread footing? All I can find so far is a table giving the required area, but not the thickness. Is there any other reference that would give this information for residential type structures? When I check it by Part 4 of the Code, the footing is too thin (7"; Part 4 requires 8" minimum for an unreinforced footing).
RE: Snow load on leeward side of 45 degree gable roof
Try clause 9.15.3.8, that's the NBCC reference. It states Footing thickness shall be a minimum of 100mm or the width of the projection of the footing beyond the supported element(i.e. face of pier). That's how I've always designed my pads anyways, a 30"x30" pad supporting a 10"x10" column needs to be 10" thick, if it supported a 16"x16" column it would only need to be 7.
RE: Snow load on leeward side of 45 degree gable roof
RE: Snow load on leeward side of 45 degree gable roof
Elsewise, you as the engineer would have to take responsibilities for deviations from the code. A lawyer would be sure to highlight any deviations from the code if something were to go wrong, even if the deviation were acceptable to most (or all) engineers. You would need to back up your deviation with concrete (no pun intended) proof that the installation meets CSA design codes for the appropriate material.
RE: Snow load on leeward side of 45 degree gable roof