Snow load on leeward side of 45 degree gable roof

[ajk1]

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?

 

[ajk1]

Can anyone respond to this? Thanks.

 

[Jerehmy]

I get the same roof slope factor per ASCE.

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.

 

[jayrod12]

And if you don't have a slippery roof finish you never get sliding snow. If you are putting asphalt shingles then no snow slides, even at a 12/12 pitch.

 

[ajk1]

Jerehmy - 1/2 on 12 is 1/24 or 2.4 degrees. Is that right? Seems like a very small angle. Why not just say zero, or am I misunderstanding something?

Jayrod - why do you say that? That is not what the National Building Code says. Is that your personal observation?

 

[BAretired]

jayrod12,
Unless the code has changed recently, there should be a reduction for all roofs with a slope exceeding 30^o but a larger reduction for slippery roofs.

BA

 

[jayrod12]

I've always neglected the reduction from my own experience. Conservative yes, but I've found asphalt shingles don't allow as much sliding as you'd expect.

 

[Jerehmy]

ajk1 - I'm just regurgitating what the ASCE states

 

[jayrod12]

And, new houses rarely have a 12/12 pitch that terminates at the eave, it is now typically a feature for a dormer or entrance and the 12/12 pitched roof terminates onto a roof with a lower slope. This causes valley loading at the interface which causes the slope factor to revert to 1.

 

[allgoodnamestaken]

Using the NBC 2010 Structural Commentaries, Figure G-1 on page G-6 I get the same numbers as you, provided your roof is not slippery (metal roofing, etc).

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.

 

[ajk1]

To Jayrod - The cottage that I am checking indeed does have roof sloping down at 45 degrees, that terminates at the eaves. And it has already been built. The owner, whom I know, has some qualms and asked me to check some things.

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.

 

[jayrod12]

A cottage style roof like that you may need to use the reduction due to sliding in order for the roof rafters to work. I generally ignore sliding at first and if I need to go back and account for it I do. However like I said, in new design I ignore it, you aren't saving that much in terms of material.

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.

 

[ajk1]

Unfortunately the contractor built something different than what was on the drawings. But it is not the rafters that are my immediate concern, it is the perimeter built-up beam at the main floor, that spans between foundation piers and supports the stud bearing wall above. He reduced the number of supporting piers although he did increase the beam size, so I am running the load down and checking the beam and the pier footing.

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).

 

[jayrod12]

You can check it yourself for 1 way and 2 way shear and bending. There's a good chance at only 7" thick it will not work unless it was just a leveling pad poured on bedrock. If it is on clays then I doubt the footing will have the bending capacity to spread the load to the outer edges.

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.

 

[ajk1]

I am aware of all that but it won't work by Part 4 for several reasons. Making the depth equal to the projection cannot be justified by calcaultions if you are on rock. For 2500 psi concrete, making the depth equal to the projection only works up to about 7000 psf bearing values, when checked by calcs. Anyway, what I am really interested in is if there is anything in Part 9 of the OBC that would determine the depth (other than the 100 mm minimum.

 

[jayrod12]

I'm not sure exactly what you are after then. It spells it out clearly in that clause. If you need higher than 7000 psf I'd be more concerned about bearing of the beam on your concrete(assumed) pier. You haven't given us any sizes of piers or pads not the kind of load you are dealing with so if you want to be within the prescriptive nature of part 9 then you must abide by that clause.

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.

 

[ajk1]

ok, thanks. Much appreciated.