Snow drift nbcc part 9

[Pmtottawa]

I'd like clarification whether we need to consider snow drift on a FLAT LOWER ROOF based on part 9 of the national.bldg code of Canada.
9.4.2.1 seems to indicate you do.
A9.4.2.2 seems to say you don't

I've been including snow drift on all my part 9 projects but beginning to question this.

Thanks in advance..

 

[skeletron]

My take is that if your situation is FLAT LOWER ROOF then "Yes" include snow drift.
Note A9.4.2.2 identifies large buildings and roof configurations that seriously exacerbate snow accumulation as examples where the simplified snow load (Cb = 0.55) cannot be used. It also goes back to the old "smaller light-frame buildings constructed according to Part 9 have not failed" argument. So, in the end there is some judgement applied based on the size of the building but if the step is significant, or if there are significant parapets, then I would include snow drift. Sentence 9.4.2.1(f) also gives that equation to determine if the simplified snow load is sufficient for flat roof.
I also think it is important to consider if Ss + Sr > 4.0kPa.
Here is a good document to read as well -> LINK

 

[Pmtottawa]

Skel
I have that document and very informative.

And agree with you that drift occurs, we've had several collapses 5 years ago...
But getting aggravated when other engineering colleuages say they never consider it for part 9.
This is a large custom homes with large wide open walls, irregular configuration a massive 15ft cantilever for the master...

I also feel that part 9 is alittle outdated with today's residential homes with large 2 story open walls, massive open spaces, irregular configurations etc.. feel like the typical rectangular regular homes are no longer the norm


Best

 

[SinStrucEng]

I consider snow drift for all my designs, including P9. Honestly, especially P9 - those buildings are pretty shoddy to begin with and a flat roof with Ottawa snow makes it much worse. I have multiple flat roof bungalows that have been designed and built, and right now, they are sitting with 2+ feet of drift on them in the Ottawa region. You NEED to factor that in.