Snow drift load on porch

[Alan CA]

Hi,

Should we account for snow drift load from the higher roof (house) on the porch roof when the higher roof (house roof) slopes to the other sides away from the lower roof (not towards the lower roof)?

 

[jimstructures]

Yes, absolutely. Review ASCE - 7 - (appropriate # per local code ) if you are in the USA.

Jim

 

[jayrod12]

I believe the OP is in Canada.

Technically part 9 doesn't require buildup. However in situations where i know that buildup is a real possibility I will use the part 4 provisions but use a cb=0.55

 

[r13]

Drift is caused by wind blow the loose snow over the unobstructed edge of the high roof, so it needs to be considered.

 

[Alan CA]

Thank you very much guys for the valuable help

 

[Pmtottawa]

Jayrod
If in Canada and under part 9 you still need to consider drift loads if its a flat roof per 9 4.2.1.f

 

[Alan CA]

Yes I'm in Ontario, Canada. I tried to check the relavent NBC 4.1.6.5 but the copy I have jumps from 4.1.6.4. To 4.1.7! I couldn't check the details.

 

[Canuck65]

I agree that technically under Part 9 of the OBC, drifting and buildup do not need to be considered for residential situations such as described by the OP. In fact, buried somewhere in Appendix A of the OBC (likely somewhere near this code clause #) it states something to the effect that even though it is recognized that valleys and step heights in residential houses may collect additional snow drifts, there has never in the history of the code been a failure attributable to excessive drifting. That being said, if your judgment tells you that there may be additional snow drifting, use a conservative value for the snow design load, round up, etc.... to give you the comfort that you need. Part 4 structures are totally different and you would be designing for quite a large snow load on the lower roof.

 

[Pmtottawa]

One odd thing with part 9 if the formula in 9.4.2.1.f
It makes 0 sense for a flat roof
Agreed that historically nothing happened, just have to wonder how many design events actually happened according to code.

P