Sloped roof reduction of load in Drifting snow

[ewing]

Good afternoon all,
I am working on a structure that is attached to a higher structure. The new structure will have an 8/12 slope.
We area allowed to take a reduction in the base snow load because of the slope. But to date I have not taken a reduction in the drift snow load for the slope.

The reason as I understand it for taking the reduction in base snow is that the wind and slipery surface cause a percentage of the snow to go directly to the ground (ASCE-7-98).

Is it not resonable to assume that the same thing would hold true for drifting snow?

Thank you for your input.
Ewing

 

[Trussdoc]

It could be that in many cases the lower surface is shielded from wind and sun by the higher structure, which would prevent the snow from sliding, even though the lower roof surface is sloped.

 

[RockEngineer]

It is my understanding that you can take the reduction in the drift load also. If using 97 UBC Appendix Chapter 16 section 1644 determines the drift height based of the lower roof Pf which is defined as the minimum snow load. Minimum snow load would include slope reductions.
Don't forget about impact loads when you have a higher roof and then a drop over 4 ft to a lower roof and over 70 psf snow load. The impact loads are usually 150% to 200% of the snow load and can be more critical than your drift loads. Here in Washington state I have to deal with some very high snow loads up to 300 psf in the mountain areas. We have very little drifting due to the wet snow but impact loads from one roof to another can be critical.

Good Luck