Snow Drift when High Roof has Long Cantilever

[enigma2]

I’m designing a structure with adjacent high and low roofs. Both roofs are flat. The high roof has a soffit that cantilevers about 5’-0 beyond the wall plane at the high-low interface. Figure 7-8 in ASCE 7 shows the peak of the surcharge load against the wall for leeward drifts onto lower roofs. This is understandable if there isn’t a large overhang. But with a big enough overhang, common sense tells me there would need to be some sort of vortex effect to have that much surcharge at the wall. Has anyone dealt with this before? Should I just follow the code and stop over thinking this or is there another rational way to calculate the drift load?

 

[JAE]

The drift can be caused by either windward or leeward winds so the cantilever wouldn't necessarily help when the wind drives snow from the low roof up against the high roof wall.

I don't think ASCE 7 distinguishes between windward and leeward snow so you would essentially ignore the cantilever for the drift calc below.

Don't forget to increase your high roof snow for the overhang of the cantilever.

 

[MarcbSE]

ASCE 7 does have different drift calculations for windward and leeward drift. The leeward drift is 75% of the windward drift.

I typically ignore the cantilever for the drift configuration below.

 

[MarcbSE]

Correction... the windward drift is 75% of Hd when using the lower roof length for Lu