Snow drift per ASCE 7

[allisch]

IBC 2000 1608.7 says to use ASCE 7 section 7.7 for computing drifts on lower roofs.
ASCE 7 section 7.7 sends you to a figure which shows that the leeward snowdrift is a function of the ground snow load and the length of the upper roof. It does NOT include height difference criteria. Therefore it appears that you use the same drift for a 2' roof height difference that you do for a 20' roof height difference.

My question is - how do you fellows deal with roofs where the roof height difference is relatively small?

On the 2' height difference, by the time that the snowdrift builds up to the higher roof, then the surface is now nearly flat- so no additional drifting should occur due to a roof height difference. (In effect the new surface will be tapered from the low to the high roof.)

Do you ignore roof height differences of less than a certain height or use a reduced drift loading in this case? Or do you apply the designated drift which may be higher than the difference to the high roof? (which seems unrealistic to me)?

 

[MarcbSE]

The text in the code limits the drift height to the clear height (roof elevation difference less the uniform snow height). The length of the snow drift is modified when the drift height is limited in this situation.

 

[MarcbSE]

Section 7.7.1 of ASCE 7-05 (same as previous versions):
"If this height (calculated Hd) exceeds Hc, the drift width, w, shall equal 4Hd^2/Hc and the drift height shall equal Hc."

Hope that helps.

 

[allisch]

Thanks for the info.
I had read this as that was only referring to the windward drift since that sentence followed the windward drift notes.

Going back and re-reading this, I can see your perspective.
It does not state that it is only for the windward drifting.

Your interpretation makes more sense.

Thanks