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Re: Snow Drifting

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I do not know of any minimum ground snow, below which drifting is not required to be calculated. However, as a practical mater, for ground snow loads less than 15 psf, drift loads are rarely significantly greater that the minimum roof load.

Also, there is some reason to think drifting is minimal in many low load areas. Although there have not been sufficient studies to prove the theory, snow in low load areas usually accumulates from a single storm, and there is not sufficient repeated wind action to create significant drifts. Also, the snow in these areas tends to occur at higher temperatures, making it wet and heavy, reducing drifting.

The drifting I am referring to is from aerodynamic shade (wind action) addressed in ASCE7 sections 7.7 and 7.8. Sliding snow from upper roofs (section 7.9), and buildup at valleys (not addressed in ASCE7) is still an important concern for low snow load areas. In fact, it may be more of an issue for wet, heavy snow, and it may be prudent to use the ground snow, rather than the roof snow for the calculation of sliding. Heavy, wet snow from a single storm is less likely to be reduced by wind action. The 0.7 factor in the conversion of ground snow to roof snow assumes wind action reduces roof snow by 30%, subject to some modification for exposure.

Dmitri Wright, PE
Cascade Engineering, Inc.
245 SE 4th Ave, Suite B
Hillsboro, OR  97123-4033

From: Jason Christensen <jason(--nospam--at)>
To: "seaint(--nospam--at)" <seaint(--nospam--at)>
Subject: Snow Drifting

Does anyone know where or if in ASCE 7 there is a "cut off" snow load that =
snow drifting requirements are no longer required?


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