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Leeward snow drift and minimum fetch

Leeward snow drift and minimum fetch

Leeward snow drift and minimum fetch

(OP)
I have a situation where I am popping up an existing building. The pop up is introducing a snow drift on the lower roof. As far as I can tell- this existing roof is capable of supporting the windward drift on the low roof. The leeward drift on the low roof, however, exceeds the existing roof load capacity. This leeward drift is affected by the fetch derived by the pop up roof length. What I want to know is- can I create a 'thick' parapet wall on the upper roof that effects a smaller fetch dimension in my leeward calculation? When I say thick- I mean extreme thickness- like 5'. Put it another way, is there a minimum fetch dimension that must be considered? Think of a row of 20' wide townhouses from left to right that is 100' long in length. The townhomes saw tooth in height- 10' up then 10' down. What is the leeward fetch for a townhome at the lower height?

RE: Leeward snow drift and minimum fetch

In new design, I'd be assuming the entire length. In an existing situation, it would depend on how badly the roof fails.

RE: Leeward snow drift and minimum fetch

I don't think the Code allows it to be considered, but I have always thought a parapet sticking up in the middle of a roof should reduce the length of roof which will cause a leeward drift. If a portion of the snow blowing across a higher roof gets stopped by a parapet, that portion of snow won't be deposited on the lower roof.

DaveAtkins

RE: Leeward snow drift and minimum fetch

I've never found a good source or code reference to suggest what obstruction size or height is good enough to stop a snow drift. This is one of those situations where I grab my "engineering judgement" book...

RE: Leeward snow drift and minimum fetch

As I recall, a fire wall typically extends 3'-0" above the roof. In many areas, I think that is sufficient to stop snow from blowing across a roof.

But as DETstru said, this is "engineering judgment."

DaveAtkins

RE: Leeward snow drift and minimum fetch

The Canadian code does provide some relief to a lower roof leeward drift due to the presence of a parapet on the upper roof. US code does not recognize the existence of a parapet for the purpose of lowering the drift load.
The Massachusetts code also provides some guidelines for the low roof/high roof/low roof scenario in terms of how much vertical separation distance is required to ignore the roofs farther away from the step in question.
The basic problem with all of these sorts of discussions is that the amount of snow that ends up in a typical leeward drift is about 35% of the available snow from the upper roof(s). The remaining snow is either staying on the upper roof or blowing beyond the area of the snow drift (and thus might be available to drop into a later drift). Parapet (projection) type drifts follow the same type of rules limited by the size of the projection, so some snow may continue beyond the parapet and fall into a leeward drift. Is it the same amount as a non-parapet condition? Who knows?

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