Snow-shelf structure

[GalileoG]

I have a new roof that is approximately 3 metres higher than an existing roof. I have looked at many options to address the issue of accumulated snow load that would result on the existing low roof, but found that the most economical approach is to build a snow shelf. This structure is in Canada.

The snow shelf will be supported entirely by the new roof and will extend above the existing low roof by a distance of approximately 3.5m. I am adding a high parapet on the new high roof to reduce the required projection of the snow shelf to 3.5m.

What is the criteria for locating the height of the snow shelf?

Top of deck to match height of the basic snow load of the low roof?

Underside of shelf structure to match height of the basic snow load of the low roof?

 

[jayrod12]

I've always thought your snow shelf must essentially act like a bridge roof such that the high roof doesn't cause buildup on the shelf, and the shelf doesn't cause buildup on the low roof.

 

[KootK]

I was told that the profile of the shelf should be of the same slope as the snow drift that would otherwise be there, although not necessarily at the same elevation. The idea being that slope would be the one that, aerodynamically, would result in lamellar wind flow over the roof surface and no snow deposition. Frankly, the whole thing's always seemed like a bit of a theoretical stretch to me. I'd love to see some research on it.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[JAE]

At least here in the US, ASCE 7 doesn't really suggest that an upper parapet affects the drifting below. Maybe in the Canada codes perhaps.


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[BadgerPE]

Is there a horizontal separation between new/old buildings? If not, how do you get away from windward drifting? A sketch would be awesome since I have never heard of this before.

 

[DST148]

If my memory serves right, per Canadian Code, height of upper parapet does affect leeward drift on the lower roof.
@GalileoG>>>> You can avoid leeward drift on the lower roof by building a new screen wall / parapet of sufficient height.
However, as BadgerPE posted above, you still need to deal with windward drift on the lower roof (because of higher adjacent new construction).
Does the existing lower roof have parapet? This may provide some relief.
Qualitatively, the build-up of the windward drift on the lower roof will be reduced depending on the projection of the new snow shelf and the reduced length of the lower roof(lu per ASCE 7) upwind of the drift.