Snow Drift Windward Fetch

[Elberon]

For calculating snow drift on the windward side of a lower roof, ASCE 7-05 tells you to use the length of the lower roof in Fig 7-9 (ASCE 7-05 7.7.1). If you have a situation where there is a long higher roof, followed by a relatively short lower roof, followed by another high roof, would you use only the length of the lower roof, or would you use both the first higher roof plus the lower roof on the windward side of the second low/high step?

 

[JAE]

I'd use both the higher plus the lower roofs that are upwind from the roof-drop. This is a situation that is not clearly indicated in the code so using the conservative interpretation is the way to go. That and it is also logical that both the high and low upwind roofs would contribute to blowing snow.

 

[lkjh345]

Would definately use the total length of the first segment of higher roof plus the lower roof to calculate the windward drift.

You may, depending on you exact configuration, also have a leeward drift at the first step from high to low.

 

[msquared48]

I'd redesign the roof.

Mike McCann
McCann Engineering

 

[hokie66]

I doubt that any code can effectively deal with the situation you describe. The lower roof section between two high sections will have no ability to shed snow, so whatever finds its way there will stay. Depending on the amount of snow, it could be expected to fill up until the higher level is reached. A lot of load, and then when it melts, probably a lot of leaks. Agree with Mike, not a good design.

 

[Elberon]

Thank you all for your input. The roof in question is an existing roof designed in 1967. At that time our building code did not require the consideration of localized drift load. The multi-level roof configuration, however, is very common, an example being a gymnasium adjacent to a single story lobby/cafeteria, office area, adjacent to a multi-story classroom wing.

For determination of drift surcharge, I will use the more conservative approach of using the lower roof area and everything adjacent to it at an equal or higher elevation. If anyone knows of any Code or Code Committee that has taken a position on this question, I would be interested in that.