Snow Drift on sloped parapet

[ephil]

Does anyone know how to calculate snow drift behind a sloped parapet structure? I have an existing building that is being remodeled and they are adding a 6' tall parapet to the front. However they are kicking back to the flat roof structure and are going to roof over the kickers to create a slope. It seems logical to me that there would be a greater accumulation of snow at the transition from flat to sloped but I can't find anything that defines what that might be.

 

[SteelPE]

Not knowing what code you are trying to use, but if it's the ASCE 7, they leave much up to interpretation on how to calculate snow drift at odd roof conditions. Of course in this instance it is bad..... but, if pushed for clarification, I can envision one day the ASCE 7 snow drift calculations being similar to the wind calculations.

Depending on how steep/tall the slope was, I may just calculate the drift as if the sloping knee wall wasn't there (using the full uniform snow and full drift height). Depending on how the knee wall is framed, you may have problems with your deck at the location where the knee wall meets the sloping roof.

 

[SteveGregory]

It makes logical sense to overlay the snow drift profile with the profile for the kickers bracing the parapet. You may discover that you have the equivalent snow load of a flat roof without a drift or maybe not.

 

[ephil]

Yes, I am looking at ASCE 7. The slope of the structure is roughly 25 degrees (4' tall and 8' long with 2' of vertical parapet sticking up above that). I somewhat agree that if the shape of the kickers matches that of the parapet that it may be the equivalent of a flat roof load. I think that I'm more concerned with sliding of the snow off of the sloped kickers accumulating at the transition between the flat roof and the kickers. There is a provision in ASCE 7 for sliding snow from an upper roof to a lower roof but I'm not sure how to apply it to this condition or if it would be applicable.

 

[SteelPE]

I sloped roof charts to see if you would actually get any sliding snow.

Also, I thought there was a provision that stated that you could eliminate sliding snow if there was an obstruction blocking the snow from sliding.... like more snow. This may be in the commentary of the code (which I don't currently have).

In this case you might not have the sliding snow condition... in fact, you may not even have drift (depending on how much ground snow you have at your location).

However, you have to remember that if you parapet is made out of lt. ga. members, that you may have a line load at the end of your sloped kneed wall. The members will carry the load back to their supports which will be the back side of the parapet and the end of the sloping roof.

This is a little tricky, but if you use common sense and some engineering judgment then you should be fine.

 

[SteelPE]

I wish they had an option to edit your posts.