Snow Drift on Low Roof from a Much Higher Roof
Snow Drift on Low Roof from a Much Higher Roof
(OP)
Couldn't find anything on this subject, so here i am.
Picture this: High Flat Roof, 200ft long, no parapet. Lower Roof is 10ft lower, also flat, and only 10 ft long. Leeward caused snow drift would accumulate, as well as Windward.
My question: At what high roof - low roof height differential does leeward stop causing snow drift on a lower roof? 35ft? 100ft? Never?
Thanks!
Picture this: High Flat Roof, 200ft long, no parapet. Lower Roof is 10ft lower, also flat, and only 10 ft long. Leeward caused snow drift would accumulate, as well as Windward.
My question: At what high roof - low roof height differential does leeward stop causing snow drift on a lower roof? 35ft? 100ft? Never?
Thanks!






RE: Snow Drift on Low Roof from a Much Higher Roof
I would say never though, as I thought the leeward buildup was caused by the wind speeding up over the high roof and picking up snow, and then losing speed therefore dropping snow on the leeward side.
RE: Snow Drift on Low Roof from a Much Higher Roof
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.
RE: Snow Drift on Low Roof from a Much Higher Roof
RE: Snow Drift on Low Roof from a Much Higher Roof
The reduction is shown for lower roofs less than 25 sq. meters and at least 10 meters below the elevation of the upper roof. That paragraph provides a linear scale from normal drift at 10 m height differential to 0 drift at 20 m height differential.
To me the Commentaries have always been confusing because some parts are clearly code language while others are clearly commentary only and everything in between.
Both ASCE7 and the new 2015 Canadian code are silent on any adjustment due to large height differential.
RE: Snow Drift on Low Roof from a Much Higher Roof
ajh's Canadian commentary makes sense, as obviously a small roof with a large height differential would result in any added snow from the upper roof likely missing the lower roof. However, I still think there would be shielding of the lower roof.
Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries
www.americanconcrete.com
RE: Snow Drift on Low Roof from a Much Higher Roof
Dik
RE: Snow Drift on Low Roof from a Much Higher Roof
RE: Snow Drift on Low Roof from a Much Higher Roof
When going through the entire calculation, the difference in roof elevation only comes into play when determining the drift width.
RE: Snow Drift on Low Roof from a Much Higher Roof
There has to be a minimum height difference at which the step fills to the top no? I'm not familiar with ASCE so I don't know.
RE: Snow Drift on Low Roof from a Much Higher Roof
I was more thinking along the lines of "In real life", similar to what KootK was saying, where if the difference was 100ft from low roof to high roof, does the snow just "blow away"
RE: Snow Drift on Low Roof from a Much Higher Roof
hc: height of step above balanced snow load
hb: depth of balanced snow load
RE: Snow Drift on Low Roof from a Much Higher Roof