Snow Drifting Around Mechanical Units and Signs

[WIEngr]

ASCE 7 states that drifting does not need to be considered if the projection is less than 15 feet wide. How close can these 15ft projections be to each other (in any direction)? 15 ft, 2 ft, 1/2 inch? If the projection is 13'-4" wide would 5ft be OK? I personally think 13'-4" should be the distance in this case but have no research to back it up.

Also if you lift a projection off of the roof can drift loading be negated? how high? height of snow, height of drift, or even higher? For instance how much drifting results from a sign that is 30' above the ground?

My client has referenced a Canadian engineer and said that their product does not increase the snow loading on a roof but I would think that anything slowing down the wind creates a drift.

 

[TXStructural]

The commentary says that aerodynamic shade can result from buildings and adjacent higher roofs as far as 20 feet away. It also reminds us that most roof failures under snow result from local drifts, not uniform snow. Check the commentary for some items about elevated surfaces, like solar panels.

Depending upon the shape of the projection, you can get drifting beneath or beside something. The two things that will get a leeward drift:
1) a projection which interferes with wind sweeping of the surface of the roof or the snow cover.
2) a projection which creates a low pressure zone, resulting in snow deposition.
There are also windward drifts against solid objects/surfaces, where the snow doesn't get swept by the wind.

So if you have snow depositing on the roof or snow cover, and it doesn't get swept away by wind, you get a drift. Projections high above the roof or snow cover may or may not get drift, depending on lots of factors. I would think that your 30 feet is probably sufficient unless the shape is like a wing or angled plate of sufficient size.