Snow drift on lower roofs.
Snow drift on lower roofs.
(OP)
HI everyone, I was wondering if i could get a little guidance as I am looking at the designing a canopy structure next to an existing building that has a 7'-0" parapet, and the height difference between the top of parapet and roof of new canopy is about 10'-6". I'm particularly looking at the potential snow drift, but I wasn't able to see anything along those lines in the ASCE 7. I did however, come across a diagram from the NBCCa 2015, were it has an exact depiction of what i am looking at. Any advice and guidance is well appreciated. I have attached a screenshot of the diagram in the NBCC 2015.

RE: Snow drift on lower roofs.
RE: Snow drift on lower roofs.
RE: Snow drift on lower roofs.
Well, simply the overall provisions for drifting on lower roofs.
Also, is there a Parapet height that will null the snow drift leeward on the canopy structure i am designing?
We've always interpreted it this way:
1. The lower roof drifting is due to turbulent snow above the high roof, thrown into the air from the high roof or from the sky.
2. This turbulent snow eventually drops to the lower roof from above, not directly laterally where a parapet might affect it.
3. Thus, the parapet above does not affect the drifting below - other than create a higher wall on which snow can drift.
4. So in some sense, the higher parapet actually could intensify the height of drift, rather than lower it.
RE: Snow drift on lower roofs.
I know I have had arguments proposed where the parapet will block the drift from forming on the canopy..... but I have never really bought into this argument.
RE: Snow drift on lower roofs.
RE: Snow drift on lower roofs.
RE: Snow drift on lower roofs.
You could perhaps assume that the upper parapet snow drift represents a reduction in available high roof snow but that quantity is generally fairly small compared to the overall available snow.
RE: Snow drift on lower roofs.
RE: Snow drift on lower roofs.
1) Standard roof snow load = 36 PSF
2) Build-up accounting for Parapet = 134 PSF extending out 25 ft
3) Build-up ignoring Parapet = 189 PSF extending 40ft
Pretty significant reduction in loads taking this into account. The build-up on the high roof from the parapet would be 84 PSF extending 15ft.
RE: Snow drift on lower roofs.
The angle of repose of snow is 30 degrees, so the snow drift (on top of the balanced snow) on the canopy is limited to a 30 degree angle from the edge of the canopy back toward the building.
DaveAtkins