NBCC - Snow Drift Loading at Low Roof

[efFeb]

Good Morning,
I am determining the snow drift load on a low roof, which is adjacent to a high roof.
I know that the leeward case is covered by snow load case I for lower level roofs (4.1.6.5), and the windward case is covered by Case II.
Out of interest, I wanted to see how the load would compare if I treated the high roof as a projection (cl. 4.1.6.7). The snow load in this case would be significantly higher.
I just want to confirm that there is no need to check an upper / lower roof case as though the upper roof is a projection?
Thanks very much for your input!

 

[CDLD]

No you do not need to check the upper roof as if it is a projection.
In fact, NBC 2015 clause 4.1.6.5.(2), allows you to calculate the snow load adjacent to projections using the lower roof clauses instead of the projection clauses.

In summary:
- When you have a low roof, use the low roof clauses.
- When you have a projection use the lesser snow load calculated as a projection or a low roof.

 

[larcix]

These are just some thoughts on snow drifts in general, as I'm not familiar with the Canadian codes, however:

Per ASCE, you check both the windward and leeward drifts. Both drifts are calculated exactly the same, based on the upwind snow fetch area (the length of roof leading up to the projection that provides the snow for the drift), except the windward (lower roof) calculation has a 3/4 factor slapped on, presumably as the wind knocks some of the drift away, whereas in the leeward case (wind from the upper roof) the projection creates a wind shadow that will actively accumulate snow.

So, drifts can come from both directions, and either way may control the design depending on the length of roof before hitting the projection. Just some food for thought.