Standards · The “Why” Behind NBCC’s Snow Load Factors

If you’re a structural engineer in Canada, you’ve stared at this equation more times than you can count:

$$S = I_s[S_s(C_b C_w C_s C_a) + S_r]$$

It’s the backbone of our snow load calculations, a formula we trust to keep our buildings standing through the harshest Canadian winters. But have you ever stopped to think about the stories behind those factors? Why is the basic roof snow load factor, \(C_b\), usually 0.8? Why does it get more complicated for larger roofs? And why do we almost always end up using a wind exposure factor, \(C_w\), of 1.0, even when the code offers a reduction?

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