In Quebec, snow isn't just something to shovel: it's one of the largest loads a roof has to carry. A poorly assessed accumulation — or one redistributed by the wind — can overload a structure. Here's how the Code approaches it, without the jargon.

Snow as a calculated load

The National Building Code of Canada (NBC) doesn't treat snow as a single value. The specified snow load on a roof combines several factors: the ground snow load specific to the locality (a statistical value based on history, the "1-in-50-year" value), a factor for wind exposure, the roof's slope, its shape, and the associated rain load.

In other words, two identical roofs in two different cities — or even a flat roof and a sloped roof in the same place — don't carry the same design load.

Why the region changes everything

The ground snow load varies widely across the province. The values tabulated in the NBC are higher in the Quebec City region and northern areas than in Greater Montreal, for example. A building designed for Montreal isn't necessarily compliant in Quebec City or the Mauricie. That's one more reason to rely on the official values for the locality rather than a rule of thumb.

Where snow piles up

The danger isn't only the total amount: it's redistribution. Wind moves snow and creates drifts where you'd least expect them:

  • against a taller wall or parapet;
  • in valleys and changes of slope;
  • below an upper roof (snow that slides and piles onto a lower roof);
  • around mechanical equipment.

These zones of local overload are a classic cause of structural problems and must be accounted for from the design stage.

When to consult an engineer

For houses and small buildings, Part 9 of the Code simplifies the calculation. But as soon as a project falls outside that scope — a commercial building, a complex roof, an addition, adding heavy equipment to an existing roof, or doubt about an older structure — snow-load calculations should be reviewed or prepared by a structural engineer.

Our team designs and verifies roof structures for residential and commercial projects in the Quebec City and Lévis area, accounting for the site-specific loads for your location.

Have a project or a concern about your roof? Get in touch.

This article explains the principles of the National Building Code of Canada in plain terms. The values and calculations applicable to your project must be established by a professional according to the edition of the Quebec Construction Code in force and the climatic data specific to your locality.