Snow Load and Roof Design in North Idaho

Why this is the first structural question we ask

Snow load drives more of a North Idaho house plan than owners expect. It determines rafter or truss depth and spacing, beam sizing, post and footing loads, how far you can cantilever a deck, whether large open spans are affordable, and how roof-to-wall junctions must be detailed. Getting it right at design stage costs drawing time; getting it wrong costs structure.

Ground snow load versus roof snow load

Two different numbers, often confused:

• Ground snow load is the value published for your area, expressed in pounds per square foot.
• Roof snow load is what the structural design actually uses, derived from the ground load and adjusted for roof slope, exposure, thermal conditions and importance factor.

A steep, cold, exposed roof sheds differently than a low-slope, warm, sheltered one. This is why two houses in the same neighbourhood can carry different design roof loads.

Drift and sliding: the details that cause damage

Most snow-related failures we see are not whole-roof collapses. They are localised:

• Drift loads where a lower roof meets a taller wall, or against parapets and dormers. Snow accumulates far deeper here than on the open field of the roof, and the framing beneath must be designed for it.
• Sliding snow off a metal roof onto a deck, an entry, a walkway, a heat pump or a parked vehicle. This is a design decision, not an accident — decide early where snow will land.
• Ice damming at eaves, driven by heat loss through the roof assembly. The cure is air sealing and insulation, not just gutter heat cable.
• Valleys and entries, where accumulation concentrates and water gets driven back under roofing.

Design decisions worth making early

1. Roof form. Simple forms with fewer valleys and level changes handle snow better and cost less to frame.
2. Where snow lands. Position entries, decks, mechanical equipment and parking out of slide paths, or plan snow retention.
3. Roof assembly. A well-air-sealed, well-insulated, properly vented assembly does more to prevent ice dams than any add-on.
4. Deck framing. Decks carry snow too, and are frequently under-designed.
5. Access. Plan how the roof will be cleared if it ever needs to be.

What to confirm before you finalise plans

Ask your jurisdiction — Kootenai County, Bonner County, Stevens County, or the relevant city — for the design ground snow load applicable to your parcel and elevation. Give that figure to your designer and engineer at the start. For background on how snow loads are derived, the governing standard is ASCE 7, Minimum Design Loads and Associated Criteria for Buildings and Other Structures, published by the American Society of Civil Engineers and referenced by the building codes adopted in both Idaho and Washington.