Designing Roofs for High Snow Loads

In areas with ground snow loads of 40 psf and above, the uniform balanced load is only the starting point. A complete snow load design must also address drifts, unbalanced load, sliding, and the minimum load, any one of which can govern the framing of a specific element.

Start with the ground snow load

Pull Pg from the ASCE 7 Hazard Tool for your site's lat/lon. Confirm with the local building department whether they have adopted a different value. Use the local adopted value wherever one exists; the Hazard Tool is not the final word if your jurisdiction has passed an amendment.

Compute all applicable load cases

ASCE 7 Chapter 7 requires separate checks for: (1) balanced flat-roof load Pf; (2) sloped/pitched load Ps after Cs reduction; (3) drift load at roof steps, parapets and walls (§7.7); (4) sliding load on lower roofs from adjacent slippery upper roofs (§7.9); (5) unbalanced load on gable roofs (§7.6); and (6) the minimum load (§7.3.4). Identify which cases apply to each structural element and design to the worst.

Roof geometry choices in heavy-snow regions

A steeper slope reduces the balanced load via Cs but increases the potential for sliding and unbalanced load. Avoid complex multi-level roofs with many step transitions; each step is a drift source. Simple gable or hip roofs with consistent slope minimise drift exposure. Open terrain exposure reduces the balanced load (Ce as low as 0.7) but increases the potential for wind-blown drifting in unprotected areas.

Connection design matters as much as the framing

Roof collapses in high-snow events often fail at connections: ridge-to-rafter, rafter-to-wall top plate, and wall top-plate-to-stud. Use engineered connector plates at every connection for critical load paths. Verify that stud walls can transfer the vertical and lateral loads from the roof snow load to the foundation.

Maintenance is part of the design strategy

In regions with Pg > 50 psf, include a maintenance plan with the building design: specify roof access points for safe snow removal, confirm rake coverage from the ground, and plan for an annual post-season inspection of framing and connections. Well-designed buildings still need monitoring in extreme winters.