What Is Snow Load?
Snow load is the downward force exerted on a roof by accumulated snow and ice, measured as weight per unit area and governed by structural engineering principles.
Definition
Snow load refers to the weight of snow and ice that accumulates on a roof surface and transfers force to the building structure below.
It is typically expressed in pounds per square foot (psf) or kilonewtons per square meter (kN/m²) and varies by climate region, roof geometry, and exposure.
Why Snow Load Matters in Roofing
Roofs must safely resist snow load to prevent structural deflection, damage, or collapse.
In cold climates, snow load is often the dominant design force acting on residential roofing systems for extended periods.
Factors That Influence Snow Load
- Climate zone: Average snowfall and snow density vary regionally
- Roof slope: Steeper roofs shed snow more readily
- Roof geometry: Valleys and obstructions cause drift accumulation
- Thermal behavior: Heat loss can cause refreezing and ice buildup
- Wind exposure: Wind redistributes snow unevenly
Uniform Load vs Drift Load
Snow load is not always evenly distributed.
- Uniform snow load covers the roof evenly
- Drift snow load accumulates in localized areas such as valleys, lower roof sections, or near walls
Drift loading can create concentrated stress even when overall snowfall appears moderate.
Snow Load and Roofing System Design
Roofing systems influence how snow behaves once it lands on the roof.
Surface material, texture, and panel continuity affect whether snow sheds gradually or accumulates unevenly.
Long-Term Performance Considerations
Repeated snow loading and unloading contributes to material fatigue, fastener loosening, and structural movement.
Understanding snow load is essential when evaluating long-life or permanent roofing systems in snow-prone regions.