Snow load is one of the most important structural considerations in Canadian roofing design. With winters that routinely bring heavy snowfall, rapid accumulation, freeze–thaw cycles, and ice formation, roofs in Ontario and other cold regions must withstand significant pressure over long periods of time.

This guide explains how snow load is measured, how it affects roof structures, and why certain materials perform better under heavy winter stress.

1. What Is Snow Load?

Snow load refers to the weight of accumulated snow and ice that a roof must safely support. The weight varies depending on:

Temperature
Snow density
Compaction
Moisture content
Ice formation
Roof slope

While fresh powder is relatively light, wet snow and ice accumulation can become extremely heavy.

2. Why Snow Load Matters in Roofing

Roofs must be engineered to support snow without:

Sagging
Cracking
Structural deformation
Water infiltration
Ice dam formation
Failure at trusses or rafters

Snow load is one of the primary structural requirements in Canadian building codes.

3. Factors That Influence Snow Load on a Roof

A) Snow Density

Powder snow may weigh only 3–5 lbs per cubic foot.
Wet snow can exceed 20–25 lbs per cubic foot.

B) Ice Accumulation

Ice weighs significantly more than snow and can form due to:

Freeze–thaw cycles
Warm attic surfaces
Direct sunlight melting snow in patches

C) Roof Pitch

Steeper slopes shed snow more easily.
Low-slope roofs accumulate more snow and ice.

D) Roof Orientation

North-facing surfaces retain snow longer.

E) Wind Drift

Wind can create uneven snow patterns, increasing load in certain areas.

4. Typical Canadian Snow Load Ranges

Snow load varies widely across the country. Ontario typically sees:

Winter minimum: 20–25 lbs/ft²
Storm accumulation peaks: 40–50 lbs/ft²
Extreme events: 60+ lbs/ft²

Roofing materials must be evaluated for their ability to withstand these conditions.

5. How Snow Load Affects Different Roofing Materials

Asphalt Shingles

Flexible
Prone to cracking under cold temperatures
Vulnerable to ice dams
Can deform under long-term snow pressure
Moisture absorption can weaken underlying decking

G90 Steel Roofing

High structural rigidity
Does not absorb moisture
Maintains shape under load
Resists freeze–thaw deterioration
Effective at shedding snow due to smooth surface

Metal Shingles vs. Standing Seam

Metal shingles: small panels distribute stress evenly
Standing seam: requires proper fastening to manage movement

6. Ice Dams and Their Structural Impact

Ice dams form when:

Heat escapes from the attic
Snow melts along the roof surface
Water refreezes at the eaves

The resulting ice barrier forces water backward under roofing materials.

Primary risks:

Rotting roof decking
Moisture penetration
Damage to insulation
Mold formation
Premature failure of roofing systems

Proper ventilation and insulation lower ice dam risk.

7. Snow Load and Attic Ventilation

Ventilation affects snow behavior because:

Cold attic = reduces melt rate
Balanced airflow = minimizes ice dam formation
Dry attic = protects lumber and decking

Balanced systems typically include:

Intake ventilation (soffit)
Exhaust ventilation (ridge, roof vents)

Ventilation standards ensure that heat does not concentrate beneath the roof.

8. How Roof Geometry Influences Snow Load

Simple gable roofs:

Predictable load distribution.

Hip roofs:

More stable in high-wind regions, but retain more snow.

Complex roof structures:

Valleys, dormers, and shapes can trap snow in concentrated areas.

Flat or low-slope roofs:

Require higher engineered load capacity.

9. How Engineers Calculate Snow Load

Engineers reference:

CSA (Canadian Standards Association)
Building Code specifications
Exposure factors
Roof pitch
Snow drift probabilities
Load duration

Calculations ensure the roof structure can handle both typical and extreme events.

10. Summary: Why Snow Load Knowledge Matters

Understanding snow load helps homeowners recognize:

Structural stress points
Winter risk factors
Appropriate roofing materials
The importance of ventilation
How freeze–thaw cycles damage roofs

In regions with harsh winters, snow load is one of the strongest reasons homeowners choose durable materials such as steel.

Learn More at ROOFNOW™

For homeowner resources, structural insights, and roofing service information, visit the main ROOFNOW™ site:

https://www.roofnow.ca