Snow load is one of those “boring” numbers that becomes very exciting the first time a roof sags, leaks,
or gets engineered twice because someone guessed. This calculator gives you a fast, clear estimate of
specified roof snow load using Ontario-style factors (location, roof width, slope, exposure, importance),
plus the breakdown so you can see exactly what’s driving the result.
Compare locations (Barrie ≠ Windsor — not even close).
Understand slope impact (steeper roofs shed more — sometimes).
See the drivers (width factor, exposure, importance category).
Have a smarter conversation with your designer/engineer before permit time.
Use this as a planning tool. Final structural design still belongs to a licensed professional engineer.
Before you hit “Calculate”
Have these ready
Closest Ontario location (snow varies a lot across the province)
Roof width (meters) — changes the basic factor
Roof pitch and surface type (metal behaves differently than shingles)
Exposure and building importance category
If your roof has valleys, multiple levels, drift zones, big dormers, parapets, or anything “architecturally interesting,”
you’re officially in engineer territory.
Snow Loads Calculator
Embedded exactly as provided
CalculatorWrapped for page layout — your math and widget code remain unchanged.
🧮 Ontario Snow Load Calculator
Calculate specified snow loads for roof design according to Ontario Building Code 2024. This calculator follows OBC Section 4.1.6.2 and Section 9.4.2.2 requirements for structural design.
Building Information
Select your city or nearest location. Ground snow load (Ss) varies by region across Ontario.
The full width of the roof. Roofs ≤4.3m wide have a basic factor (Cb) of 0.45; wider roofs use 0.55.
Roof pitch expressed as rise over run. 8/12 means 8 inches of rise for every 12 inches of horizontal run. Steeper roofs shed snow more easily.
Slippery roofs (metal, glass) allow snow to slide off completely, reducing the slope factor (Cs).
Most residential buildings use “Normal Importance” with a factor (Is) of 1.0. Critical facilities require higher factors.
Exposed locations in open terrain have wind that blows snow off the roof, reducing the exposure factor (Cw) to 0.75.
✅ Calculation Results
Specified Snow Load (S)
0.00 kPa
(0.0 psf — pounds per square foot)
Calculation Breakdown
📋 Important Notes:
Minimum specified snow load is 1.0 kPa per OBC requirements
This calculation is for uniform loads on simple roof geometries
When designing structural members per Part 4, multiply S by a factor of 1.5
Additional loads may apply for drifting, sliding snow, or complex roof shapes
⚠️ Professional Engineering Disclaimer
This calculator provides simplified snow load estimates for educational purposes only. It does not account for all factors including drift loads, unbalanced loads, rain-on-snow, partial loading, site-specific conditions, or complex roof configurations (multi-level roofs, valleys, dormers).
For structural design and building permits, you MUST consult a licensed professional engineer who can perform complete calculations according to OBC Part 4. The creator assumes no liability for the use of these calculations.
Understanding Ontario Snow Loads
What is Snow Load?
Snow load is the downward force on a roof due to the weight of accumulated snow and ice. In Ontario, roofs must be designed to safely support snow loads that could occur once every 50 years (1-in-50 year event).
The OBC Formula Explained
S = Is[Ss(Cb × Cw × Cs × Ca) + Sr]
S = Specified snow load (kPa) — the final design value
Is = Importance factor (0.8 to 1.25 based on building type)
Ss = Ground snow load (varies by location: 1.3 to 3.4+ kPa in Ontario)
Cb = Basic roof factor (0.45 for narrow roofs ≤4.3m, 0.55 for wider roofs)
Cw = Wind exposure factor (typically 1.0, or 0.75 if very exposed)
Cs = Slope factor (reduces with steeper slopes; snow sheds more easily)
Ca = Shape factor (1.0 for simple roofs; varies for complex shapes)
Sr = Associated rain load (typically 0.4 kPa in Ontario)
Key Factors Affecting Snow Load
Location: Northern Ontario receives much heavier snow than southern regions (Thunder Bay: 3.2 kPa vs Windsor: 1.3 kPa)
Areas with potential snow drifting from adjacent roofs or buildings
Roofs with features like large skylights, chimneys, or equipment
Buildings in areas with variable elevation or microclimates
Code References
Ontario Building Code 2024, Division B, Part 4, Section 4.1.6.2 (Specified Snow Load)
Ontario Building Code 2024, Division B, Part 9, Section 9.4.2.2 (Specified Snow Loads for Houses)
MMAH Supplementary Standard SB-1 “Climatic and Seismic Data”
National Building Code of Canada 2020 (NBC 2020)
Why snow load matters (even if you “never had an issue before”)
Old roofs don’t complain… until they do
In Ontario, snow load isn’t just “how much snow you get.” It’s the design load your roof structure must safely carry,
factoring in location-based snow, rain-on-snow allowance, roof width, wind exposure, slope, and building importance.
That’s why two houses that look identical can have different required loads, especially if one is exposed, wider, or in a heavier snow region.
The practical reason you want this number early is simple: roof structure decisions cascade. Truss design, rafter sizing,
beam sizing, connection details, and even how you handle complex roof geometry can change once the snow load is nailed
