Snow Load Calculator · Roof Snow Load & Permissible Load

Snow Load Calculator

Estimate roof snow load, total snow weight & permissible load

Your roof
:12
deg
Roof pitch reduces the effective snow load because snow slides off steeper slopes. Steeper roofs shed more snow.
Snow load on roof
cm
kN/m²
kg
Snow load = snow depth × density × 9.807 ÷ 1000 · Total weight = snow load × roof area ÷ cos(pitch).
The location of your house
Values are the 1-in-50-year ground snow load Ss from NBC 2020 Table C-2. Confirm exact values with your local building code.
Show advanced options
Permissible load on roof
kN/m²
cm
kg
The permissible load is the code-specified roof snow load S from NBC 2020 §4.1.6: S = Is × [Ss × (Cb × Cw × Cs × Ca) + Sr].
Results
✅ Safe — Under Limit
Snow Load on Roof
—
—
Ground snow load Ss
—
Total snow weight
—
Permissible load
—
Load ratio
—
Roof area
—
Snow depth
—
Snow density
—
Pitch angle
—
📋 Summary
Enter roof dimensions, snow cover and location to see the snow load and permissible load.
Reference only. Always verify with the current NBC 2020, your provincial building code and a licensed structural engineer.
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Faiq Ur Rahman

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Faiq Ur Rahman is a web designer, digital product developer, and founder of Toolraxy, a growing platform of web-based calculators and utility tools. He specializes in building structured, user-friendly tools focused on health, finance, productivity, and everyday problem-solving.

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Snow is heavier than it looks. A modest 30 cm layer of settled snow weighs roughly 75 kg per square metre of roof surface, which on a typical 100 m² bungalow amounts to seven and a half tonnes of dead load more than the weight of four family cars sitting on the trusses. Wet snow roughly doubles that figure, and wind-packed snow doubles it again. Every building code in a snow-prone country therefore specifies a design roof snow load, and every building is engineered against it. This snow load calculator brings that check to the homeowner level. It uses the 1-in-50-year ground snow load values published in the National Building Code of Canada 2020, combined with the code’s exposure, slope and importance factors, to produce a permissible roof snow load and compare it against the load actually sitting on the roof right now. It is intended for homeowners deciding whether to clear a roof, for property managers monitoring several buildings through a winter, and for anyone who wants to understand why a roof that survived one winter might not survive the next.

 

How to Use the Snow Load Calculator

  1. Enter your Roof length and Roof width, choosing the matching unit for each.

  2. Set the Roof pitch as an X:12 ratio or an angle in degrees, the other field updates automatically.

  3. Measure the Snow cover thickness at a representative spot on the roof, in centimetres.

  4. Pick the Snow type that best matches current conditions, from fresh light snow through to wind-packed.

  5. Select your Province or territory and then the nearest Location from the dropdown.

  6. Read the snow load in the hero display, with the ground snow load and total weight beneath.

  7. Open Show advanced options to adjust exposure and importance factors if your building is unusually sheltered or has a higher consequence classification.

  8. Check the Load ratio tile, anything under 80% is comfortable, above 100% requires immediate action.

 

How the Snow Load Calculator Formula Works

The calculation combines two independent figures: the load actually sitting on the roof, derived from the snow’s depth and density, and the load the code permits, derived from the location’s ground snow load and a series of modification factors.

Formula: Snow Load (kPa) = Depth (m) × Density (kg/m³) × 9.807 ÷ 1000

Formula: Total Snow Weight (kg) = Depth (m) × Density (kg/m³) × Plan Area (m²)

Formula: Roof Area (sloped) = Plan Area ÷ cos(Pitch Angle)

Formula: Permissible Load S = Is × [Ss × (Cb × Cw × Cs × Ca) + Sr]

Formula: Load Ratio = Snow Load ÷ Permissible Load

Roof length and width must both be greater than zero, snow depth cannot be negative, and pitch must fall between 0° and 89°. Density is fixed at one of four preset values, 100, 250, 400 or 500 kg/m³ depending on the snow type selected. The NBC factors default to Cb = 0.8, Cw = 1.0, Ca = 1.0 and Sr = 0.4, with Cs dropping from 1.0 at 30° to 0 at 70°, and Is defaulting to 1.0 for a normal-occupancy building.

 

Worked Example

A homeowner in Barrie, Ontario, is checking a 14 m × 9 m gable roof at a 6:12 pitch after a heavy snowfall. The snow on the roof is wet and about 45 cm deep.

  • Pitch angle = atan(6 ÷ 12) = 26.57°

  • Plan area = 14 × 9 = 126 m²

  • Slope factor Cs = 1.0 (pitch below 30°)

  • Snow load = 0.45 × 400 × 9.807 ÷ 1000 = 1.77 kN/m²

  • Equivalent psf = 1.77 × 20.885 = 36.9 psf

  • Total snow weight = 0.45 × 400 × 126 = 22,680 kg (over 22 tonnes)

  • Barrie ground snow load Ss = 2.4 kN/m² (NBC 2020 Table C-2)

  • Permissible load = 1.0 × [2.4 × (0.8 × 1.0 × 1.0 × 1.0) + 0.4] = 2.32 kN/m²

  • Load ratio = 1.77 ÷ 2.32 = 76%

The takeaway: at 76 percent of the permissible load, this roof is approaching its limit but not yet at it. The calculator would flag it as “Approaching Limit” rather than “Safe,” which is the point at which a prudent owner starts arranging to clear the roof rather than waiting for another snowfall. The maximum snow cover before the load limit is reached works out at about 59 cm of wet snow, a further 14 cm of accumulation would push the structure into the exceedance band.

Frequently Asked Questions

How much snow load can a roof hold?

It depends on the building code the roof was designed to and the location. A typical Canadian residential roof is designed for between 1.5 and 3.5 kN/m² of roof snow load, which corresponds to roughly 60 cm to 1.4 m of settled snow. The permissible load displayed by this calculator is the code figure for your location, the actual capacity of an existing roof may be lower if it has deteriorated or was under-designed.

 

What is the density of snow?

Snow density ranges from about 50 kg/m³ for fresh powder to over 500 kg/m³ for wind-packed or rain-saturated snow. The four presets in the calculator cover the practical range: 100 for fresh, 250 for settled, 400 for wet and 500 for wind-packed.

 

How is snow load different from ground snow load?

Ground snow load Ss is the weight of snow on open ground, measured at weather stations over a 50-year return period. Roof snow load is the load on a roof after accounting for wind removal, slope shedding, drifting and rain-on-snow. The roof load is usually lower than the ground load, but on sheltered or drifting-prone roofs it can be higher.

 

At what snow depth should I clear my roof?

When the load ratio reaches about 80 percent of the permissible load. For a typical Canadian location that corresponds to roughly 45 to 60 cm of settled snow, or less if the snow is wet. The calculator displays the exact maximum depth for your roof and location in the advanced panel.

 

Does roof pitch reduce snow load?

Yes. The NBC applies a slope factor Cs that reduces the roof load on pitches above 30° and eliminates it entirely at 70°. The reasoning is that snow slides off steeper roofs under its own weight, so it does not accumulate to the depth it would on a shallow roof.

 

Why is wet snow so much heavier than dry snow?

Because water is roughly 1,000 kg/m³ while ice crystals are much lighter. A snowpack saturated with meltwater contains less air and more water, and its density can exceed 500 kg/m³. The same depth of wet snow therefore weighs two to five times as much as dry powder.

 

Can I use this calculator outside Canada?

The formulas are universal, but the ground snow load table is specific to Canadian locations from NBC 2020. For US locations, the ground snow load comes from ASCE 7 and the values differ. If you use the calculator outside Canada, override the ground snow load with a value from your local code rather than relying on the closest Canadian city.

 

What is rain-on-snow and why does it matter?

Rain-on-snow is a design condition in which rainfall lands on an existing snowpack and freezes or is absorbed without draining. It adds an extra load on top of the snow typically 0.4 kN/m² in the NBC formula, though it varies with location and roof type. It matters because rain-on-snow events are a common trigger for roof collapses that had previously survived deeper snowfalls.

 

How do I measure snow depth on my roof?

Push a ruler or tape vertically into the snow until it reaches the roof surface, then read the depth. Take several measurements across the roof and use the average, but note that drifting causes local variations of 2:1 or more. If you cannot access the roof safely, measure on the ground nearby and subtract about 20 percent to account for settling.

 

What is the importance factor Is?

The importance factor raises the required design load for buildings whose failure would have serious consequences. Homes and offices use 1.0, schools and assembly buildings use 1.15, and hospitals, emergency shelters and post-disaster facilities use 1.25. Barns and low-occupancy agricultural buildings use 0.8.

 

Does roof shape change the snow load?

Yes. The NBC formula includes a shape factor Ca for accumulations caused by adjacent surfaces, a taller wall or a change in roof level can cause drifting that doubles the local load. The basic factor Cb of 0.8 in this calculator assumes a simple roof under 50 m × 70 m with no unusual drift sources.

 

Can I clear the snow myself?

Only if you can do so safely from the ground, using a roof rake with an extended handle. Climbing onto a snow-covered roof is extremely dangerous and is a common cause of winter fatalities. For anything beyond the reach of a ground-level rake, hire a professional snow removal contractor with fall-arrest equipment and insurance.

Disclaimer

This snow load calculator provides a reference estimate based on the National Building Code of Canada 2020 ground snow loads and the code’s simplified roof load formula. It does not replace a structural engineering assessment, does not verify the capacity of any existing structure, and does not account for drift accumulations, complex roof geometries, or the condition of the roof framing. Snow density is a judgement call and can vary by a factor of five within a single storm. Always confirm the design snow load for your specific building with a licensed structural engineer and with your provincial or territorial building code before making decisions about roof safety or snow removal.

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