Roof Snow Load Calculator

Understand Your Roof Snow Load

This roof snow load calculator estimates the uniform snow load on a roof in pounds per square foot (psf) based on ground snow load, exposure, thermal conditions, and roof slope. It is intended as an educational planning aid for homeowners, contractors, and designers who want a rough idea of how winter snow might translate into loading on a roof surface.

Important: This tool provides an approximation only. It does not replace a licensed structural engineer, detailed ASCE 7 calculations, or local building code requirements.

Basic Snow Load Concepts

Ground snow load (Pg)

The ground snow load, usually written as Pg, is the weight of snow that can accumulate on flat, unobstructed ground in a specific region. It is expressed in pounds per square foot (psf). Building codes and design standards publish maps or tables of Pg values based on long-term weather records.

Typical ranges include:

• Mild climates: 0–20 psf
• Cold, snowy regions: 20–60 psf
• Mountainous or very snowy areas: 60 psf and higher

Roof snow load (Pf)

The roof snow load, written as Pf, is the design load applied to the roof surface. It differs from Pg because roof conditions—slope, exposure to wind, and how warm the building is—change how much snow actually remains on the roof.

How This Calculator Estimates Roof Snow Load

This tool uses a simplified approach inspired by the ASCE 7 snow load standard. It combines ground snow load with exposure and thermal factors and then applies a reduction for roof slope:

Text formula: Pf = 0.7 × Ce × Ct × Pg × φ

The same relationship in MathML is:

where each symbol means:

Exposure and Thermal Factors

Exposure factor (Ce)

Roofs in open, windy locations tend to lose snow to drifting and blowing, while roofs sheltered by nearby buildings or trees may retain more snow. The exposure factor Ce adjusts for this:

• Ce < 1.0 – Very exposed roof where wind can easily blow snow off.
• Ce ≈ 1.0 – Typical suburban exposure.
• Ce > 1.0 – Sheltered or partially enclosed roof that tends to hold more snow.

Thermal factor (Ct)

The thermal factor Ct addresses how building heat affects snow buildup. Warm roofs can melt snow from below, sometimes reducing retained load; very cold, well-insulated roofs may keep snow longer.

• Ct ≈ 1.0 – Typical heated residential or commercial building.
• Ct < 1.0 – Situations where less snow is expected to remain, depending on code guidance.

Actual Ce and Ct values should be taken from the governing building code or from a qualified engineer. The values you enter here simply scale the ground snow load up or down to reflect your judgment about exposure and thermal conditions.

Effect of Roof Slope

Roof slope has a major impact on how much snow remains on the surface. Flat and low-slope roofs tend to retain snow, while steeper roofs allow snow to slide off more easily. The calculator represents this with a slope factor φ (phi) between 0 and 1.

• Flat to low slope (0–5°): φ is close to 1.0, so the roof load is nearly the same as a flat-roof design case.
• Moderate slope (about 15–30°): φ is somewhat below 1.0, reducing the design snow load.
• Steep slope (>30°): φ may be much smaller, representing significant shedding of snow.

In reality, snow sliding depends on roofing material (metal vs. asphalt shingles), surface roughness, ice dams, and sun exposure. The simple slope factor in this tool captures only the general trend: steeper roofs usually carry less snow.

Interpreting the Calculator Result

The result displayed by the calculator is an estimated roof snow load Pf in psf. This is a uniform load, meaning it is assumed to act evenly over the roof surface.

Use the output as follows:

• As a rough check on the magnitude of snow loads for planning, budgeting, or educational purposes.
• To compare scenarios, such as different slopes, exposure conditions, or hypothetical ground snow loads.
• As a talking point when consulting a structural engineer or local building official.

Do not use the calculated Pf value by itself to declare a roof safe or unsafe, to size structural members, or to override any requirements of your building code, insurance company, or design professional.

Worked Example

Imagine a house in a moderately snowy region with the following characteristics:

• Ground snow load, Pg = 40 psf
• Exposure factor, Ce = 1.1 (somewhat sheltered by nearby trees and houses)
• Thermal factor, Ct = 1.0 (typical heated home)
• Roof slope = 30°

Step 1: Start with the basic flat roof relationship:
Pf(flat) = 0.7 × Ce × Ct × Pg

Plug in the values (ignoring slope for the moment):
Pf(flat) = 0.7 × 1.1 × 1.0 × 40 = 0.7 × 44 = 30.8 psf

Step 2: Apply a slope factor φ for a 30° roof. In this simplified example, suppose φ for 30° is about 0.8. Then:

Pf = Pf(flat) × φ = 30.8 psf × 0.8 ≈ 24.6 psf

The calculator would therefore report an approximate roof snow load of about 25 psf. A designer using full ASCE 7 procedures might obtain a somewhat different value depending on the exact method and local code adjustments, but this example shows how the inputs interact.

Comparison: Flat vs. Sloped Roofs (Illustrative)

The table below illustrates how the estimated roof snow load might change with slope for one specific set of assumptions (Pg = 40 psf, Ce = 1.0, Ct = 1.0). The φ values are representative only and not code values.

This example shows that for the same climate and exposure, a steeply pitched roof may carry substantially less snow than a flat roof. However, real design must also consider local drifting, unbalanced loads, and details such as parapets and valleys.

Limitations and Assumptions

This calculator is highly simplified and relies on user-supplied assumptions. Key limitations include:

• Not a code calculation: The method is inspired by ASCE 7 concepts but does not reproduce the full procedure. Actual code calculations can be more complex and may produce different results.
• No drifting or sliding analysis: The tool assumes a uniform load. It does not account for snow drifting against higher roofs, parapets, mechanical equipment, or other projections, and it does not model snow sliding from upper roofs onto lower roofs.
• No unbalanced or partial loading: Real roofs can experience uneven loading due to wind, sun exposure, or drifting into valleys and corners. This calculator treats the load as evenly distributed.
• Simplified slope factor: The slope adjustment used here is generic and does not consider specific roofing materials, ice formation, or friction effects.
• No structural capacity check: The output is an estimate of applied load, not an evaluation of what your roof structure can safely support. Existing buildings may or may not be adequate for the estimated load.
• User-entered inputs: Results are only as reliable as the inputs. Ground snow load and factors should come from local codes or qualified professionals whenever possible.

Because of these limitations, always treat the calculator as a screening and educational tool, not as a design or approval method.

Safety, Codes, and When to Call a Professional

If you are worried about the safety of a roof during or after a heavy snow event, contact a licensed structural engineer or local building official. Warning signs such as new cracks, unusual noises, sagging, or doors that suddenly stick can indicate distress and require immediate professional attention.

Before making structural changes or deciding whether to remove snow from a roof, consider:

• Your local building code requirements for snow load design values.
• The age and condition of the structure.
• Whether snow removal could damage the roof or create unsafe conditions for people on the ground.

Always follow official guidance and manufacturer recommendations, and use this calculator only as a supplemental reference.