Snow Load Calculator
Professional roof snow load analysis based on ASCE 7 standards
Input Parameters
Windswept
Open terrain
Normal
Typical exposure
Partial Shelter
Some protection
Sheltered
Dense shelter
Heated
Normal heat
Low Heat
Minimal heating
Unheated
No heat source
Greenhouse
High heat loss
Results Summary
Ground Snow
30 psf
Roof Snow
21 psf
Total Weight
25,200 lbs
Per Sq Ft
21 psf
Important Notice
High snow load detected. Professional engineering review recommended.
Snow Drift Analysis
Understanding Snow Load Calculations for Safe Roof Design
A snow load calculator is an essential engineering tool that determines the weight of snow a roof structure must safely support. This calculation is critical for ensuring building safety, preventing structural failures, and meeting building code requirements. Our calculator uses the ASCE 7 standard methodology to provide accurate snow load estimates for your specific location and building characteristics.
What is Snow Load?
Snow load represents the downward force exerted by the accumulation of snow and ice on a building’s roof. Unlike permanent dead loads, snow loads are variable and depend on multiple factors including geographic location, elevation, roof geometry, and local climate patterns. The weight can range from 10-15 pounds per square foot for light, fresh snow to over 40 pounds per square foot for wet, compacted snow.
Key Snow Load Formula
pf = 0.7 × Ce × Ct × Is × pg
ps = Cs × pf
Where: pf = flat roof snow load, ps = sloped roof snow load
Ce = exposure factor, Ct = thermal factor, Is = importance factor
Cs = slope factor, pg = ground snow load
Ground Snow Load vs. Roof Snow Load
Ground snow load (pg) is the weight of snow measured on the ground at your location, typically determined from historical weather data and ASCE 7 maps. Roof snow load is calculated from ground snow load but accounts for factors that affect how snow accumulates on roofs. Generally, flat roof snow load is 70% of ground snow load, with additional modifications for exposure, thermal conditions, and roof slope.
Critical Factors in Snow Load Calculations
Exposure Factor (Ce)
Accounts for wind effects on snow accumulation. Ranges from 0.7 for windswept areas to 1.2 for densely sheltered locations. Wind can significantly reduce snow accumulation on exposed roofs or increase it in protected areas.
Thermal Factor (Ct)
Considers heat loss through the roof affecting snow melting. Heated buildings typically use 1.0, while unheated structures use 1.2. Greenhouses with high heat loss may use 0.85.
Importance Factor (Is)
Based on building occupancy and consequences of failure. Ranges from 0.8 for low-hazard buildings to 1.2 for essential facilities like hospitals.
Slope Factor (Cs)
Accounts for snow sliding off sloped roofs. Flat roofs retain full load (Cs = 1.0), while steep roofs shed snow more readily, reducing the design load.
Roof Slope Impact on Snow Loads
| Roof Slope | Slope Factor | Snow Behavior | Design Consideration |
|---|---|---|---|
| 0° – 5° | 1.0 | Full retention | Maximum load, drainage critical |
| 5° – 20° | 1.0 – 0.9 | High retention | Near full load, some sliding |
| 20° – 40° | 0.9 – 0.7 | Moderate sliding | Reduced load, unbalanced conditions |
| 40° – 70° | 0.7 – 0.0 | Significant sliding | Minimal retention, sliding hazards |
Special Loading Conditions
Snow Drift Loads
Snow drifts form at roof elevation changes, parapets, and adjacent to taller structures. These drifts can create loads 2-3 times the uniform snow load. Leeward drifts are typically larger than windward drifts and must be carefully considered in structural design.
Unbalanced Snow Loads
Wind can create uneven snow distribution, particularly on gabled and curved roofs. Design must consider both uniform and unbalanced loading conditions, with unbalanced loads often governing the design of individual structural members.
Rain-on-Snow Surcharge
In regions with ground snow loads of 20 psf or less and relatively flat roofs, a 5 psf rain-on-snow surcharge may apply. This accounts for the additional weight of rain absorbed by the snow pack.
Regional Snow Load Considerations
High Snow Load Regions
- • Mountain areas: 100+ psf
- • Lake-effect zones: 60-90 psf
- • Northern states: 40-80 psf
- • High elevation: increases with altitude
Moderate Snow Load Regions
- • Mid-Atlantic: 20-40 psf
- • Central Plains: 15-30 psf
- • Pacific Northwest: 20-50 psf
- • Transition zones: highly variable
Building Code Requirements
The International Building Code (IBC) and most local codes reference ASCE 7 for snow load determination. Key requirements include:
- Minimum snow loads regardless of calculated values
- Site-specific case studies for certain locations
- Special provisions for essential facilities
- Requirements for snow retention devices on sloped roofs
- Consideration of sliding snow impacts on lower roofs
Snow Load and Roof Types
Flat Roofs
Flat roofs experience the full design snow load with no reduction for slope. Proper drainage is critical to prevent ponding, which can lead to progressive collapse. Consider snow drift at parapets and equipment.
Gabled Roofs
Gabled roofs benefit from slope reduction factors but must be designed for unbalanced loads. Wind can create significant accumulation on leeward slopes while clearing windward slopes.
Metal Roofs
Metal roofs typically have smooth surfaces that promote snow sliding. While this reduces load, it creates hazards below. Snow guards or retention systems may be required by code.
Snow Removal Considerations
When to Remove Snow
- • Snow depth exceeds design assumptions
- • Rain is forecast on existing snow
- • Visible sagging or structural distress
- • Doors or windows becoming difficult to operate
- • Unusual creaking or popping sounds
Frequently Asked Questions
How often should I check my roof during winter?
Monitor your roof after each significant snowfall, especially if accumulation exceeds 12 inches. Pay special attention after rain events on existing snow, as this dramatically increases load.
What’s the difference between fresh snow and packed snow weight?
Fresh snow typically weighs 5-10 psf per foot of depth, while packed or wet snow can weigh 20-30 psf per foot. Ice formation can exceed 50 psf per foot of depth.
Do solar panels affect snow load calculations?
Solar panels add dead load and can create snow retention areas. They may also cause differential melting patterns. The added weight must be considered in the overall structural capacity.
How do I determine my local ground snow load?
Check ASCE 7 snow load maps, consult your local building department, or hire a structural engineer. Many areas require site-specific case studies for accurate determination.
Professional Engineering Considerations
While snow load calculators provide valuable preliminary estimates, professional engineering review is essential for:
- Complex roof geometries with multiple levels
- Buildings in high snow load regions (>50 psf)
- Essential facilities and high-occupancy structures
- Retrofits or additions to existing buildings
- Areas prone to rain-on-snow events
- Structures with large roof spans or unusual configurations
Safety Reminder
This calculator provides estimates based on standard methodologies. Always consult with qualified structural engineers for final design decisions. Local building codes may have additional requirements beyond ASCE 7 standards.
Conclusion
Understanding and accurately calculating snow loads is crucial for safe building design and maintenance. Our snow load calculator simplifies this complex process by applying industry-standard factors and formulas. Regular monitoring, proper maintenance, and timely snow removal when necessary will help ensure your roof performs safely throughout winter conditions.
Remember that snow load calculation is just one aspect of comprehensive structural design. Factors like wind loads, seismic forces, and dead loads must all be considered together. When in doubt, consult with professional engineers who can provide site-specific analysis and ensure your structure meets all safety requirements.