Calculate Roof Snow Load
Enter your snow depth, roof dimensions, and snow type to calculate the total weight and load on your roof. Results update instantly as you type.
Multiple Roof Sections
If your roof has multiple sections with different snow depths or types, add each section here for a combined total.
Snow Types and Densities
Snow density varies widely depending on temperature, moisture, age, and compression. The table below lists typical values used by engineers and building officials:
| Snow Type | Density (lb/ft³) | Density (kg/m³) | Weight per Inch per ft² | Characteristics |
|---|---|---|---|---|
| Fresh / Fluffy | 5 -- 12 | 80 -- 192 | 0.4 -- 1.0 psf | Light, powdery, just fallen in cold weather |
| Settled | 12 -- 18 | 192 -- 288 | 1.0 -- 1.5 psf | A few days old, partially compacted by its own weight |
| Packed / Wind-blown | 18 -- 30 | 288 -- 481 | 1.5 -- 2.5 psf | Compressed by wind or foot traffic, granular texture |
| Wet / Heavy | 30 -- 40 | 481 -- 641 | 2.5 -- 3.3 psf | Rain-soaked or spring thaw snow, very heavy |
| Ice | ~57 | ~913 | ~4.75 psf | Solid ice layer from freezing rain or refreezing |
How density affects load
12 inches of fresh snow at 7 lb/ft³ creates a load of about 7 psf -- well within safe limits. But 12 inches of wet snow at 35 lb/ft³ creates a load of 35 psf -- in the danger zone. Always consider the type of snow, not just the depth, when assessing roof safety.
Ground Snow Load vs. Roof Snow Load
Building codes use the ground snow load (measured at weather stations) as the starting point for calculating roof loads. However, the actual snow load on a roof is typically less than the ground snow load due to several factors:
| Factor | Effect | Typical Reduction |
|---|---|---|
| Heat loss through roof | Melts snow from below, reducing accumulation | 10 -- 20% |
| Wind exposure | Blows snow off open roofs | 10 -- 30% |
| Roof slope | Snow slides off steeper roofs | 0 -- 50% (depends on pitch) |
| Roof surface | Metal roofs shed snow more readily | 5 -- 15% |
The standard formula
Building codes (ASCE 7) calculate flat roof snow load as:
pf = 0.7 × Ce × Ct × Is × pg
- pg = ground snow load
- Ce = exposure factor (0.8 for exposed, 1.0 for partially exposed, 1.2 for sheltered)
- Ct = thermal factor (1.0 for heated buildings, 1.1 for unheated, 1.3 for freezer buildings)
- Is = importance factor (1.0 for normal, 1.1 for essential facilities)
This calculator estimates roof snow load directly from measured depth and density. For structural engineering or building permits, consult your local building code and a licensed engineer.
How This Calculator Works
The snow load calculation follows these steps:
Step 1: Determine snow density
Select a snow type to use its average density, or enter a custom value. The calculator uses the midpoint of each range: Fresh = 8.5 lb/ft³, Settled = 15, Packed = 24, Wet = 35, Ice = 57.
Step 2: Calculate load per square foot
Load (psf) = Snow Depth (ft) × Snow Density (lb/ft³)
For example, 18 inches (1.5 ft) of packed snow at 24 lb/ft³ = 1.5 × 24 = 36 psf.
Step 3: Apply roof pitch adjustment
The slope factor (Cs) reduces the effective load on pitched roofs. A flat roof retains 100% of the load. A 6/12 pitch retains roughly 72%. A 12/12 pitch retains about 44%. The factor is calculated as:
Cs = 1 - (pitch/12) × 0.56
where pitch is the rise per 12 inches of run.
Step 4: Calculate total weight
Total Weight = Load (psf) × Cs × Roof Area (ft²)
Safety Guidelines: When to Remove Snow
These general guidelines can help you decide when roof snow removal is warranted. Always prioritize safety -- hire professionals for steep or high roofs.
| Load Level | Condition | Action |
|---|---|---|
| Below 20 psf | Safe for most residential roofs | No action needed. Monitor if snow continues falling. |
| 20 -- 30 psf | Approaching structural limits for some roofs | Monitor closely. Plan removal if more snow is forecast. Check for ice dams. |
| Above 30 psf | Exceeds design load for many residential roofs | Remove snow as soon as safely possible. Watch for warning signs. |
Warning signs of excessive snow load
- Interior doors sticking or hard to open/close
- Cracking or popping sounds from the ceiling or walls
- Visible sagging of the roof or ceiling
- New cracks appearing in drywall or plaster
- Roof leaks from ice dam buildup at the eaves
- Bowing of utility pipes or sprinkler heads
Snow removal tips
- Use a roof rake from the ground when possible -- avoid climbing onto a snow-loaded roof
- Remove snow evenly from both sides to avoid unbalanced loads
- Leave 1-2 inches of snow rather than scraping to bare shingles
- Clear snow away from the base of the building as well
- Never use salt or chemicals on a roof -- they can damage shingles
- For commercial or flat roofs, hire a professional snow removal service
Frequently Asked Questions
How much does snow weigh on a roof?
Snow weight varies greatly by type. Fresh fluffy snow weighs about 5-12 lb/ft³, while wet packed snow can weigh 30-40 lb/ft³. A single inch of wet snow over 1,000 sq ft of roof can weigh over 2,500 lbs. The depth alone does not tell the full story -- always consider the snow type and density.
What is a safe snow load for a residential roof?
Most residential roofs are designed to handle 20-30 psf (pounds per square foot) of snow load, though this varies by region and building code. Loads below 20 psf are generally safe. Between 20-30 psf warrants close monitoring. Above 30 psf is dangerous and may require immediate snow removal. Check your local building code for your area's design snow load.
How does roof pitch affect snow load?
Steeper roofs shed snow more easily, reducing the effective snow load. A flat roof bears the full load, while a moderately steep roof (6/12 pitch) may only retain about 72% of the ground load. Very steep roofs (12/12 or 45 degrees) can shed most snow naturally. However, shed snow can create dangerous ground-level accumulations near the building.
What is the difference between ground snow load and roof snow load?
Ground snow load is the weight of snow measured on flat ground at weather stations. Roof snow load is typically 60-80% of ground snow load due to heat loss through the roof, wind exposure, and roof slope. Building codes provide formulas to convert ground snow load to roof snow load using exposure, thermal, and importance factors.
When should I remove snow from my roof?
Consider removing snow when the load approaches or exceeds 20 psf, when you notice doors sticking or cracking sounds from the structure, when ice dams form at the eaves, or when the snow depth exceeds 2 feet of packed snow or 4 feet of fresh snow. Always hire a professional for steep or high roofs.
Can rain on existing snow increase the load?
Yes, significantly. Rain is absorbed by snow and can dramatically increase its weight without increasing its depth. A light rain on a deep snow pack can add several psf of load. This is one of the most dangerous scenarios because the roof load increases without visible change. After rain-on-snow events, reassess the load immediately.
Does this calculator store my data?
No. All calculations run entirely in your browser. No data is sent to any server, and nothing is stored.
Privacy & Limitations
Privacy: This calculator runs entirely in your browser. No data is transmitted or stored anywhere.
Limitations: This tool provides estimates based on typical snow density values and simplified roof pitch adjustment. Actual snow loads can vary based on wind drifting, uneven accumulation, ice dam formation, and local conditions. For structural engineering decisions or building permits, consult your local building code and a licensed structural engineer. This calculator is not a substitute for professional assessment.
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Snow Load Calculator FAQ
How much does snow weigh on a roof?
Snow weight varies greatly by type. Fresh fluffy snow weighs about 5-12 lb/ft3, while wet packed snow can weigh 30-40 lb/ft3. A single inch of wet snow over 1,000 sq ft of roof can weigh over 2,500 lbs.
What is a safe snow load for a residential roof?
Most residential roofs are designed to handle 20-30 psf (pounds per square foot) of snow load. Loads below 20 psf are generally safe. Between 20-30 psf warrants monitoring, and above 30 psf is dangerous and may require immediate snow removal.
How does roof pitch affect snow load?
Steeper roofs shed snow more easily. A flat roof bears the full ground snow load, while a roof with a 6/12 pitch may only retain about 72% of the ground load. The slope factor (Cs) reduces the effective load on pitched roofs.
What is the difference between ground snow load and roof snow load?
Ground snow load is the weight of snow on a flat surface at ground level. Roof snow load is typically 60-80% of ground snow load due to wind exposure, heat loss through the roof, and the slope of the roof surface.
When should I remove snow from my roof?
Consider removing snow when the load exceeds 20 psf, when you notice doors sticking or cracking sounds, when ice dams form at the eaves, or when the snow depth exceeds 2 feet of packed snow or 4 feet of fresh snow.
Does this calculator store my data?
No. All calculations run entirely in your browser. No data is sent to any server, and nothing is stored.