How to Calculate Snow Load on Your Roof
Snow accumulation on roofs can create enormous structural loads. Understanding how to calculate snow load helps you determine if your roof is at risk, when to remove snow, and what design standards apply to your area. This guide covers the basic formulas and practical guidelines.
Snow Weight Basics
Snow weight varies dramatically based on type:
| Snow Type | Weight per Cubic Foot | Weight per Inch per Sq Ft |
|---|---|---|
| Fresh light/fluffy snow | 3 – 5 lbs | 0.25 – 0.42 lbs |
| Packed/settled snow | 10 – 20 lbs | 0.83 – 1.67 lbs |
| Wet heavy snow | 20 – 40 lbs | 1.67 – 3.33 lbs |
| Ice | 57 lbs | 4.75 lbs |
The Basic Formula
Snow Load (psf) = Snow Depth (inches) x Snow Density (lbs per inch per sq ft)
For a quick estimate with average packed snow: Snow Load = Depth (inches) x 1.25 lbs/sq ft
Example
24 inches of average packed snow: 24 x 1.25 = 30 lbs per square foot (psf)
For a 1,500 sq ft roof: 30 x 1,500 = 45,000 lbs (22.5 tons) of snow on the roof.
Ground Snow Load vs Roof Snow Load
Building codes use ground snow load (pg) as the starting point. Roof snow load is typically less than ground snow load because wind removes some snow and heat from the building melts some from below. The conversion formula from ASCE 7:
Where Ce = exposure factor, Ct = thermal factor, Cs = slope factor, Is = importance factor, pg = ground snow load. For typical residential buildings, the roof snow load is approximately 50–70% of the ground snow load.
Typical Roof Design Loads
| Region | Ground Snow Load (psf) | Typical Roof Design Load |
|---|---|---|
| Southern US | 0 – 10 psf | 10 – 20 psf |
| Mid-Atlantic | 15 – 30 psf | 20 – 30 psf |
| New England | 30 – 70 psf | 25 – 50 psf |
| Mountain West | 40 – 200+ psf | 30 – 150 psf |
| Pacific Northwest | 15 – 50 psf | 15 – 40 psf |
Warning Signs Your Roof is Overloaded
- Doors or windows that suddenly stick or won't close
- Visible sagging in the roofline or ceiling
- Cracking or popping sounds from the attic structure
- New cracks in interior walls or drywall
- Water stains appearing on ceilings (from ice dam backup)
If you notice any of these signs, evacuate the area below the roof and call a structural engineer or your local building department immediately.
Roof Pitch and Snow Load
Steeper roofs shed snow more effectively. The slope reduction factor (Cs) in the building code reduces design snow load for steeper pitches. However, this creates a different danger: snow slides. Steeper metal and slate roofs require snow guards to prevent dangerous avalanches.
When to Remove Snow from Your Roof
Consider snow removal when:
- Accumulation exceeds 2 feet of heavy/wet snow or 4 feet of light snow
- Ice dams are forming at the eaves
- You notice any structural warning signs
- Your local ground snow load exceeds your roof's design capacity
Use a roof rake from the ground to remove snow from eaves. Never get on a snow-covered roof. Hire a professional for heavy removal.
Frequently Asked Questions
How much snow can my roof handle?
Most residential roofs in snow country are designed for 20–50 psf of snow load. Check your local building code for the exact design requirement. Your home's building plans (if available) will specify the design snow load. When in doubt, consult a structural engineer. See our building code requirements for more information.
Does roof pitch affect how much snow accumulates?
Yes. Pitches above 6/12 shed snow more effectively, especially with smooth materials like metal. Flat and low-slope roofs (under 3/12) accumulate the most snow because gravity cannot overcome friction. Check material weights on our material weights reference.
Are ice dams a snow load concern?
Ice dams add significant concentrated load at the eaves. A 2-inch-thick ice dam along 100 feet of eave adds approximately 4,750 pounds of load concentrated at the weakest part of the roof structure. Proper ventilation and insulation prevent ice dams — see our ventilation guide.