Roof Snow Load Calculator
Calculate the vertical snow pressure on your sloped or flat roof based on local ground snow levels, pitch, and structural exposure.
- Flat Roof Load (Pf)
- 14.0 psf
- Roof Angle
- 18.4°
- Slope Reduction Factor (Cs)
- 1.000
- Design Safety Limit
- 17 psf
Enter your values and press Calculate to see your result.
How it works
Follow the exact steps to get your result instantly and privately.
Step 1: Find Ground Snow Load
Identify the ground snow pressure (in psf) for your county or city.
Step 2: Select Roof Pitch
Choose your roof's vertical slope ratio (rise-over-run).
Step 3: Input Surface & Exposure
Specify whether your roof is slippery (like metal) and how exposed it is to wind.
Step 4: Calculate Load
Get the flat and sloped roof snow loads, plus design safety recommendations.
Step 1: Find Ground Snow Load
Identify the ground snow pressure (in psf) for your county or city.
How to Calculate Roof Snow Load
Our calculator uses standard ASCE 7 methodologies. First, we compute the flat roof snow load ($P_f$) using ground snow load ($P_g$), exposure factor ($C_e$), and thermal factor ($C_t$). Then, we apply a slope factor ($C_s$) based on pitch and roof materials to get the final sloped snow load ($P_s$).
Measuring Methods
- Ground Snow Load (Pg): The standard weight of snow expected on flat ground, measured in pounds per square foot (psf).
- Slope Factor (Cs): A multiplier that decreases as pitch increases, representing the snow's natural tendency to slide off steep roofs.
Flat Load (Pf) = 0.7 * Ce * Ct * Pg
Sloped Load (Ps) = Cs * PfFor a heated house (Ct = 1.0) in a suburban area (Ce = 1.0) with a 20 psf ground snow load: Pf = 0.7 * 1.0 * 1.0 * 20 = 14 psf. If the roof has a 6/12 pitch (26.57°) and standard shingles, Cs is 1.0, so the sloped roof load (Ps) is 14 psf. If the pitch increases to 12/12 (45°), Cs drops to 0.625, reducing the load to 8.85 psf.
Worked examples
Here are some common regional configurations showing how thermal conditions and pitch influence sloped roof snow load.
| Scenario | Calculation | Result |
|---|---|---|
| Suburban Home, 4/12 Pitch, 30 psf Ground Snow | Flat Load = 21 psf, Shingle Slope Factor = 1.00 | 21.0 psf |
| Open Field Farmhouse, Metal Roof, 12/12 Pitch, 40 psf Ground Snow | Flat Load = 25.2 psf, Slippery Slope Factor = 0.45 | 11.5 psf |
| Unheated Barn, Shingle Roof, 8/12 Pitch, 20 psf Ground Snow | Flat Load = 16.8 psf, Standard Slope Factor = 0.88 | 14.8 psf |
Conversion reference
This quick guide shows how snow depth converts to structural weight. Wet, packed snow is significantly heavier than fresh powder.
| Snow Type | Weight Per Inch of Depth | Depth to Reach 20 psf Load |
|---|---|---|
| Light, Fresh Powder | 0.5 lbs / sq ft | 40 inches |
| Average Packed Snow | 1.0 - 1.5 lbs / sq ft | 13 - 20 inches |
| Wet, Heavy Snow | 1.5 - 2.5 lbs / sq ft | 8 - 13 inches |
| Solid Ice | 4.7 lbs / sq ft | 4.2 inches |
Quick facts
- One inch of solid ice weighs about 4.7 pounds per square foot—equivalent to almost 10 inches of fresh snow.
- Metal roofs with unobstructed eaves shed snow much faster than shingle roofs, resulting in lower structural design load requirements.
- Unheated structures like garages support heavier snow loads because they do not benefit from heat escaping the building to melt snow.
Frequently asked questions
Most standard residential roofs built to modern codes are designed to support a minimum snow load of 20 to 40 pounds per square foot (psf). In heavy snow zones, roofs are designed for 50 to 100+ psf.
Yes. Fresh, dry powder weighs about 5 lbs per cubic foot, while wet, compacted snow can weigh up to 20-30 lbs per cubic foot. Solid ice is the heaviest, weighing approximately 57 lbs per cubic foot.
Because metal roofs slide snow off very quickly, they can cause dangerous 'snow avalanches' over walkways and doorways. Snow guards are recommended in these locations to break up or hold back the snow.
Warning signs include sagging ceiling tiles, new cracks in drywall around door frames, doors that stick or won't close, creaking/cracking sounds from rafters, and severe water leaks.