How to Calculate Roof Snow Load

The ASCE 7 Snow Load Formula

Building codes use ASCE 7 (Minimum Design Loads for Buildings) to calculate roof snow loads from ground snow data.

Step 1: Find Your Ground Snow Load (Pg)

Ground snow load varies dramatically by location. Check your local building code or use ASCE 7 Figure 7.2-1. General ranges:

Step 2: Exposure Factor (Ce)

Accounts for how exposed or sheltered the building site is to wind.

• Fully exposed (open terrain): Ce = 0.8 — wind blows snow off the roof
• Partially exposed: Ce = 1.0 — standard suburban conditions
• Sheltered (dense trees/buildings): Ce = 1.2 — snow accumulates more

Step 3: Thermal Factor (Ct)

Accounts for heat loss through the roof that melts snow.

• Heated building: Ct = 1.0 — heat melts some snow
• Unheated/ventilated attic: Ct = 1.1 — less melting
• Unheated structure (barn, warehouse): Ct = 1.2 — minimal melting

Step 4: Slope Factor (Cs)

Steeper roofs shed snow faster. Below 15° (roughly 3/12 pitch), Cs = 1.0. Above that, it decreases:

Worked Example

Location: Denver, CO (Pg = 30 PSF), suburban (Ce = 1.0), heated home (Ct = 1.0), 6/12 asphalt shingles (Cs = 0.87):

Combined Design Load

Total roof design load combines dead load (materials) + live load (workers) + snow load:

• Dead load: 8-25 PSF depending on roofing material
• Live load: 20 PSF minimum (IRC code)
• Snow load: Calculated above
• Total: Verify your framing can handle the combined load