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Roof Snow Load Calculator

Estimate ASCE 7 compliant roof snow loads. Adjusts for roof pitch, thermal factors, and exposure categories.

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About

Structural failures in winter are often caused by underestimating the weight of accumulated snow. This tool implements the simplified logic from ASCE 7 standards to convert Ground Snow Load (Pg) into Roof Snow Load (Pf). It accounts for the critical reduction factors: slope (snow slides off steeper roofs), thermal status (heated roofs melt snow faster), and exposure (wind sweeps snow off open roofs). Accurate calculation is essential for sizing rafters and trusses correctly to prevent collapse.

roofing snow load structural engineering asce 7

Formulas

The Flat Roof Snow Load is calculated first:

Pf = 0.7 × Ce × Ct × I × Pg

If the roof is sloped, a Slope Factor Cs is applied based on the angle θ (pitch). For standard shingles:

Ps = Pf × Cs

Reference Data

FactorSymbolTypical ValueCondition
Ground Snow LoadPg20-100 psfVaries by zip code (Map data)
Exposure FactorCe1.0Partially exposed (Suburban)
Exposure FactorCe0.9Fully exposed (Windy/Open)
Exposure FactorCe1.2Sheltered (Trees/Buildings)
Thermal FactorCt1.0Heated structures (Standard)
Thermal FactorCt1.2Unheated structures (Barn/Garage)
Importance FactorIs1.0Risk Category II (Residential)
Importance FactorIs0.8Risk Category I (Agriculture)
Importance FactorIs1.2Risk Category IV (Hospitals)

Frequently Asked Questions

This value is determined by local building codes based on historical weather data. You must consult the ASCE 7 hazard tool map or your local county building department website. It is typically between 20 psf (South) and 100+ psf (North/Mountains).
No. Snow drift occurs where lower roofs meet higher walls or parapets, creating deep accumulations. Drift load calculations are complex and require a separate analysis of the roof geometry.
PSF stands for "Pounds per Square Foot". It is the standard unit of pressure in US structural engineering. 20 psf is roughly equivalent to 1 kPa (kiloPascal).
The 0.7 factor accounts for the density difference and wind scouring. Snow on a roof is generally less dense and less accumulated than snow measured on the ground.