Snow Load Calculator

Structural Engineering & ASCE 7-22 Roof Snow Load Mechanics

In structural engineering, the American Society of Civil Engineers (ASCE 7) and the International Building Code (IBC) specify design snow loads to ensure roof framing, trusses, and purlins withstand gravity dead and live winter loads.

Primary ASCE 7 Snow Load Equations

• Flat Roof Snow Load (p_f):

p_f = 0.7 × C_e × C_t × I_s × p_g

• Minimum Flat Roof Snow Load (p_f,):

p_f, = cases I_s · p_g & if p_g 20 psf \\ 20 · I_s & if p_g > 20 psf cases

• Sloped Roof Snow Load (p_s):

p_s = C_s × p_f

• Total Snow Weight: W = p_s × Roof Area (sq ft)

Critical Engineering Modification Factors

• Exposure Factor (C_e): Accounts for wind scouring. Fully exposed roofs have lower snow retention (0.9), while sheltered roofs accumulate deeper drifts (1.2).

• Thermal Factor (C_t): Heated buildings lose heat upward to melt snow (1.0), while unheated barns (1.2) and commercial freezers (1.3) sustain heavy unmelted snow packs.

• Slope Factor (C_s): Steep roofs shed snow quickly.

How to Use This Calculator

Enter your Ground Snow Load (p_g — PSF) (e.g. 30 in psf), Roof Pitch & Slope Angle — choose from Flat to 4/12 Pitch (0° to 18.4° — Cs = 1.00), 6/12 Pitch (26.6° — Cs = 1.00), 8/12 Pitch (33.7° — Cs = 0.90), Terrain & Wind Exposure (C_e) — choose from Partially Exposed (Standard Suburban/Trees — Ce = 1.0), Fully Exposed (Open Plains / Windy Coastal — Ce = 0.9), Sheltered (Dense Tall Conifers / Urban Canyons — Ce = 1.2), Thermal Insulation & Heating (C_t) — choose from Heated Building (Standard Residential/Commercial — Ct = 1.0), Unheated Garage / Pole Barn / Shed (Ct = 1.2), Kept Below Freezing / Cold Storage (Ct = 1.3), Occupancy Category & Importance (I_s) — choose from Category II — Standard Homes & Buildings (Is = 1.0), Category IV — Essential / Hospital / Police / Fire (Is = 1.2), Category I — Agricultural / Temporary Sheds (Is = 0.8), and Total Roof Surface Area (Sq Ft) (e.g. 1500 in sq ft). Results update instantly as you type — adjust any value to compare scenarios.

The Formula

ASCE 7 Flat roof snow load: pf = 0.7 Ce Ct Is pg

Sloped roof snow load: ps = Cs * pf

Worked Example

A 1,500 sq ft home in a 30 PSF ground snow zone experiences a design roof load of 21.0 PSF, supporting over 15.75 tons of snow without structural rafter deflection.

Understanding Your Result

Evaluate your result in the context of your specific use case and relevant industry standards or reference values. A single calculated result is a starting point — test two to three scenarios to understand how sensitive the outcome is to your key input assumptions. For high-stakes decisions, verify your result against a secondary source or subject-matter expert before acting.