Convert Ground Snow Load to Roof Snow Load: A Practical Guide for U.S. Buildings

The article explains how to convert the published ground snow load (pg) into the design roof snow load used for structural design, highlights the controlling factors from standard building codes, and shows practical calculations and common adjustments designers must consider. The guidance references industry practice and emphasizes checking the local building code and ASCE standards for jurisdictional specifics.

Term	Definition	Typical Value/Range
pg	Ground Snow Load From Map Or Local Authority	Varies By Location (psf)
Pf (Roof Snow Load)	Uniform Design Snow Load On Roof (psf)	Computed From pg And Factors
Ce	Exposure Factor	0.9–1.2
Ct	Thermal Factor	0.9–1.2
Is	Importance Factor	1.0–1.15 (Higher For Essential Facilities)
0.7	Conversion Coefficient In ASCE 7 For Flat Roofs	Constant Multiplier

Why Convert Ground Snow Load To Roof Snow Load

Building codes publish ground snow load (pg) as a geographic value representing the weight of snow on level ground and open areas. Structural design requires a roof snow load that accounts for the roof’s exposure, thermal behavior, importance, and geometry. Converting pg to roof load ensures safe, economical structural members and roof system design under realistic loading conditions.

Governing Standard And Basic Conversion Formula

Designers typically follow the ASCE standard and local building codes. The commonly used conversion for uniform roof snow on many roof types is: Pf = 0.7 × Ce × Ct × Is × pg. This formula yields the uniform roof snow load (Pf) in pounds per square foot (psf). The factor 0.7 comes from the code’s conversion of ground snow to a flat-roof uniform load under typical conditions.

Explanation Of Factors

Ground Snow Load (pg)

pg Is The Ground Snow Load Published On Maps Or Provided By Local Jurisdiction. Values Vary Widely Across The U.S., So Designers Should Use The Local Code Or Authority Having Jurisdiction (AHJ) Value.

Exposure Factor (Ce)

Ce Accounts For The Wind And Shelter Effects That Influence Snow Accumulation On Roofs. Open Exposures Can Increase Snow Retention, While Sheltered Sites May Reduce It. Typical Ce Values Range From About 0.9 (sheltered) To 1.2 (exposed), As Defined By The Standard.

Thermal Factor (Ct)

Ct Reflects Heat Loss Through The Roof That Causes Snowmelt. Warm Roofs With Significant Heat Loss Can Reduce Snow Accumulation; Thus Ct Can Be Less Than 1.0 For Heated Buildings And Near 1.0 For Cold Or Well-Insulated Roofs.

Importance Factor (Is)

Is Adjusts The Design Load For Building Importance Or Occupancy Category. Essential Facilities (Hospitals, Emergency Services) May Have Higher Is Values To Increase Design Reliability.

When The Standard Conversion Does Not Apply

The 0.7×Ce×Ct×Is×pg formula is for a uniform roof snow load under ordinary conditions. It does not replace special checks such as snow drifts, roof unbalanced loads, sliding snow, or accumulation due to parapets and rooftop obstructions. These conditions often require separate localized loads or analysis and can produce much higher design loads than the uniform Pf.

Handling Roof Slope And Snow Reduction

Many codes allow snow load reduction for steep roofs. For roofs with slopes above a threshold (commonly 30 degrees), a slope factor reduces the uniform roof snow load. The exact slope-reduction formula and thresholds are provided in ASCE and local codes, so designers should apply the code-specific slope coefficient rather than assuming a generic reduction.

Common Special Cases: Drifts, Sliding, And Unbalanced Snow

Snow drifting against parapets, rooftop equipment, or steeper roof planes can produce localized loads far exceeding uniform Pf. The code provides procedures to compute drift heights and resulting concentrated loads. Drift loads and sliding snow loads must be checked separately and combined with Pf when applicable.

Step-By-Step Example Calculation

Assume A Location With pg = 30 psf, Ce = 1.0, Ct = 1.0, Is = 1.0. Using The Basic Formula: Pf = 0.7 × 1.0 × 1.0 × 1.0 × 30 = 21 psf. This 21 psf Is The Uniform Roof Snow Load For A Flat Or Low-Slope Roof Under Standard Conditions.

Example With Factors: If The Same Building Has Ce = 1.1 (exposed) And Is = 1.15 (important facility), Then Pf = 0.7 × 1.1 × 1.0 × 1.15 × 30 ≈ 26.5 psf.

Calculating Drift Loads—Overview

Drift Calculations Usually Require Determining The Drift Height Based On Roof Geometry And Snow Source (e.g., ridge or adjacent upper roof). The Resulting Drift Load Is Often Applied Over A Limited Width Or As An Equivalent Uniform Load On The Affected Tributary Area. Designers Should Follow The Code Drift Procedures For Accurate Dimensions And Loads.

Practical Tips For Designers And Owners

• Always Use The Ground Snow Load (pg) From The Local Jurisdiction Or Code Map.
• Apply The Exact Factor Values (Ce, Ct, Is) From The Applicable ASCE Edition Or Local Code.
• Check For Drifts, Sliding Snow, And Roof-Edge Accumulations—These Often Control Design.
• For Roof Remodels Or Additions, Verify Existing Roof Geometry And Thermal Conditions Before Assuming Ct Values.
• Consult The AHJ And A Licensed Structural Engineer For Ambiguous Or Complex Conditions.

Example Table Of Typical Factor Values

Factor	Condition	Typical Value
Ce	Sheltered	0.9
Ce	Normal	1.0
Ce	Exposed	1.1–1.2
Ct	Warm Roof	0.8–0.95
Ct	Well Insulated / Cold Roof	1.0
Is	Ordinary Building	1.0
Is	Essential Facility	1.05–1.15

Code References And Best Practice Recommendations

The American Society Of Civil Engineers (ASCE) standard and the International Building Code (IBC) provide the procedures and tables for converting ground snow to roof snow and for calculating drifts and sliding. Designers Should Use The Specific Edition Adopted By The Jurisdiction And Confirm Factor Values With The AHJ.

Workflow Checklist For Converting Ground Snow Load To Design Roof Load

1. Obtain pg From Local Code Maps Or AHJ Documentation.
2. Determine Ce Based On Site Exposure And Surrounding Terrain.
3. Estimate Ct Based On Roof Thermal Characteristics And Insulation.
4. Set Is Based On Building Importance And Occupancy Category.
5. Compute Pf Using The Code Conversion (Typically Pf = 0.7×Ce×Ct×Is×pg).
6. Evaluate Roof Slope Reduction And Apply As Required By The Code.
7. Perform Drift, Sliding, And Unbalanced Snow Analyses Where Geometry Or Obstacles Indicate Additional Loads.
8. Document Assumptions And Confirm With The AHJ Or A Licensed Structural Engineer.

Common Pitfalls And How To Avoid Them

Errors Often Occur By Using Incorrect pg Values, Applying Generic Factor Values Without Site Verification, Or Ignoring Local Code Revisions. Always Confirm The Governing Code Edition, Verify Factor Selections With Site Observations, And Address Special Conditions Such As Drifts Or Parapets Explicitly.

Additional Resources

For Detailed Procedures, Consult The Current ASCE Standard On Minimum Design Loads And The Local Building Code. Many State And Local Websites Publish Ground Snow Maps And Jurisdictional Amendments. When In Doubt, Engage A Licensed Structural Engineer To Validate Load Calculations And Combinations.