ASCE 7-22 Chapter 7 - Snow Loads: The Engineering Standard Behind Colorado Garage Door Structural Requirements

Colorado snow loads are higher than most of the country, and they vary dramatically by elevation and location. ASCE 7-22 Chapter 7 defines how those snow loads are calculated and applied in structural design. For garage structures and door systems, the chapter provides the engineering basis for header and framing requirements.

What this standard says

ASCE 7-22 Chapter 7 governs the determination of snow loads for structural design. The chapter works from a ground snow load (pg), which is the weight of snow on the ground for a specific return period and location. Chapter 7 then provides conversion factors to determine the design snow load on a roof.

SEAC describes the relationship between ground and roof snow loads:

"The flat roof snow load is calculated by applying a 0.7 roof-to-ground conversion factor, adjusted for exposure, thermal conditions, and risk category."

The flat roof snow load (pf) is calculated as:

pf = 0.7 × Ce × Ct × Is × pg

Where: - 0.7 is the basic ground-to-roof conversion factor - Ce is the exposure factor (reduces load for exposed sites, increases for sheltered areas) - Ct is the thermal factor (increases load for unheated structures) - Is is the importance factor based on risk category - pg is the ground snow load from the ASCE 7-22 map or an authority having jurisdiction (AHJ) value

The 0.7 factor accounts for the fact that roofs typically carry less snow than the ground because of wind removal and heat loss. For unheated structures like detached garages, the thermal factor Ct is 1.3 instead of 1.0, which increases the design snow load by 30 percent.

Ground snow loads in Colorado vary widely with elevation and terrain. The ASCE 7-22 national map provides regional values, but Colorado's complex terrain requires site-specific values. The Structural Engineers Association of Colorado (SEAC) published the 2016 Colorado Design Snow Loads map, which provides ground snow load values at much finer geographic resolution than the national ASCE 7 map. Most Colorado AHJs, including Denver, Jefferson County, and Larimer County, require use of the SEAC 2016 values or locally specified values.

Denver's 2025 Building Code increased the ground snow load to 43 psf (up from 35 psf in previous editions). This affects the design load for all new structures in Denver, including garages.

When it applies

ASCE 7-22 Chapter 7 is referenced by both the IBC and IRC for snow load design. It applies to any structure that requires structural engineering review, including garages with wide spans, attached structures with large roof areas, or facilities where snow accumulation over a garage door opening is a concern.

For a standard residential garage door, the primary structural impact of snow load is on the header: the structural element spanning the door opening. The header must be sized to carry the roof load above the opening, including snow load. This is a framing and structural design issue, not a garage door design issue directly.

For carports and structures open on one side, drifting snow provisions in ASCE 7-22 Section 7.8 may produce design loads higher than the flat roof snow load.

What this means for you

Denver's ground snow load increased in 2025. The 2025 Denver Building Code uses 43 psf ground snow load. This is a significant increase from the previous 35 psf. New garage structures designed under the 2025 code must be sized for this higher load.

Unheated garages have a higher design load. The thermal factor for unheated structures is 1.3. A detached, unheated garage in Denver has a flat roof design snow load of approximately 0.7 x 1.3 x 43 = 39 psf (before exposure and importance adjustments). This is the structural basis for header and rafter sizing.

The snow load affects the header, not the door panel. A residential sectional garage door panel does not carry roof snow load. The structural load from snow is carried by the header beam above the door. The door must fit properly in the opening, but its panel strength is governed by wind load, not snow load.

Drifting can add substantial load at certain locations. Snow drift at an adjacent higher roof, against a wall, or at a change in roof elevation can produce design loads significantly greater than the flat roof snow load. A structural engineer should evaluate any garage with adjacent elevated surfaces.

G Brothers can identify when a header above a garage door may be undersized for current Denver snow loads and recommend engineering review.

Full text and source

ASCE 7-22 is a copyrighted standard published by the American Society of Civil Engineers and available at asce.org. The SEAC 2016 Colorado Design Snow Loads document and related guidance are available at https://www.seacolorado.org/publications.php. Denver's 2025 snow load update is at the Denver Community Planning and Development source above.

ASCE 7-22 Chapter 7 snow load provisions apply to structural design. The snow load on a garage roof affects the header design above the door opening, not the garage door panel structural rating.

Want to put numbers to this? Use the interactive roof snow load calculator below, or open the full roof snow load calculator with examples and notes.