DASMA TDS 193 - Wind Exposure Categories for Garage Door Design

Two identical garage doors in different locations can have different wind load requirements. The reason is wind exposure. DASMA TDS 193 explains the three exposure categories and how each one affects the design pressure your door must meet.

What this data sheet says

DASMA TDS 193 describes the wind exposure categories defined in ASCE 7. ASCE 7 is the structural engineering standard behind the IRC and IBC wind load chapters. There are three categories:

• Exposure B: urban or suburban areas with closely spaced obstructions at least 30 feet high. Neighborhoods with mature trees and adjacent buildings are typically Exposure B.
• Exposure C: open terrain with scattered obstructions. This covers open country, grasslands, and all locations within 1,500 feet of open water. Most Colorado Front Range locations are Exposure C because of the open prairie to the east.
• Exposure D: flat, unobstructed areas directly exposed to large bodies of water. This category rarely applies in Colorado.

"The exposure category shall be determined at the building site based on the surface roughness that prevails in the upwind direction for a distance sufficient to affect the mean hourly design wind speed."

A door in Exposure C must be rated for a higher design pressure than the same door in Exposure B. Both may be in a 115 mph wind zone, but open terrain produces less buffering of the wind. The result is a higher required psf rating at the same wind speed.

When it applies

Denver and most Front Range suburbs use Exposure C. The 2025 Denver Building Code calls for Exposure C in Table R301.2(1). Jefferson, Arapahoe, Douglas, and Larimer Counties also use Exposure C for most locations.

At 115 mph Vult, Exposure C produces a higher effective velocity pressure than Exposure B. That difference shows up in the psf rating required for the door. Two homes in "115 mph zones" in different states may require different door ratings because of exposure.

Foothills locations west of Denver (Morrison, Evergreen, Bailey) may have site-specific exposure conditions. A structural engineer may need to evaluate those sites. ASCE 7 allows the exposure category to vary by direction, which matters in complex terrain.

What this means for you

Ask for the exposure category when comparing door specifications. A door rated at 24 psf may meet requirements in Exposure B but fall short in Exposure C. The psf alone does not tell the whole story.

Denver permit applications require Exposure C documentation. The pressure certificate submitted to the city should reflect the Exposure C design pressure for your opening size.

Foothills and mountain communities may need site-specific review. Standard ASCE 7 table values assume uniform terrain. Complex topography near the mountains can accelerate wind in ways the standard tables do not capture.

G Brothers uses Exposure C as the design basis for all Denver metro and Front Range door specifications. We can provide the supporting documentation for permit submittals.

Full text and source

Download DASMA TDS 193 from the official TDS index at https://www.dasma.com/technical-data-sheets/.

This entry explains wind exposure categories for residential and commercial garage door specification. The category determinations follow ASCE 7-22 and the definitions adopted by reference in the 2024 IBC and IRC.