The ground elevation factor (Ke) is a wind-design factor used in ASCE 7 to account for changes in air density caused by a building’s elevation above sea level. It is particularly relevant when calculating wind velocity pressure for roofs and other building components. For coastal buildings, however, Ke is usually close to 1.00 because many coastal sites are located at or near sea level.
What Does the Ground Elevation Factor Ke Mean?
Air becomes less dense as elevation increases. Because wind pressure depends partly on air density, the same wind speed can produce slightly less pressure at a high-elevation site than it would at sea level.
ASCE 7-16 introduced Ke directly into the velocity-pressure calculation. The equation is:
qz = 0.00256 × Kz × Kzt × Ke × Kd × V²
Here, qz is velocity pressure, Kz accounts for exposure and height, Kzt accounts for topographic effects, Kd is the wind directionality factor, Ke accounts for ground elevation, and V is the basic wind speed.
The current ASCE 7 provisions also permit designers to conservatively use Ke = 1.00 at all elevations. When the actual elevation is considered, Ke generally decreases as elevation above sea level increases. For example, ASCE 7-16 lists approximately 0.96 at 1,000 ft, 0.90 at 3,000 ft, and 0.80 at 6,000 ft above sea level.
Why Is Ke Important for Coastal Roof Design?
Most coastal properties are relatively close to sea level, so the numerical effect of Ke is normally small. At a site near sea level, Ke is approximately 1.00, meaning there is little or no reduction in calculated velocity pressure from elevation.
That does not mean coastal wind design is less important.
Coastal buildings can experience significant wind pressures because of high basic wind speeds, hurricane exposure, open terrain, and strong wind effects around roof edges and corners. For roof systems, these factors can influence the required resistance of membranes, insulation, fasteners, edge metal, copings, and other components.
In other words, Ke is only one part of the wind-load calculation. A coastal roof should not be considered adequately designed simply because its elevation factor is close to 1.00.
How Ke Works With Other Wind Factors
For a commercial roof, engineers typically consider multiple factors when determining design wind pressures. These can include:
- Basic wind speed (V): The wind speed specified for the building location and risk category.
- Exposure category (Kz): Accounts for surrounding terrain and the height of the roof or component above ground.
- Topographic factor (Kzt): Accounts for wind speed-up caused by qualifying hills, ridges, or escarpments.
- Ground elevation factor (Ke): Adjusts for air-density changes with elevation.
- Wind directionality factor (Kd): Accounts for the probability that the maximum wind will occur from a critical direction.
- External and internal pressure coefficients: Used to determine pressures acting on specific roof and wall areas.
For low-slope roofs, the resulting velocity pressure is then used with applicable pressure coefficients to determine design pressures, including wind uplift.
What Does This Mean for Coastal Property Owners?
For a coastal commercial building, Ke will commonly have little effect because the property is near sea level. The more significant design considerations are often the local wind speed, exposure category, building geometry, roof zones, and hurricane-related wind uplift.
This is especially important around roof perimeters and corners, where wind pressures can be substantially higher than in the interior roof field. Proper design and installation of the complete roofing assembly—including membrane attachment, insulation, fasteners, edge securement, and perimeter details—can therefore be critical to long-term performance.
The Bottom Line
The ground elevation factor (Ke) adjusts wind velocity pressure to account for the lower air density found at higher elevations. At coastal locations near sea level, Ke is generally about 1.00, so its direct effect on wind pressure is limited. However, coastal roof design still requires careful evaluation of wind speed, exposure, roof geometry, pressure zones, and attachment systems.
For commercial properties in hurricane-prone coastal areas, understanding Ke is useful—but it should always be evaluated as part of the complete ASCE 7 wind-load calculation, rather than as a standalone measure of roof performance.
Related Questions
- How does risk category affect the design wind speed for my roof?
- How does building height affect roof uplift pressures?
- What is the difference between design wind speed and ultimate wind speed?
- How does wind-driven sand abrasion affect membranes near the beach?
- How does a building classified as “partially enclosed” affect roof uplift design?
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