Wind uplift occurs when the air pressure below a roof system is greater than the air pressure above a system. As wind flows over a building, the pressure directly above the roof surface decreases. Simultaneously, atmospheric pressure from below attempts to equalize this pressure differential, resulting in an upward push of air from below the roofing system. This phenomenon is referred to as wind uplift.
Additional uplift forces are generated on roof membranes whenever air can readily move from the building's interior to the underside of the roofing assembly, commonly through air-permeable decking without an air barrier, such as steel or wood, or through cracks, penetrations or perimeters in monolithic decks. In these situations, the membrane itself acts as an effective air barrier, receiving the full impact of the uplift load.
The height of the building, surrounding terrain and position relevant to the wind direction can further affect wind uplift forces. Turbulent vortices at the building's perimeter and corners are produced when updrafts, created as wind is deflected up the side of the building, meet the horizontally moving air stream at rooftop level. The resulting vortices not only cause violent turbulence, but can further magnify the differential between external and internal air pressures, resulting in even greater uplift forces.
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